Dissertations / Theses on the topic 'Spintronica'

In questo lavoro di tesi è stata studiata l'anisotropia magnetica di film sottili epitassiali di La0.7Sr0.3MnO3 (LSMO), cresciuti con la tecnica Channel Spark Ablation su substrati monocristallini di SrTiO3 (001). L'interesse nei confronti di questi materiali nasce dal fatto che, grazie alla loro proprietà di half-metallicity, sono usati come iniettori di spin in dispositivi per applicazioni in spintronica, l'elettronica che considera elemento attivo per l'informazione non solo la carica elettrica ma anche lo spin dei portatori. Un tipico esempio di dispositivo spintronico è la valvola di spin (un dispositivo costituito da due film ferromagnetici metallici separati da uno strato conduttore o isolante) il cui stato resistivo dipende dall'orientazione relativa dei vettori magnetizzazione (parallela o antiparallela) degli strati ferromagnetici. E’ quindi di fondamentale importanza conoscere i meccanismi di magnetizzazione dei film che fungono da iniettori di spin. Questa indagine è stata effettuata misurando cicli di isteresi magnetica grazie ad un magnetometro MOKE (magneto-optical Kerr effect). Le misure di campo coercitivo e della magnetizzazione di rimanenza al variare dell'orientazione del campo rispetto al campione, permettono di identificare l'anisotropia, cioè gli assi di facile e difficile magnetizzazione. I risultati delle misure indicano una diversa anisotropia in funzione dello spessore del film: anisotropia biassiale (cioè con due assi facili di magnetizzazione) per film spessi 40 nm e uniassiale (un asse facile) per film spessi 20 nm. L'anisotropia biassiale viene associata allo strain che il substrato cristallino induce nel piano del film, mentre l'origine dell'uniassialità trova la giustificazione più probabile nella morfologia del substrato, in particolare nella presenza di terrazzamenti che potrebbero indurre una step-induced anisotropy. Il contributo di questi fattori di anisotropia alla magnetizzazione è stato studiato anche in temperatura.

Lo scopo di questa tesi è la fabbricazione di ossidi complessi aventi struttura perovskitica, per mezzo della tecnica Channel Spark Ablation (CSA). Più precisamente sono stati depositati film sottili di manganite (LSMO), SrTiO3 (STO) e NdGaO3 (NGO). Inoltre nel laboratorio ospite è stata effettuata la caratterizzazione elettrica e dielettrica (spettroscopia di impedenza), mentre per l'analisi strutturale e chimica ci si è avvalsi di collaborazioni. Sono stati fabbricati dispositivi LSMO/STO/Co e se ne è studiato il comportamento magnetoresistivo e la bistabilità elettrica a seconda del carattere epitassiale od amorfo dell'STO. I risultati più promettenti sono stati ottenuti con STO amorfo. Sono stati costruiti diversi set di condensatori nella configurazione Metallo/Isolante/Semiconduttore (MIS), con M=Au, I=STO o NGO ed S=Nb:STO, allo scopo di indagare la dipendenza delle proprietà dielettriche ed isolanti dai parametri di crescita. In particolare ci si è concentrati sulla temperatura di deposizione e, nel caso dei film di STO, anche sulla dipendenza della costante dielettrica dallo spessore del film. Come ci si aspettava, la costante dielettrica relativa dei film di STO (65 per un film spesso 40 nm e 175 per uno di 170 nm) si è rivelata maggiore di quella dei film di NGO per i quali abbiamo ottenuto un valore di 20, che coincide con il valore del bulk. Nonostante l'elevata capacità per unità di area ottenibile con l'STO, la costante dielettrica di questo materiale risulta fortemente dipendente dallo spessore del film. Un ulteriore aspetto critico relativo all'STO è dato dal livello di ossidazione del film: le vacanze di ossigeno, infatti, possono ridurre la resistività dell'STO (nominalmente molto elevata), ed aumentarne la corrente di perdita. Al contrario l'NGO è meno sensibile ai processi tecnologici e, allo stesso tempo, ha un valore di costante dielettrica più alto rispetto ad un tipico dielettrico come l'ossido di silicio.

I dispositivi elettronici che sfruttano proprietà legate allo spin elettronico costituiscono un settore molto promettente per lo sviluppo della nanoelettronica del futuro. Recentemente, gli isolanti topologici tridimensionali (IT-3D), quando posti a contatto con materiali ferromagnetici (FM), giocano un ruolo centrale nel contesto del miglioramento dell’efficienza di conversione tra spin e carica elettronici in eterostrutture di tipo FM/TI. L’oggetto principale di questa tesi è lo studio delle interazioni chimico-fisiche tra l’IT-3D Sb2Te3, nelle sue forme granulare ed epitassiale, con film di Fe e Co attraverso l’uso di tecniche di Diffrazione/Riflettività di raggi-X, spettroscopia di risonanza ferromagnetica (FMR) e pompaggio di spin in risonanza ferromagnetica (SP-FMR). In concomitanza con l’ottimizzazione delle proprietà dei materiali, un particolare interesse è stato rivolto verso l’impatto industriale della ricerca presentata. Per questo motivo, per la produzione di Sb2Te3 e di alcuni dei FM impiegati, sono state impiegate tecniche di deposizione di materiali su larga scala ( 4 pollici), quali la Metal Organic Chemical Vapor Deposition (MOCVD) e l’Atomic Layer Deposition (ALD) rispettivamente. Una approfondita caratterizzazione chimica, strutturale e magnetica dell’interfaccia Fe/ Sb2Te3-granulare ha evidenziato un marcato intermixing tra i materiali e una generale tendenza degli atomi di Fe nel legare con l’elemento calcogenuro quando presente in un IT. Attraverso trattamenti termici rapidi e a bassa temperatura sottoposti sui film di Sb2Te3 granulare prima della crescita del Fe, l’interfaccia Fe/Sb2Te3-granulare è risultata morfologicamente più netta e chimicamente stabile. Lo studio di film sottili di Co cresciuti attraverso ALD su Sb2Te3 granulare ha permesso la produzione di interfacce Co/Sb2Te3-granulare di alta qualità, con la possibilità inoltre di modificare le proprietà magneto-strutturali dei film di Co attraverso una selezione appropriata di substrati. Con l’obbiettivo di migliorare le proprietà dei film di Sb2Te3, dei trattamenti termici specifici sono stati condotti su Sb2Te3 granulare appena cresciuto, ottenendo film di Sb2Te3 altamente orientati con una qualità cristallina vicina al cristallo singolo di tipo epitassiale. Questi substrati di Sb2Te3 sono stati utilizzati per produrre eterostrutture di Au/Co/Sb2Te3-epitassiale e Au/Co/Au/Sb2Te3-epitassiale per studiare la loro risposta di FMR. I dati di FMR per il campione Au/Co/Sb2Te3-epitassiale sono stati interpretati considerando un contributo di Two Magnon Scattering (TMS) dominante, verosimilmente a causa della presenza di rugosità magnetica all’interfaccia Co/Sb2Te3-epitassiale. L’introduzione di un interlayer di Au per evitare il contatto diretto tra Co e Sb2Te3 si è dimostrato vantaggioso per la totale eliminazione del contributo di TMS. Misure di SP-FMR sono state condotte sulla struttura ottimizzata Au/Co/Au/Sb2Te3-epitassiale, sottolineando il ruolo giocato dallo strato di Sb2Te3-epitassiale nel processo di SP. I segnali di SP ricavati da campioni di Au/Co/Au/Si(111) e Co/Au/Si(111) sono stati utilizzati per determinare l’efficienza di conversione spin-carica ottenuta dall’introduzione dello strato di Sb2Te3. L’efficienza estratta è stata calcolata interpretando i dati di SP-FMR attraverso i modelli di effetto Edelstein inverso ed effetto di Spin-Hall inverso, i quali hanno dimostrato che l’IT-3D Sb2Te3 è un candidato promettente per essere impiegato nella prossima generazione di dispositivi spintronici.
Spin-based electronic devices constitute an intriguing area in the development of the future nanoelectronics. Recently, 3D topological insulators (TI), when in contact with ferromagnets (FM), play a central role in the context of enhancing the spin-to-charge conversion efficiency in FM/TI heterostructures. The main subject of this thesis is the study of the chemical-physical interactions between the granular and epitaxial Sb2Te3 3D-TI with Fe and Co thin films by means of X-ray Diffraction/Reflectivity, Ferromagnetic Resonance spectroscopy (FMR) and Spin Pumping-FMR. Beside the optimization of the materials properties, particular care was taken on the industrial impact of the presented results, thus large-scale deposition processes such as Metal Organic Chemical Vapor Deposition (MOCVD) and Atomic Layer Deposition (ALD) were adopted for the growth of the Sb2Te3 3D-TI and part of the FM thin films respectively. A thorough chemical, structural and magnetic characterization of the Fe/granular Sb2Te3 interface evidenced a marked intermixing between the materials and a general bonding mechanism between Fe atoms and the chalcogen element in chalcogenide-based TIs. Through rapid and mild thermal treatments performed on the granular Sb2Te3 substrate prior to Fe deposition, the Fe/granular-Sb2Te3 interface turned out to be sharper and chemically stable. The study of ALD-grown Co thin films deposited on top of the granular-Sb2Te3 allowed the production of high-quality Co/granular-Sb2Te3interfaces, with also the possibility to tune the magneto-structural properties of the Co layer through a proper substrate selection. In order to improve the structural properties of the Sb2Te3, specific thermal treatments were performed on the as deposited granular Sb2Te3, achieving highly oriented films with a nearly epitaxial fashion. The latter substrates were used to produce Au/Co/epitaxial-Sb2Te3 and Au/Co/Au/epitaxial-Sb2Te3 and the dynamic of the magnetization in these structures was investigated studying their FMR response. The FMR data for the Au/Co/Sb2Te3 samples were interpreted considering the presence of a dominant contribution attributed to the Two Magnon Scattering (TMS), likely due to the presence of an unwanted magnetic roughness at the Co/epitaxial-Sb2Te3 interface. The introduction of a Au interlayer to avoid the direct contact between Co and Sb2Te3 layers was shown to be beneficial for the total suppression of the TMS effect. SP-FMR measurements were conducted on the optimized Au/Co/Au/epitaxial-Sb2Te3 structure, highlighting the role played by the epitaxial Sb2Te3substrate in the SP process. The SP signals for the Au/Co/Au/Si(111) and Co/Au/Si(111) reference samples were measured and used to determine the effective spin-to-charge conversion efficiency achieved with the introduction of the epitaxial Sb2Te3 layer. The extracted SCC efficiency was calculated interpreting the SP-FMR data using the Inverse Edelstein effect and Inverse Spin-Hall effect models, which demonstrated that the Sb2Te3 3D-TI is a promising candidate to be employed in the next generation of spintronic devices.

Nel campo dei semiconduttori, lo studio delle proprietà spin dipendenti forniscono informazioni fondamentali per la realizzazione di dispositivi che uniscano spin, fotonica ed elettronica. In questi dispositivi l’informazione è codificata nel grado di libertà (DOF) dello spin, sfruttando l’accoppiamento spin-orbita (SOC) tra il momento angolare del fotone e lo spin del portatore. Ho concentrato la mia ricerca sullo studio del SOC con spettroscopia ottica in Si, Ge, Sn e loro leghe. Questi materiali possiedono proprietà promettenti per applicazioni di spintronica, tra cui lunghi tempi di vita e lunghezze di diffusione dello spin. I processi di fabbricazione aprono la strada all’ingegnerizzazione del bandgap e dello strain come DOF addizionali per sintonizzare i fenomeni spin-dipendenti. La spettroscopia ottica permette di superare i problemi delle misure elettriche, come la qualità dei contatti, che impediscono una stima corretta dei parametri cinetici. I pozzi quantici (QW), sono valide piattaforme per unire i DOF sopracitati e permettere la manipolazione dello spin con campi elettrici. Nei sistemi a QW che mancano della simmetria di inversione di punto o di quella strutturale (BIA/SIA), la degenerazione di spin è rimossa dai campi Dresselhaus o Rashba. In quanto campi magnetici, possono agire agiscono sullo spin cambiandone l’orientazione. La SIA può sorgere da un drogaggio asimmetrico. In questo caso, il dispositivo possiede anche un campo elettrico che può essere sfruttato. Infatti, con un campo elettrico esterno si può modulare il campo Rashba, manipolando lo spin. Questa possibilità è significativa per la spintronica, si pensi allo spin-FET, dove la tensione di gate seleziona l’orientazione dello spin e quindi lo stato on/off. Ho eseguito misure di fotoluminescenza (PL) su campioni costituiti da uno stack di 50 QW di Ge/Si0.15Ge0.85 cresciuto nella zona intrinseca di un diodo p-i-n. Il drogaggio asimmetrico introduce la SIA, permettendo di manipolare elettricamente lo spin. Tramite due contatti in Al, ho sintonizzato un campo elettrico esterno e studiato gli effetti sulla popolazione di spin tramite PL. Inoltre, un’analisi in funzione della potenza ha mostrato una dipendenza della polarizzazione della PL dalla pompa ottica. Ho studiato anche una singola modulation-doped QW Ge0.91Sn0.09/Ge. Il profilo di banda confina delle lacune e permette la formazione di un gas bidimensionale di lacune. La struttura asimmetrica introduce la SIA, permettendo di studiare meccanismi di conversione spin-carica. Tramite litografia ho realizzato una barra di Hall ed ho eseguito misure di effetto spin-Hall inverso, da cui ho potuto estrarre l’angolo di spin-Hall. Ho anche compiuto misure magneto-ottiche a 4K, sfruttando l’effetto Hanle, per ottenere il tempo di vita (T) del portatore, che risulta di pochi ns. L’effetto Hanle è stato applicato per la prima volta nei materiali del quarto gruppo in epistrati di Ge1-xSnx (sotto), e l’ho esteso alle QW, dimostrandosi una tecnica affidabile per determinare T. Ho studiato anche campioni di epistrato di Ge1-xSnx. Il contenuto di Sn varia da 0 a 10 %, e lo strain compressivo assicura una natura indiretta del bandgap. Ho estratto T e ho trovato un comportamento non banale al variare della percentuale di Sn, la cui origine può essere attribuita alla presenza di difetti del cristallo. Questi difetti sono dovuti possibilmente alla crescita fortemente fuori equilibrio necessaria per la realizzazione di campioni di Ge1-xSnx ricchi in Sn. Per concludere, in questa tesi mi sono occupato di misure ottiche per indagare il SOC in eterostrutture di materiali del IV gruppo. I risultati ottenuti sono un passo in avanti nell’investigazione della dinamica dello spin elettronico del gruppo IV e aprono la strada verso studi futuri sulla manipolazione elettro-ottica dello spin in tecnologie quantistiche basate sull’interazione spin-luce, come spin-FET, spin-laser
In the field of semiconductors, the study of spin-dependent properties provides fundamental information needed for the realization of devices that merge spin, photonic and electronic functionalities. In these devices the information is encoded in the spin degree of freedom (DOF), exploiting the interaction between the angular momentum of the photon and the carrier spin via the spin-orbit coupling (SOC). I focused on the study of SOC in Si, Ge, Sn and their alloys using optical spectroscopy. These materials possess promising properties for spintronics applications such as long spin lifetime, diffusion length and decoherence time. Notably, the advanced manufacture also opens the way to bandgap and strain engineering as further DOF to tune spin-dependent phenomena, whereas the application of optical spectroscopy allows to overcome typical problems of electrical measurements, e.g., the quality of contacts, that hamper the estimation of carrier kinetics parameters Quantum well (QW) systems are valid platforms to merge all the aforementioned DOF and to also introduce a way to manipulate the spin via electric fields. Indeed, in QW systems that possess bulk or structure inversion asymmetry (BIA/SIA), the spin degeneracy is removed due to the Dresselhaus or Rashba fields. As effective magnetic fields, they can act on the spin of a carrier, ultimately changing its orientation. SIA can arise from an asymmetric doping of the device. In this case, the device also possesses an intrinsic electric field, which can be of practical use for applications. Indeed, an external field can be applied to tune the Rashba field, achieving spin manipulation. This opportunity has a strong impact in spintronics devices, such as the spin-FET, where the gate voltage selects the orientation of the spin and switch between on/off states I carried out photoluminescence (PL) investigations on a stack of 50 Ge/Si0.15Ge0.85 QWs grown within the intrinsic region of a p-i-n diode. The asymmetric doping introduces the SIA, necessary for achieving electrical manipulation of the spin. Via a pair of Al contacts, I was also able to study the effect of a tunable external electric field on the spin population via continuous-wave as well as time-resolved PL. Additionally, a power dependent analysis unveiled a strong effect of the light pump on the polarization I also performed PL measurements on a single modulation-doped Ge0.91Sn0.09/Ge QW. The band edge profile confines holes in the well, resulting in the formation of a two-dimensional hole gas. The asymmetric structure introduces the SIA and allows for the observation of spin-to-charge conversion mechanisms in this 2D system. I patterned a Hall bar on the sample and performed inverse spin-Hall effect measurements, extracting the spin-Hall angle. I also performed magneto-optics measurement, namely the Hanle effect, to unveil the carrier lifetime (T) of the material, which is in the ns regime at 10 K. This optical technique was applied for the first time to group IV materials in Ge1-xSnx epilayers (below), and was extended also to the QW system, proving it to be a reliable and easy method to determine T I have also studied Ge1-xSnx epilayers. The Sn content was varied from 0 to 10 %, while the compressive strain ensured an indirect bandgap nature. I applied Hanle effect to extract T and I unveiled a non-trivial behaviour with the Sn content, whose origin is ascribed to the presence of crystal flaws possibly due to the strong out-of-equilibrium growth conditions required for the realization of Sn-rich Ge1-xSnx samples In conclusion, this thesis is devoted to an all-optical investigation of SOC in heterostructures of group IV materials. The results obtained here are a step forwards in the investigation of spin dynamics of electrons in group IV and pave the way to future exploration of electrical-optical manipulation of spins in quantum technologies based on spin-photon interaction such as spin-FETs and spin-lasers

Il lavoro svolto si concentra sul trasporto di carica e spin in dispositivi trilayer La0.7Sr0.3MnO3/SrTiO3/Co multifunzionali. Questi dispositivi mostrano sia magnetoresistenza che resistive switching, con un'interessante interazione fra i due effetti. Le giunzioni SrTiO3 sono state scelte per questo lavoro sia per via dei precedenti studi su SrTiO3 come barriera in dispositivi spintronici (cioè dispositivi con magnetoresistenza), sia perché sono promettenti come materiale base per costruire memristor (cioè dispositivi con resistive switching). Il lavoro di tesi è stato svolto all'Istituto per lo studio dei materiali nanostrutturati (ISMN-CNR) a Bologna. Nella prima parte di questa tesi illustrerò la fisica dietro al resistive switching e alla magnetoresistenza di dispositivi trilayer, mostrando anche risultati di studi su dispositivi simili a quelli da me studiati. Nella seconda parte mostrerò la complessa fisica degli ossidi utilizzati nei nostri dispositivi e i possibili meccanismi di trasporto attraverso essi. Nell'ultima parte descriverò i risultati ottenuti. I dispositivi La0.7Sr0.3MnO3/SrTiO3/Co sono stati studiati tramite caratterizzazione elettrica, di magnetotrasporto e con spettroscopia di impedenza. Le misure ottenute hanno mostrato una fisica molto ricca dietro al trasporto di spin e carica in questi dispositivi, e la mutua interazione fra fenomeni spintronici e di resistive switching rappresenta una chiave per comprendere la fisica di questi fenomeni. Analisi dati della dipendenza della resistenza della temperature e caratteristiche corrente-tensioni saranno usati per quantificare e descrivere il trasporto in questi dispositivi.

Since a decade, spintronics and related physics have attracted considerable attention due to the massive research conducted in these areas. The main reason for growing interest in these fields is the expectation to use the electrons spin instead of or in addition to the charge for the applications in spin-based electronics, quantum information, and quantum computation. A prime concern for these spins to be possible candidates for carrying information is the ability to coherently control them on the time scales much faster than the decoherence times. This thesis reports on the spin dynamics in two-dimensional electron gases hosted in artificially grown III-V semiconductor quantum wells. Here we present a series of experiments utilizing the techniques to optically control the spin polarization triggered by either optical or electrical methods i.e. well known pump-probe technique and current-induced spin polarization. We investigated the spin coherence in high mobility dense two-dimensional electron gas confined in GaAs/AlGaAs double and triple quantum wells, and, it\'s dephasing on the experimental parameters like applied magnetic field, optical power, pump-probe delay and excitation wavelength. We have also studied the large spin relaxation anisotropy and the influence of sample temperature on the long-lived spin coherence in triple quantum well structure. The anisotropy was studied as a function sample temperature, pump-probe delay time, and excitation power, where, the coherent spin dynamics was measured in a broad range of temperature from 5 K up to 250 K using time-resolved Kerr rotation and resonant spin amplification. Additionally, the influence of Al concentration on the spin dynamics of AlGaAs/AlAs QWs was studied. Where, the composition engineering in the studied structures allows tuning of the spin dephasing time and electron g-factor. Finally, we studied the macroscopic transverse drift of long current-induced spin coherence using non-local Kerr rotation measurements, based on the optical resonant amplification of the electrically-induced polarization. Significant spatial variation of the electron g-factor and the coherence times in the nanosecond scale transported away half-millimeter distances in a direction transverse to the applied electric field was observed.
Há uma década, a spintrônica e outras áreas relacionadas vêm atraindo considerável atenção, devido a enorme quantidade de pesquisa conduzidas por elas. A principal razão para o crescente interesse neste campo é a expectativa da aplicação do controle do spin do elétron no lugar ou em adição à carga, em dispositivos eletrônicos e informação e computação quânticas. A possibilidade destes spins carregarem informação depende, primeiramente, da habilidade de controlá-los coerentemente, em uma escala de tempo muito mais rápida do que o tempo de decoerência. Esta tese trata da dinâmica de spins em gases de elétrons bidimensionais, em poços quânticos de semicondutores III-V, crescidos artificialmente. Nós apresentamos uma série de experimentos, utilizando técnicas para o controle ótico da polarização de spin, desencadeadas por métodos óticos ou eletrônicos, ou seja, técnicas conhecidas de bombeio e prova e polarização de spin induzida por corrente. Nós investigamos a coerência de spin em gases bidimensionais, confinados em poços quânticos duplos e triplos de GaAs/AlGaAs e a dependência da defasagem com parâmetros experimentais, como campo magnético externo, potência ótica, tempo entre os pulsos de bombeio e prova e comprimento de onda da excitação. Também estudamos a grande anisotropia de relaxação de spin como função da temperatura da amostra, potência de excitação e defasagem entre bombeio e prova, medidos para uma vasta gama de temperatura, entre 5K e 250K, usando Rotação de Kerr com Resolução Temporal (TRKR) e Amplificação Ressonante de Spin (RSA). Além disso estudamos a influência da concentração de Al na dinâmica dos poços de AlGaAs/AlAs, para o qual a engenharia da composição da estrutura permite sintonizar o tempo de defasagem de spin e o fator $ g $ do elétron. Por fim, estudamos a deriva transversal macroscópica da longa coerência de spin induzida por corrente, através de medidas de Rotação de Kerr não-locais, baseadas na amplificação ressonante ótica da polarização eletricamente induzida. Observamos uma variação espacial significante do fator $ g $ e do tempo de vida da coerência, na escala de nanosegundos, deslocada distâncias de meio milímetro na direção transversa ao campo magnético aplicado.

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Le sujet de cette thèse est l étude de nanofils de cobalt dans une matrice d oxyde de cérium (CeO2) épitaxiée sur SrTiO3(001). L auto-assemblage de nanofils a été mis en évidence lors de la croissance de couches minces de CeO2 fortement dopées au cobalt par ablation laser pulsée. Le caractère métallique du cobalt a été vérifié par des mesures d absorption X au seuil K du cobalt réalisées au synchrotron. La formation de nanofils a été mise en évidence par des études de microscopie électronique en transmission en mode haute résolution et en mode dénergie filtrée. Ces études combinées montrent la formation de fils métalliques de Co dans la matrice, orientés le long de la direction de croissance, de longueur limitée par l épaisseur de la couche et de diamètre dans la gamme 3-7 nm. Ces nanofils constituent des systèmes modèles en nanomagnétisme. Deux assembles de fils (diamètre 3 nm et 5 nm) ont été étudiées en détail. La structure interne des fils a été déterminée par microscopie électronique et le renversement de l aimantation au moyen de mesures magnétiques statiques et dynamiques. L anisotropie magnétique de ces systèmes a été sondée par résonance ferromagnétique. Ces mesures et leurs interprétations ont permis de mettre en évidence la localisation du renversement de l aimantation dans les fils. Ce phénomne de localisation a été corrélé à la structure interne des fils, plus précisément à l existence de grains hexagonaux au sein desquels l anisotropie magnétocristalline est en compétition avec l anisotropie de forme. L ensemble de ces résultats a permis de corréler le comportement magnétique à la structure interne réelle de ces objets.
O objeto de estudo da presente tese é o estudo de nanofios de Cobalto auto-formados em matriz de Óxido de Cério (CeO2) epitaxiado sobre substrato de SrTiO3 (001). A formação espontânea de nanofios de Co metálico foi observada em filmes finos fortemente dopados produzidos por abação laser. O caráter metálico do cobalto presente no filme foi evidenciado através da análise de espectros de absorção de Raios-X na borda K do cobalto realizados no síncrotron SOLEIL. Aglomeração na forma de nanofios pôde ser comprovada através de microscopia eletrônica em transmissão de elétrons nos modos de alta resolução e de filtragem em energia. Combinando os resultados, chega-se a conclusão de formação de nanofios metálicos de Cobalto orientados paralelamente à direção de crescimento do filme com comprimento podendo alcançar até toda espessura do filme e com diâmetro entre 3 e 7 nm. Tais nanofios são sistemas modelos para estudo em nanomagnetismo. Propriedades de dois conjuntos de nanofios (com diâmetros de 3 e de 5 nm) foram detalhadamente estudadas. A estrutura interna foi determinada por microscopia eletrônica e a reversão de magnetização através de medidas estáticas e dinâmicas. A anisotropia magnética dos filmes foi investigada através de ressonância ferromagnética. A interpretação dos resultados permite evidenciar a localização da reversão de magnetização nos nanofios. O fenômeno de localização foi relacionado à estrutura interna dos nanofios, precisamente à existência de grãos de cobalto hcp, nos quais, as anisotropias de forma e magnetocristalina competem. O conjunto de resultados permitiu correlacionar o comportamento magnético com a estrutura real dos nanofios.

The Cu/NiFe/Cu/Co/(CoOx) spin-valves have been prepared by the ion-beam sputtering method. Their GMR ratio and the time stability have been investigated by the magnetoresistance and the MOKE measurements at room temperature. The reproducibility of the preparation of the samples have been studied as well, i.e. two identically configurations of the layers should have the same magnetotransport properties.

Antiferromagnetici sintetici (SAF) costituiti da due strati ferromagnetici separati da un sottile strato metallico non magnetico hanno recentemente suscitato un rinnovato interesse come potenziali candidati per una serie di applicazioni innovative e avanzate nell’ambito della spintronica e della biotecnologia. I SAF sono componenti chiave nei dispositivi spintronici e una significativa attenzione è stata recentemente prestata alla preparazione di tali sistemi su substrati flessibili in virtù dei significativi vantaggi che offrono rispetto a dispositivi fabbricati su substrati rigidi convenzionali, come la capacità di piegare e regolare la forma del substrato, un minor peso e costi contenuti. Sebbene il progresso e lo sviluppo di sistemi spintronici con anisotropia magnetica longitudinale su substrati non planari sia stato notevole nel corso degli ultimi anni, eterostrutture magneto-resistive flessibili con anisotropia magnetica perpendicolare (PMA) sono piuttosto inesplorate nonostante consentano funzionalità aggiuntive e prestazioni migliorate. D'altra parte, per applicazioni diagnostiche e terapeutiche, microdischi SAF con magnetizzazione perpendicolare preparati con approcci litografici top-down sono stati recentemente proposti come valida alternativa alle più studiate particelle superparamagnetiche sintetizzate per via chimica in quanto soddisfano tutti i criteri chiave richiesti per applicazioni biomedicali, consentendo al contempo un significativo grado di controllo e modulazione delle proprietà magnetiche. In questo contesto, la tesi si propone di sviluppare e studiare dispositivi magneto-resistivi su substrati flessibili e microdischi per applicazioni biomedicali basati su SAF a film sottile con PMA. L'attenzione si è concentrata su sistemi basati su Co/Pd e Co/Ni in virtù dell’elevata anisotropia magnetica (~106 J/m3) e della possibilità di regolare in maniera fine le proprietà magnetiche variando lo spessore dei singoli strati e il numero di ripetizioni N del doppio strato di Co/Pd(Ni). In particolare, multistrato flessibili con struttura spin-valve e magnetoresistenza gigante, costituiti da un free layer di [Co/Pd(Ni)]N e da un reference layer SAF con struttura [Co/Pd(Ni)]N/Ru/[Co/Pd(Ni)]N separati da uno strato di Cu, sono stati preparati sia mediante deposizione diretta su substrati flessibili che attraverso l’utilizzo di strategie transfer-and-bonding di tipo wet e dry. Sono state inoltre eseguite misurazioni in condizioni di flessione per indagare la robustezza delle spin-valve flessibili e la possibilità della loro integrazione su superfici curve. Film SAF ottimizzati sono stati infine impiegati per la preparazione di multistrato a film sottile costituiti da ripetizioni multiple di singole unità SAF con struttura [Co/Pd]N/Ru/[Co/Pd]N e anisotropia magnetica perpendicolare allo scopo di fabbricare microdischi SAF free-standing mediante processi litografici.
Although discovered about three decades ago, the peculiar properties of synthetic antiferromagnetic (SAF) thin films consisting of two ferromagnetic layers separated by a non-magnetic metal spacer have recently revived a renewed interest as potential candidates for a number of innovative and advanced applications including spintronics and biotechnology. SAFs are key component in spintronic devices and a significant attention has been recently paid on the preparation of such devises on flexible substrates, which provide wide advantages over their conventional rigid-substrate counterparts, such as the ability to bend and adjust the shape of a device, a light-weight and low costs. While the progress and development of systems with longitudinal magnetic anisotropy on non-planar substrates has been remarkable over the last few years, flexible magneto-resistive heterostructures with perpendicular magnetic anisotropy (PMA) are rather unexplored despite they allow for additional functionality and improved performance. On the other hand, for diagnostic and therapeutic applications, perpendicular magnetized SAF microdisks prepared by top-down lithographic approaches have been recently proposed as a valid alternative to the most investigated superparamagnetic particles synthetized by chemical routes as they fulfill all the key criteria required for biomedical applications while allowing a significant degree of control and tunability of the magnetic properties. Within this context, this thesis aims at developing and studying magneto-resistive spintronic devices on flexible substrates and microdiscs for biomedical applications based on SAF thin film stacks with PMA. The focus was on Co/Pd- and Co/Ni-based systems due to their strong PMA (~106 J/m3) and the possibility to finely tune their magnetic properties by varying the thickness of the individual layers and the number of repetitions N of the Co/Pd(Ni) bilayer. In particular, flexible Co/Pd(Ni)-based giant magnetoresistance spin-valve thin film stacks consisting of a [Co/Pd(Ni)]N free layer and a fully compensated [Co/Pd(Ni)]N/Ru/[Co/Pd(Ni)]N synthetic antiferromagnet reference electrode separated by a Cu spacer, were prepared by direct deposition on flexible substrates and by exploiting both wet and dry-etching transfer-and-bonding approaches. Measurements under bending conditions were also performed to investigate the robustness of the flexible spin-valves and the possibility for their integration on curved surfaces. The optimized SAF stacks were also used for the preparation of thin fil stacks consisting of multiple repeats of single [Co/Pd]N/Ru/[Co/Pd]N SAF units with perpendicular magnetic anisotropy with the aim to fabricate free-standing SAF microdisks by using lithographic processes.

Dans cette thèse, les interactions magnétoélectriques et optomagnétiques transmises par les contraintes dans les structures ferroélectriques/ferromagnétiques sont étudiées. Nous montrons que la dynamique des déformations du Pb(ZrxTi1-x)O3 aboutit à la manipulation électrique sous-coercitive de multi-états ferroélastiques rémanents. La mesure par une jauge résistive de ces états, ainsi que l'écriture et l'effacement électriques et le stockage ferroélastique, sont démontrés. La configuration des contraintes de matériaux ferroélectriques créée électriquement, permet de modifier l'anisotropie magnétique d'une couche ferromagnétique. Ce phénomène est utilisé pour contrôler le champ magnétique coercitif des composants magnétostrictifs des vannes de spin au moyen des déformations. L’irradiation lumineuse est également utilisée pour entraîner une photostriction rémanente dans le BiFeO3. Cette déformation rémanente est transférée à une couche ferromagnétique et permet un contrôle optique de la coercivité magnétique. Nous montrons comment les états magnétiques peuvent être écrits au moyen de la lumière et effacés par un champ électrique
In this thesis, the strain-mediated magnetoelectric and optomagnetic interactions in ferroelectric/ferromagnetic structures are studied. The strain dynamics in Pb(ZrxTi1-x)O3 is shown to result in the sub-coercive electrical manipulation of its remanent ferroelastic multi-states. The resistive readout of these states provided by the strain gauge layers, together with the electrically-triggered ferroelastic writing, storage, and erasing, are demonstrated. These strain configurations created by electric fields in ferroelectrics can effectively impact the magnetic anisotropy of a ferromagnetic adlayer. This phenomenon is shown to control the magnetic coercive field of the magnetostrictive components of spin valves via the strain. Light irradiation is shown to result in remanent photostriction effect (photo-driven deformation) in BiFeO3. Such optically-induced remanent deformations can be transferred to a ferromagnetic adlayer and result in the optical control of the magnetic coercive force. It is shown here how magnetic states can be written by light and erased by an electric field

Magíster en Ciencias, Mención Física
La spintrónica se perfila como una de las corrientes mas atractivas y prometedoras dentro de la materia condensada gracias a la diversidad de fenómenos presentes, como el efecto Hall de spin, la magneto-resistencia gigante. En la spintrónica el estudio de materiales antiferromagnéticos es interesante pues dentro de sus propiedades se encuentran su abundancia natural y la posibilidad de disminuir las escalas temporal y espacial de los fenómenos presentes en ellos. Un ejemplo es la utilización de estos materiales en memorias magnéticas, pues gracias a la ausencia de magnetización neta en un material antiferromagnético es posible almacenar información en regiones de menor tamaño debido a la nula interacción dipolar entre dominios magnéticos. Esta tesis esté compuesta de tres trabajos teóricos orientados al desarrollo de la spintrónica antiferromagnética. En la primera parte se presenta la teoría efectiva de un sistema antiferromagnético no colineal. Para esto consideramos un sistema anisotrópico y con interacción de intercambio entre spines vecinos. A través de un parámetro de orden perteneciente al grupo de rotaciones estudiamos la dinámica de las excitaciones de baja energía del sistema obteniendo como resultado una familia de solitones topológicos que están descritos por la ecuación de sine-Gordon. Finalmente comparamos nuestros resultados con simulaciones numéricas de un sistema de momentos magnéticos obteniendo resultados completamente concordantes. La segunda parte corresponde al estudio de un cristal magnónico antiferromagnético. A partir de una teoría fenomenológica estudiamos la dinámica del campo de magnetización bajo el efecto de interacción de intercambio, y anisotropía uniaxial. A través de una modulación periódica de la anisotropía y del campo magnético caracterizamos el espectro de ondas de spín y las estructura de bandas del sistema. En la tercera y última parte se presenta el estudio de la generación de corrientes de spin mediante deformaciones de una red antiferromagnética gracias a efectos cuánticos. Este fenómeno, conocido como efecto piezospintrónico, es estudiado en dos modelos de interés: grafeno antiferromagnético y zinc-blende antiferromagnético. Este efecto, en conjunto con el efecto Hall de spín inverso pueden ser útiles para la detección de corrientes de spín puras.
Spintronics is one of the most attractive and promising areas in condensed matter due to the diversity of phenomena present in it as the spin Hall e ect and the giant magnetoresistance. In spintronics the study of antiferromagnetic materials is interesting due to their natural abundance and the possibility of decreasing the temporal and spatial scale of the phenomena in which they are involved. One example of this is the use of antiferromagnetic materials in magnetic memories, where due to the absence of net magnetization it is possible to store information in smaller regions because of the null dipolar interaction between domains. This thesis is made of three theoretical works focused in di erent aspects of antiferromagnetic spintronics. In the rst chapter we present the e ective theory of a non collinear antiferromagnet. For this we consider an anisotropic system with exchange interaction among neighbor spins. By making use of an order parameter in the rotation group we study the dynamics of low energy excitations of the system obtaining as result a family of topological solitons which are described by the sine-Gordon equation. Finally we compare our results with numerical simulations of a system of magnetic moments obtaining totally concordant results. The second chapter corresponds to the study of an antiferromagnetic magnonic crystal. From a phenomenological theory we study the dynamics of the magnetization eld under the e ect of exchange interaction and uniaxial anisotropy. Through a periodic modulation of the anisotropy and of the magnetic eld we characterize the spin wave spectra and the band structure of the system. In the third and last chapter we show the study of generation of spin currents by deformation of an antiferromagnetic lattice thanks to quantum mechanical e ects. This phenomenon, known as piezospintronic e ect, is studied in two interesting models: antiferromagnetic graphene and antiferromagnetic zinc-blende. This e ect together with the inverse spin Hall e ect could be useful for the detection of pure spin currents. v
Este trabajo ha sido parcialmente financiado por Proyecto Fondecyt N° 1150072, Proyecto Basal N° FB0807- CEDENNA, y Anillo de Ciencia y Tecnología N° ACT 1117

Dans les dernières années, la miniaturisation progressive des dispositifs de stockage magnétique a rendu nécessaire de comprendre comment les propriétés physiques sont modifiées par rapport à l'état massif lorsque les dimensions sont réduites à l'échelle nanométrique. Pour cette raison, une méthode précise de préparation et caractérisation de nanostructures est extrêmement importante. Ce travail se concentre sur les propriétés magnétiques et de transport de nanoparticules de cobalt incorporées dans des matrices de cuivre. Notre dispositif expérimental nous permet de contrôler indépendamment la taille moyenne des agrégats, la concentration et la composition chimique. La production des agrégats de cobalt est basée sur la pulvérisation cathodique et l'agrégation dans la phase gazeuse. Cette source permet de produire des agrégats dans une large gamme de taille, de un à plusieurs milliers d'atomes. Dans un premier temps, nous avons étudié le rôle des interactions entre particules dans les propriétés de transport et magnétiques, en augmentant la concentration des nanoparticules de cobalt (à partir de 0.5 % à 2.5 % et 5 %). Nos résultats démontrent les précautions nécessaires et constituent une base solide pour de futures études sur les propriétés spintroniques des systèmes granulaires. Dans le but de décrire les propriétés magnétiques intrinsèques d'agrégats, nous avons préparé des échantillons fortement dilués (_0.5%) pour différents diamètres d'agrégats de 1.9 nm à 5.5 nm. Nous avons constaté que les propriétés magnétiques sont dépendantes de la taille. L'utilisation d'une caractérisation magnétique complète, sensible à la variation de l'anisotropie magnétique efficace, nous montre que l'anisotropie magnétique est dominée par les contributions de la surface ou de la forme des nanoparticules
In the last years, the progressive miniaturization of magnetic storage devices has imposed the necessity to understand how the physical properties are modified with respect to the bulk when the dimensions are reduced at the nanometric scale. For this reason an accurate method of preparation and characterization of nanostructures is extremely important. This work focuses on the magnetic and transport properties of cluster-assembled nanostructures, namely cobalt nanoparticles embedded in copper matrices. Our setup allows us to independently control the mean cluster size, the concentration and the chemical composition. The cobalt cluster production is based on magnetron sputtering and gas phase aggregation. The performance of the source permits a wide range of cluster masses, from one to several thousand atoms. As a first step we studied the role of inter-particle interactions in the transport and magnetic properties, increasing the cobalt nanoparticle concentration (from 0.5% to 2.5% and 5%). Our results demonstrate the necessary precautions and constitute a solid basis for further studies of the spintronic properties of granular systems. Finally, in order to describe the intrinsic magnetic properties of cluster-assembled nanostructures, we prepared strongly diluted samples (_0.5%) for different cluster sizes from 1.9 nm to 5.5 nm. We found that the magnetic properties are size-dependent. Using a complete magnetic characterization, sensitive to the change in the effective magnetic anisotropy, we show that the magnetic anisotropy is dominated by the contributions of the surface or of the shape of the nanoparticles

In the last years, the progressive miniaturization of magnetic storage devices has imposed the necessity to understand how the physical properties are modi- ed with respect to the bulk when the dimensions are reduced at the nanometric scale. For this reason an accurate method of preparation and characterization of nanostructures is extremely important. This work focuses on the magnetic and transport properties of cluster-assembled nanostructures, namely cobalt nanoparticles embedded in copper matrices. Our setup allows us to independently control the mean cluster size, the concentration and the chemical composition. The cobalt cluster production is based on magnetron sputtering and gas phase aggregation. The performance of the source permits a wide range of cluster masses, from one to several thousand atoms. As a rst step we studied the role of inter-particle interactions in the transport and magnetic properties, increasing the cobalt nanoparticle concentration (from 0.5% to 2.5% and 5%). Our results demonstrate the necessary precautions and constitute a solid basis for further studies of the spintronic properties of granular systems. Finally, in order to describe the intrinsic magnetic properties of cluster-assembled nanostructures, we prepared strongly diluted samples (<0.5%) for di erent cluster sizes from 1.9 nm to 5.5 nm. We found that the magnetic properties are size-dependent. Using a complete magnetic characterization, sensitive to the change in the e ective magnetic anisotropy, we show that the magnetic anisotropy is dominated by the contributions of the surface or of the shape of the nanoparticles.

This thesis explores the use of 2D materials (graphene and hBN) for spintronics. Interest on these materials in spintronics arose from theoretical predictions of high spin filtering in out-of-plane transport through graphene and hBN sandwiched by ferromagnets. Similarly, 5-layer graphene was forecast to be a perfect spin filter. In the case of in-plane spin transport, graphene was expected to be an excellent material due to its low spin-orbit coupling and low number of defects. Although there already exist experimental works that attempted to explore the aforementioned predictions, they have failed so far to comply with the expected results. Earlier experimental works in graphene and hBN out-of-plane spin transport achieved low spin filtering on the order of a few percent; while spin relaxation parameters in graphene for in-plane spin transport remained one or two orders of magnitude below the predicted values. In the case of vertical devices, the failure to meet the theoretical expectations was attributed to the oxidation of the ferromagnets and the lack of an epitaxial interface between the later and the graphene or hBN. Similarly, the exact mechanisms that lead to high spin relaxation for in-plane spin transport in graphene are not completely understood, in part due to the low-quality of the explored devices. In this thesis we analyze new architectures and procedures that allowed us to fabricate ultraclean and oxidation-free interfaces between ferromagnets and graphene or hBN. In these devices we encountered negative and reversible magnetoresistance, that could not be explained with the previous theoretical models. We propose a new model based on a thorough characterization of the devices and well-known properties of graphene that were not taken into account in the previous model. We also employed a novel type of contact to graphene (1D-contacts) and applied it for the first time to achieve spin-injection in graphene. The main advantage of this type of contact is the full encapsulation of graphene with hBN, which leads to high quality graphene spintronic devices.

In the past three decades, spintronic devices have played an important technological role. Half-metallic alloys have drawn much attention due to their special properties and promised spintronic applications. This dissertation describes some theoretical techniques used in first-principal calculations of alloys that may be useful for spintronic device applications with an emphasis on half-metallic ferromagnets. I consider three types of simple spintronic materials using a wide range of theoretical techniques. They are (a) transition metal based half-Heusler alloys, like CrMnSb, where the ordering of the two transition metal elements within the unit cell can cause the material to be ferromagnetic semiconductors or semiconductors with zero net magnetic moment, (b) half-Heusler alloys involving Li, like LiMnSi, where the Li stabilizes the structure and increases the magnetic moment of zinc blende half-metals by one Bohr magneton per formula unit, and (c) zinc blende alloys, like CrAs, where many-body techniques improve the fundamental gap by considering the physical effects of the local field. Also, I provide a survey of the theoretical models and numerical methods used to treat the above systems.

Spin electronic or spintronic devices which are used in hard disk drive (HDD) read heads are expected to replace the current silicon based transistor technology used in volatile memories. A prime example for the net advantage of employing spin rather than electric charge manipulation is found in the newly developed magnetic random access memory (MRAM) which is proposed as a replacement for the dynamic random access memory (DRAM) based on three terminal metal-oxide-semiconductor (MOS) devices. Besides the decrease of energy consumption by a factor three arising from manipulating electron angular momentum, the magnetic memories are non-volatile hence they do not require constant power to store information. This allows for additional energy saving due to data stability when the storage device is powered off.

Novel characteristics of spin-based phenomena are intensively researched in the hope of discovering effects that could be used to develop new types of high-performance spintronic devices. Recent dynamics studies have revealed new principles for spintronic devices to sense microwaves. The capabilities for detecting both microwave electric field and magnetic field could make the spintronic microwave sensor as ubiquitous as semiconductor devices in microwave applications in the future. In this thesis, the feasibility of spintronic sensors in microwave applications has been researched and developed. Thanks to the high conversion efficiency of microwave rectification in the magnetic tunnel junction (MTJ) based spintronic sensor, it can directly measure the coherent spatially scattered microwave field distribution and detect a hidden object by analyzing the reflected microwave amplitude pattern. To enable the “real-time” vector measurement of the microwave field, a sensor based rapid phase detection technique is also developed. Combining the rapid phase detection technique and the microwave holography principle, a two-dimensional microwave holographic imaging system using a spintronic sensor was built. The high sensitivity of the microwave phase measurement allows the coherent imaging of the target to be reconstructed in noisy environments. By adapting the broadband measurement, not only the shape but also the distance of the target can be determined, which implies that three-dimensional imaging is achievable using a spintronic device. Combining the broadband microwave measurement and a wavefront reconstruction algorithm with a spintronic microwave sensor in circular trajectory, the reconstructed images of targets are obtained. The reconstructed images clearly indicate the targets' positions even when the targets were immersed in a liquid to simulate an inhomogeneous tissue environment. Our spintronic techniques provide a promising approach for microwave imaging, with the potential to be used in various areas, such as biomedical applications, security services, and material characterization.
October 2016

Les matériaux ferrimagnétiques de synthèse à aimantation perpendiculaire ont été étudiés extensivement lors de la dernière décennie. Leurs propriétés d’électronique de spin, notamment en ce qui concerne les propagations de parois magnétiques par l’injection d’un courant, en font des candidats idéaux pour les applications de mémoires magnétiques de type racetrack. Du fait de propriétés remarquables concernant d’une part la génération et le transport de courant de spin par couple de spin orbite, et d’autre part le couplage d’échange de type RKKY, l’iridium est un excellent candidat en tant que matériau de spacer pour les matériaux ferrimagnétiques de synthèse. Dans ce manuscrit, nous étudions des multicouches ferrimagnétiques de synthèse composées de deux couches de cobalt séparées par un spacer d’iridium. Nous présentons d’une part l’optimisation de la croissance de tels matériaux, de sorte à obtenir un système modèle en vue d’applications pour des « racetrack memories ». Nous maximisons ainsi le couplage d’échange antiferromagnétique entre les couches de cobalt et l’aimantation à rémanence. D’autre part, nous étudions les propriétés de transport de spin de l’iridium grâce à des méthodes de résonance magnétique par pompage en spin. Nous en concluons que les matériaux ferrimagnétiques de synthèse à base d’iridium sont des systèmes modèles pour la fabrication de « racetrack memories »
Synthetic ferrimagnets with perpendicular magnetic anisotropy have been studied extensively in the past decades. Their outstanding properties in terms of spintronics, especially concerning the current-induced magnetic domain wall propagation lead us to contemplate them as promising candidates as materials for magnetic racetrack memories. Besides, considering the remarkable properties of iridium concerning the transport and the generation of pure spin currents by means of spin orbit torque, as well as its large RKKY coupling properties, this material seems to be an excellent material as a spacer for synthetic ferrimagnets. In this manuscript, we study magnetic multilayers composed of two magnetic layers of cobalt separated by an iridium spacer. We optimise the growth of these multilayers by choosing the most adequate thicknesses, so as to obtain a model system for racetrack memories applications. Thus, we maximise the antiferromagnetic exchange between the cobalt layers, and the remanence magnetisation. Besides, we study the spin current generation and transport properties of iridium by spin pumping ferromagnetic resonance means. We draw the conclusion that iridium-based synthetic ferrimagnets can be considered as model systems for racetrack memory technology

Magnetoresistance, a change in the resistance of a material in an externally applied magnetic field, is an extremely important property of magnetic materials. The discovery of giant magnetoresistance has led to a revolution in computing, driving increases in storage density of hard disks and paving the way for commerical spintronic devices. Conventionally, magnetoresistance is measured by sourcing a current through a material and measuring the voltage. Ohm’s law is used to calculate the resistance of the material with and without an external magnetic field, the difference between these results being the magnetoresistance. This technique is limited as it does not offer spatial resolution, so variations in magnetoresistance in a material can not be detected. Electrical contact must also be made to the material, which can cause damage to the material being measured. The magnetorefractive effect, the change in the reflection spectrum of a material in an external magnetic field, can be used as an alternative to the electrical measurement of magnetoresistance. The magnetorefractive effect allows non-contact measurements of magnetoresistance to be made, so the material remains undamaged, whilst also offering the possibility of spatial resolution. Modelling the spectral magnetorefractive effect can also aid in understanding the underlying physical mechanism behind the magnetoresistance, which is impossible with an electrical measurement. Infrared reflection microspectroscopy was used to observe variations in reflectivity across Fe3O4 thin films. By modelling these variations, it was possible to estimate the chemical composition of the samples as well as observe any variations in composition across them. A spatial variation in magnetoresistance was observed across a CoFe/Cu multilayer using the magnetorefractive effect, whilst also obtaining the spectral resolution necessary to model the system, the first time such a measurement has been performed. The correlation between the magnetorefractive effect and magnetoresistance had been predicted to be strong in the far-infrared by previous theoretical work. The magnetorefractive effect in the far-infrared was measured for a series of spin valves, demonstrating this strong correlation in the far-infrared for the first time, providing long awaited experimental confirmation of this theoretical prediction.

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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior
Neste trabalho estudamos tres importantes sistemas magnéticos extensamente pesquisados nas últimas décadas. Na primeira parte, a proposta recente da utilização de skyrmions magnéticos, que são excitações topológicas tipo quasi-partícula em ferromagnetos, em memórias tipo racetrack, tem atraído a atenção de pesquisadores nos últimos anos abrindo um novo campo de estudo chamado skyrmionics, que é uma tentativa de utilizar estas estruuras magnéticas como transportadores de informação na próxima geração de de dispositivos spintrônicos. Para a utilização de skyrmions magnéticos, em alguns sistemas é necessário a inclusão de interação Dzyaloshinskii-Moriya e campos magnéticos externos no sistema. Neste trabalho, nós exploramos um sistema sem estes requisitos. Primeiro, propusemos um modo controlado de criação de skyrmions e skyrmioniums impressos em em uma nanofita de material ferromagnético com magnetização fora do plano. Após isso, investigamos o destacamento da estrutura da região abaixo de um nanodisco, responsável por imprimir esta estrutura. O transporte é feito por spin transfer torque devido a pulsos de corrente elétrica spin polarizada aplicadas na nanofita. A detecção da estrutura é feita por magnetoresistência túnel. Esta estrutura que se move, após deixar a região abaixo do disco, não é mais considerada um skyrmion e, calculando como a carga topológica evolui, a estrutura foi chamada de sóliton magnético ressonante. A segunda parte cobre os efeitos de geração de correntes puras de spin por Spin Pumping e efeito Seebeck de Spin e a conversão dessas correntes de spin em correntes de carga em isolantes topológicos a temperatura ambiente. A conversão de corrente de spin em corrente de carga é devido ao efeito Edelstein Inverso (IEE) que é possivel devido ao ’spin-momentum locking’ do elétron no nível de Fermi devido ao campo de Rashba. As medidas nas duas técnicas levaram ao mesmo valor do parâmetro IEE, mostrando que ambos os resultados são maneiras eficientes de converão de corrente de spin em corrente de carga. Na terceira parte, redes de nanomagnetos projetados para assemelhar-se a gelos de spin (estados magnéticos desordenados) e são conhecidos como gelos de spin artificiais e, estudos teóricos e experimentais da termodinâmica nestas redes. Nas redes retangulares de gelos de spin artificiais espera-se que mostrem diferentes transições de fase mudando a geometria do sistema. Esta dinâmica gerada por efeitos geométricos abrem uma possibilidade de explorar diferentes estados fundamentais e geração de monopolos magnéti- cos por efeitos térmicos. Aqui, mostramos que uma rede particular de gelos de spin artificiais se mostram com menos restrições para que as nanoilhas mudem magnetização em uma rede em particular e, comparndo o impacto de efeitos térmicos em mudanças de magnetização em diferentes sistemas, é possível encontrar o fenÃt’meno chamado geometrotermodinâmica.
In this work we study three important magnetic systems extensively researched in the past decades. In the first part, the recent proposition of the use of magnetic skyrmions, which are topological particle-like excitations in ferromagnets, in racetrack memories, have attracted a lot of attention recently opening up a new field of study called skyrmionics which is an attempt to use those magnetic structures as information carriers in next generation of spintronic devices. For usage of magnetic skyrmions, in some systems is necessary to include the Dzyaloshinskii- Moriya interaction (DMI) and the out-of-plane magnetic field into the system. In this work, we explore a system without these requirements. First, we propose a controlled way for the creation of magnetic skyrmions and skyrmioniums imprinted in a perpendicular magnetized ferromagnetic nanotrack. Then we investigate the detachment of the imprinted spin textures from the underneath of the nanodisk, the transport by the spin-transfer torque imposed by spin-polarized current pulses applied in the nanotrack and the detection by Tunnel Magnetoresistance (TMR). We notice that the moving structure is not a skyrmion after is detached, and by calculating how the topological charge behaves, we have called it the resonant magnetic soliton (RMS). The second part covers the generation of spin currents by Spin Pumping and Spin Seebeck effects and the conversion of this spin current to charge current in (Bi 0.22 Sb 0.78 ) 2 T e 3 topological insulators at room temperature. The spin-to-charge current conversion is attributed to the inverse Edelstein effect (IEE) made possible by the spin-momentum locking in the electron Fermi contours due to the Rashba field. The measurements by the two techniques yield the same value for the IEE parameter, showing that those methods can be an efficient way to the spin to charge current in topological insulators. In the third part, arrays of nanomagnets designed to resemble spin ice materials (disordered magnetic states) are known as artificial spin ices (ASI). Here we study, both theoretically and experimentally the thermodynamic effects on streched arrays of spin ices. The rectangular artificial spin ices (RASI) is expected do show different phase transitions by changing the geometry of the system. This geometrically driven dynamics in ASI can open up the panorama of exploring distinct ground states and thermally generated magnetic monopole excitations. Here, it is shown that a particular RASI lattice experience less restriction to flip precisely in a kind of rhombic lattice and by comparing the impact of thermal effects on the spin flips in these three appropriate different RASI arrays, it is possible to find the phenomenon that we call ASI geometrothermodynamics.

Cette thèse a pour but d’étudier le transport de porteur de charge au sein de polymères conjugués, avec comme finalité d’identifier les propriétés des appareils d’électronique organique appropriées pour des applications dans la spintronique organique. Nous avons analysé des échantillons planaires, de géométries latérales, qui offrent la possibilité d’étudier les propriétés de transport sous l’application de différents stimulus et la détection le transport de longue distance du moment angulaire (spin), au sein de semi-conducteurs organiques (OSC). Dans cette configuration, des critères bien établis doivent être satisfait pour réaliser le transport diffusif d’un courant de spin polarisé au travers d’un matériel organique. Nous avons analysé ces diffèrent critères et trouvé des matériaux dont les propriétés physiques fournissent une solution satisfaisante. Le résultat de ce travail fut la création de transistors à effet de champ organiques dont les propriétés répondent au besoin des applications de spintronique
In this thesis, charge carrier transport in conjugated polymers is studied with the aim to identify organic electronics devices properties suitable for applications in organic spintronics. We investigate planar samples, in a lateral geometry, which offer the possibility to study transport properties under the application of different stimuli and to detect long-range spin transport in OSCs. In this configuration, well-established criteria must be satisfied to realize diffusive-like transport of a spin-polarized current through an organic material. We analyse these criteria and find possible materials properties solutions. The outcome is the realization of organic field-effect transistors with properties ad hoc for spintronics applications

In this thesis, we present the mathematical and implementation details of an ab initio method for calculating spin-polarized quantum transport properties of atomic scale spintronic devices under external bias potential. The method is based on carrying out density functional theory (DFT) within the Keldysh non-equilibrium Green's function (NEGF) formalism to calculate the self-consistent spin-densities. This state-of-the-art technique extends previous work by: i) reformulating the theory in spin-space such that the non-equilibrium charge density can be evaluated for different spin-channels, and ii) introducing k-point sampling to treat transverse periodic devices such that correct bulk as well as surface magnetism can be described. Computational details including k-point sampling to converge the Brillouin zone integration, optimization of pseudopotentials and basis sets, and efficient O(N) calculation of the Green's function are presented. We apply this method to investigate nonlinear and non-equilibrium spin-polarized transport in several magnetic tunnel junctions (MTJs) as a function of external bias voltage. Firstly, we find that for an Fe/MgO/Fe trilayer structure, the zero-bias tunnel magnetoresistance (TMR) is several thousand percent, and it is reduced to about 1000% when the Fe/MgO interface is oxidized. The TMR for devices without oxidization reduces monotonically to zero with a voltage scale of about 0.5-1V, consistent with experimental observations. We present an understanding of the nonequilibrium transport by investigating microscopic details of the scattering states and the Bloch bands of the Fe leads. Secondly, we investigate a molecular MTJ composed of Ni leads sandwiching a benzenedithiol(BDT) molecule. We find a TMR of ~27% which declines toward zero as bias voltage is increased. The spin currents are nonlinear functions of bias voltage, even changing sign at certain voltages due to specific features of the coupling between molecular states
Dans cette thèse, nous présentons les détails mathématiques et d'implémentation d'une méthode \emph{ab initio} pour le calcul des propriétés de transport quantique polarisé en spin de dispositifs spintroniques de taille atomique sous un différence de potentiel appliquée. La méthode est basée sur la théorie des fonctionnelles de la densité (DFT) dans le cadre du formalisme des fonctions de Green hors équilibre (NEGF) de Keldysh pour calculer les densités de spin auto-consistantes. Cette technique de pointe étend les travaux précédents: i) en reformulant la théorie dans l'espace de spins de telle manière que la densité de charge hors équilibre peut être évaluée pour différents canaux à spin, et ii) en présentant l'échantillonnage de k-point pour traiter les dispositifs périodiques transversaux de telle façon que le magnétisme de surface et de volume peuvent être décrits correctement. Les détails informatiques, y compris l'échantillonnage de k-points pour la convergence de l'intégration sur la zone de Brillouin, la construction et l'optimisation de la base et des pseudo-potentiels, et un calcul efficace O(N) de la fonction de Green sont présentés. Nous appliquons cette méthode pour étudier le transport polarisé en spin, non linéaire et hors équilibre, dans plusieurs jonctions tunnel magnétiques (MTJs), comme fonction du potentiel appliqué. Premièrement, nous constatons que pour une structure tri-couche de Fe/MgO/Fe, la magnétorésistance tunnel (TMR) à différence de potentiel nulle est de plusieurs milliers de pourcents, et se réduit environ à 1000% lorsque l'interface de Fe/MgO est oxydée. La TMR pour des dispositifs sans oxydation se réduit à zéro de façon monotone avec une différence de potentiel de l'ordre de 0.5-1V, conformément aux observations expérimentales. Nous interprétons nos résultats en étudiant les détails microscopiques d'états de diffusion et des bandes de Bloch dans les électrodes

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2018.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 107-128).
Domain walls are regions of spatially non-uniform magnetizations in magnetic materials. They form the boundaries between two or more uniformly magnetized regions called domains. Skyrmions are circular magnetic domains with chiral domain walls that are interesting due to their stability and potential for fast motion. These spin structures can be used to encode Os and Is in spintronic memory. Chiral domain walls and skyrmions have been seen in magnetic thin films sandwiched between non-identical non-magnetic materials which have high spin-orbit coupling and Dzyaloshinskii-Moriya interaction. An optimization of the different physical interactions involved in magnetic thin films can result in stripe and labyrinth domain patterns which can then be transformed into skyrmion lattices. In this thesis, we present a detailed understanding of single- and multi-layer magnetic thin films along with all the relevant physical interactions. We show that inplane magnetic fields stabilize domain walls in thin films with Dzyaloshinskii-Moriya interaction. The application of in-plane magnetic fields is shown to create multi-domain patterns in films where the ground state is uniform magnetization. Next, we study the formation of stripe and labyrinth domain patterns in magnetic films. The domain widths obtained are compared with the predictions of several theoretical models developed over the last 50 years. The appropriate model that works for thin films with strong Dzyaloshinskii-Moriya interaction is identified with the help of micromagnetic simulations. The appropriate model includes effects of finite domain wall width and volume charges inside Neel domain walls. This model is then used to measure the Dzyaloshinskii-Moriya interaction in experimentally grown magnetic thin films. Thereafter, we highlight the role of other design variables such as the thickness of magnetic and non-magnetic layers, the choice of materials, and the role of geometrical confinement in controlling the length scale of the domain patterns. This work generates the necessary knowledge and develops techniques to engineer chiral spin textures in single- and multi-layer magnetic thin films.
by Parnika Agrawal.
Ph. D.

In this thesis investigations of the static and dynamic properties of magnetic thin films and thin magnetic multilayers with spintronic properties are presented. A selective area chemical vapour deposition technique has been used to fabricate continuous and patterned epitaxial CrO$_2$ thin films grown on (100)-oriented TiO$_2$ substrates. Precessional magnetization dynamics were stimulated both electrically and optically, and probed by means of time-resolved Kerr microscopy (TRSKM) and vector network analyser ferromagnetic resonance (VNA-FMR) techniques. The dependence of the precession frequency and the effective damping parameter upon the static applied magnetic field were investigated. All films exhibited a large in-plane uniaxial anisotropy. The effective damping parameter was found to exhibit strong field dependence in the vicinity of the hard axis saturation field. However, continuous and patterned films were found to generally possess similar dynamic properties, confirming the suitability of the deposition technique for fabrication of future spintronic devices. Ta/CoFeB/MgO trilayers with perpendicular magnetic anisotropy were fabricated by magnetron sputtering and patterned into Hall bars by photolithography. Scanning Kerr microscopy (SKM) was combined with electrical transport measurements to gain insight into the underlying mechanisms of current-induced spin-orbit torque (SOT) switching within such devices. Switching was found to be a stochastic, domain wall driven process, the speed of which is strongly dependent on the switching current. Kerr imaging shows domain nucleation at one edge of the device which modelling reveals is likely assisted by the out-of-plane component of the Oersted field. Further domain growth, leading to magnetisation reversal, may still be dominated by spin torques, but the Oersted field provides an additional mechanism by which to control the switching process. Pulsed current TRMOKE experiments were performed on both Ta/CoFeB/MgO trilayers patterned into large Hall bars, and on patterned Ta/CoFeB/MgO trilayers formed upon planar waveguides. Magnetisation dynamics in these structures were found to be complex and to have strong dependence on bias field direction, bias field polarity, pulsed current direction, current density and device size. Two components of the dynamic response were observed in Hall bars, a fast oscillatory component and a slow unipolar deflection. A strong spatial dependence of the dynamic response was observed for patterned CoFeB/MgO devices formed upon planar waveguides with a particularly large in-plane rotation observed at the edges of a square element. These studies highlight the complexity of the SOTs generated by the spin Hall and Rashba effects. Spin pumping within Ta/Ag/Co2MnGe(5 XFMR was used to directly detect the motion of the sink layer magnetization at the source layer resonance. nm)/Ag(6 nm)/Ni81Fe19(0-5 nm)/Ag/Ta large area spin valve structures was studied by vector network analyser ferromagnetic resonance (VNA-FMR), and element-specific phase-resolved X-ray ferromagnetic resonance (XFMR). Spin current absorption as a function of Ni81Fe19 sink layer thickness was explored indirectly by VNA-FMR as a modification of the Co2MnGe source layer damping. XFMR was used to directly detect the motion of the sink layer magnetization at the source layer resonance. The bipolar form of the sink layer signal clearly indicates the action of spin transfer torque (STT) resulting from spin pumping, while comparison with a macrospin model allows the real part of the spin mixing conductance to be directly determined. The dependence of the source layer damping upon the sink layer thickness was observed to be different to previous studies, due to both spin current absorption in the outer Ta layers, and superparamagnetic relaxation in sink layers with thickness less than or equal to 0.6 nm. The XFMR measurements show that the absorption of spin current within the sink layer continues to increase up to the largest sink layer thickness of 5 nm, presumably due to improving interface quality.

Injection and detection of spin polarised current in a metal/semiconductor deviceand the measurement of the degree of injected spin polarisation are two key issuesin the development of hybrid spintronics. This thesis touches on both themes asit details the development of planar Andreev spectroscopy as a tool to measureinjected spin and the electrical characterisation of MgO tunnel barriers for efficientspin injection and detection. Point contact Andreev reflection spectroscopy has been widely used tomeasuretransport spin polarisation in magnetic materials. Planar Andreev structures havean advantage over point contacts as they offer greater control over interface qualityand the possibility of spatially resolved information about the spin polarisationusing nanojunction arrays. We find that planar junctions compare favourably topoint contacts in that they can yield low interface barriers and minimal nonthermalsmearing. We show that a low interface barrier is critical for accurate detectionof spin polarisation, particularly in semiconductorswhere large Fermi velocitymismatchcontributes to the barrier. Furthermore, the fabrication method strongly affectsall parameter values. For Pb/InAs planar junctions we demonstrate that themost feasible way to obtain interfaces suitable for spin detection is an ?etch-back?processing strategy. The processing routes are shown to be scalable to nanoarrayfabrication to allow measurement of spin accumulation. We also examine the electrical properties of ultrathin MgO barriers grown onInAs epilayers and the dependence of barrier characteristics on InAs surface pretreatmentand growth conditions. Chemical pretreatment improves the yield oftunnel junctions and changes the roughness of the interface between the oxide andthe semiconductor. Electrical characterisation confirms that tunnel barriers withappropriate values of interface resistance for efficient spin injection/detection havebeen achieved. Using the Rowell criteria and various tunnelling models we showthat single step tunnelling occurs above 150 K and a thermal smearing model suggeststhat tunnelling is the dominant transport process down to 10 K.

Spintronics is a rapidly growing research field due to scalability, integrablility within existing VLSI architecture, significantly reduced switching energy and latency while maintaining stable bit orientation (Spin-up, Spin-down). For the first time sub-5nm Spin Transfer Torque –Magnetic Tunneling Junctions (STT-MTJ) were investigated utilizing various Integrated Circuit (IC) fabrication techniques to evaluate novel concepts in logic switches. Tunneling Magnetoresistance (TMR) was measured in STT-MTJ stacks of Ta/CoFeB/MgO/CoFeB/Ta with differing diameter ferrimagnetic CoFe2O4 nanoparticles (10nm, 4nm and 2nm) embedded in the MgO layer. MR was detected in the 2nm and 4nm particle devices and demonstrated evidence of single electron transport. Tri-layer STT-MTJ devices were fabricated using a thin film stack of Ta/Ru/Ta/CoFeB(M1)/MgO/CoFeB(M2)/MgO/CoFeB(M3)/Ta. The overall diameter of the stack was reduced to sub-20nm using Focused Ion Beam (FIB) to mill away extra material. The coercivities of the ferrimagnetic CoFeB layers were modified during thin film deposition by altering sputter conditions. Field Applied- Magnetic Force Microscopy (FA-MFM) was used to detect four different magnetic intensities corresponding to three discreet resistances in the singly addressed device, making this architecture a candidate for neuromorphic computational applications. Lastly a lithographic-less architecture was developed to mass fabricate and electo-mechanically probe multi-layered, single point, sub-5nm particle based STT-MTJ devices using off-the-shelf anodized nanoporous alumina. Once fabricated, the devices were probed to measure their IV characteristics and magnetoresistance (MR). The unprecedented MR changes on the order of 50,000% at room temperature suggest quantum mechanical behavior.

La mise en évidence d’effets de polarisation d’échange (“exchange bias”, EB) ouvre de nouvelles perspectives dans le domaine émergeant de la spintronique organique. Dans une première partie de la thèse, on étudie l’EB des systèmes Co/MPc et Py/MPc (M=Mn, Co, Fe, Zn) par magnétométrie. Pour tous ces systèmes, l’EB est observé avec des températures de blocage de 100K environ. Ces études sont complétées par des mesures de résonance ferromagnétique confirmant les valeurs du champ de polarisation. Dans une troisième partie, on étudie les propriétés magnétiques des tricouches Co/Pc/Co. Les cycles d’hystérèse présentent des marches indiquant un renversement séquentiel des couches de cobalt. A basse température, on observe de l’anisotropie unidirectionnelle pour les deux couches mais leurs champs de polarisation diffèrent
Observation of exchange bias (EB) phenomenon by using molecular materials as a pinninglayer open the horizon for tremendous perspective in the field of organic spintronics. Thefirst part of the thesis is devoted to the study of EB of Co/MPc and Py/MPc (M=Mn, Co, Fe,Zn) by static magnetometry. The existence of EB is evidenced in all Pc molecules with block-ing temperature around 100K. The second part is devoted to the study of EB by dynamicFMR measurements. The values of EB measured by this method are compatible with staticmagnetometry measurements. The third part is devoted to study magnetic properties of thetrilayer Co/Pc/Co systems. Hysteresis loops exhibit a stepped shape indicative of successivereversal of each layer. Low temperature loops show that both Co layers experience unidi-rectional anisotropy after field cooling, with differing bias fields

Fabrication of molecular spin devices with ferromagnetic electrodes coupled with a high spin molecule is an important challenge. This doctoral study concentrated on realizing a novel molecular spin device by the bridging of magnetic molecules between two ferromagnetic metal layers of a ferromagnetic-insulator-ferromagnetic tunnel junction on its exposed pattern edges. At the exposed sides, distance between the two metal electrodes is equal to the insulator film thickness; insulator film thickness can be precisely controlled to match the length of a target molecule. Photolithography and thin-film deposition were utilized to produce a series of tunnel junctions based on molecular electrodes of multilayer edge molecular electrodes (MEME) for the first time. In order to make a microscopic tunnel junction with low leakage current to observe the effect of ~10,000 molecules bridged on the exposed edge of a MEME tunnel barrier, growth conditions were optimized; stability of a ~2nm alumina insulator depended on its ability to withstand process-induced mechanical stresses. The conduction mechanism was primarily 1) tunneling from metal electrode to oranometalic core by tunneling through alkane tether that acts as a tunnel barrier 2) rapid electron transfer within the oranometalic Ni-CN-Fe cube and 3) tunneling through alkane tether to the other electrode. Well defined spin-states in the oranometalic Ni-CN-Fe cube would determine electron spin-conduction and possibly provide a mechanism for coupling. MEME with Co/NiFe/AlOx/NiFe configurations exhibited dramatic changes in the transport and magnetic properties after the bridging of oranometalic molecular clusters with S=6 spin state. The molecular cluster produced a strong antiferromagnetic coupling between two ferromagnetic electrodes to the extent, with a lower bound of 20 erg/cm,2 that properties of individual magnetic layers changed significantly at RT. Magnetization, ferromagnetic resonance and magnetic force microscopy studies were performed. Transport studies of this configuration of MEME exhibited molecule-induced current suppression by ~6 orders by blocking both molecular channels and tunneling between metal leads in the planar 25μm2 tunnel junction area. A variety of control experiments were performed to validate the current suppression observation, especially critical due to observed corrosion in electrochemical functionalization step. The spin devices were found to be sensitive to light radiation, temperature and magnetic fields. Along with the study of molecular spin devices, several interesting ideas such as ~9% energy efficient ultrathin TaOx based photocell, simplified version of MEME fabrication, and chemical switching were realized. This doctoral study heralds a novel molecular spin device fabrication scheme; these molecular electrodes allow the reliable study of molecular components in molecular transport.

The field of semiconductor spintronics is relatively underdeveloped when compared to its metallic counterpart which has found great success in the computer storage industry in hard disk drive head technology and to a lesser extent non-volatile and robust (M)RAM. The use of semiconductors in spintronics is promising with the ability to modulate spins via a gate controlled spin-orbit interaction allowing for spintronic logic and computation devices. Combining this with dilute ferromagnetic semiconductors, which have been suggested as a base for semiconductor MRAM, it may even be possible to integrate MRAM and logic into a single chip to allow for further miniaturisation of devices. In this thesis we look into the suitability of Ge as a platform for semiconductor spintronics verses other popular alternatives such as Si and GaAs. The first section of this thesis looks at evidence of a strong spin-orbit interaction in low temperature magnetoresistance curves measured in a high mobility (777,000 cm2V-1s-1) Ge 2DHG. Despite the lack of beatings in Subnikov de Haas oscillations, which often serves as an indicator of a strong spin-orbit coupling, weak antilocalisation like behaviour is seen at low fields. The spin splitting energy extracted from the weak antilocalisation and found to be 0.8meV with a Rashba parameter of 3.2x10-28 eVm3. The spin split energy is comparable to other Ge and III-V 2DHGs, however the Rashba parameter is in fact higher, which we believe is attributed to the high level of remote doping. The second section focuses on the treatment of ferromagnet/oxide spin tunnel contacts on Ge deposited by evaporation or sputtering, both of which are common in industrial scale fabrication. Annealing XPS studies are conducted on these contacts to examine the temperature limits at which annealing treatments can be done before contacts degrade. Transition line measurements are also examined to look at the electrical characteristics of the deposited contacts. The final section focuses on the fabrication and measurements of Hanle devices on n-Ge. The extracted spin lifetimes are of the same order as those measured in similarly doped Ge devices, however strong temperature dependencies of both the spin lifetime and spin accumulation signal suggest that the signal measured is strongly influenced by local states within the oxide barrier.

Le travail exposé dans cette thèse de doctorat présente des expériences à basse température dans le domaine de la spintronique quantique sur des hétérostructures à base de germanium. Tout d’abord, les avantages attendus du germaniumpour la spintronique quantique sont exposés, en particulier la faible interaction hyperfine et le fort couplage spin-orbite théoriquement prédits dans le Ge. Dans un second chapitre, la théorie des boites quantiques et systèmes à double boite sont détaillés, en se focalisant sur les concepts nécessaires à la compréhension des expériences décrites plus tard, c’est-à-dire les effets de charge dans les boites quantiques et double boites, ainsi que le blocage de spin de Pauli. Le troisième chapitre s’intéresse à l’interaction spin-orbite. Son origine ainsi que ses effets sur les diagrammes d’énergie de bande sont discutés. Ce chapitre se concentre ensuite sur les conséquences de l’interaction spin-orbite spécifiques aux gaz bidimensionnels de trous dans des hétérostructures de germanium, c’est-à-dire l’interaction spin-orbite Rashba, le mécanisme de relaxation de spin D’Yakonov-Perel ainsi que l’antilocalisation faible.Le chapitre quatre présente des mesures effectuées sur des nanofils coeur coquillede Ge/Si. Dans ces nanofils une boite quantique se forme naturellement et celui-ci est étudié. Un système à double boite quantiques est ensuite formé par utilisation de grilles électrostatiques, révélant ainsi du blocage de spin de Pauli.Dans le cinquième chapitre sont détaillés des mesures demagneto-conductance de gas de trous bidimensionnels dans des hétérostructures de Ge/SiGe contraints dont le puit quantique se situe à la surface. Ces mesuresmontrent de l’antilocalisation faible. Les temps de transport caractéristiques sont extraits ainsi que l’énergie de séparation des trous 2D par ajustement de courbe de la correction à la conductivité due à l’antilocalisation. De plus, les mesures montrent une suppression de l’antilocalisation par un champ magnétique parallèle au puit quantique. Cet effet est attribué à la rugosité de surface ainsi qu’à l’occupation virtuelle de sous-bandes inoccupées.Finalement, le chapitre six présente des mesures de quantisation de la conductancedans des hétérostructures de Ge/SiGe contraints dont le puit quantique est enterré. Tout d’abord, l’hétérostructure est caractérisée grâce à des mesures de magneto-conductance dans une barre de Hall. Ensuite, un second échantillon dessiné spécialement pour la réalisation de points de contact quantiques est mesuré. Celui-ci montre des marches de conductance. La dépendance en champ magnétique de ces marches est mesurée, permettant ainsi une extraction du facteur gyromagnétique de trous lourds dans du germanium
This thesis focuses on low temperature experiments in germaniumbased heterostructure in the scope of quantumspintronic. First, theoretical advantages of Ge for quantum spintronic are detailed, specifically the low hyperfine interaction and strong spin orbit coupling expected in Ge. In a second chapter, the theory behind quantum dots and double dots systems is explained, focusing on the aspects necessary to understand the experiments described thereafter, that is to say charging effects in quantum dots and double dots and Pauli spin blockade. The third chapter focuses on spin orbit interaction. Its origin and its effect on energy band diagrams are detailed. This chapter then focuses on consequences of the spin orbit interaction specific to two dimensional germaniumheterostructure, that is to say Rashba spin orbit interaction, D’Yakonov Perel spin relaxation mechanism and weak antilocalization.In the fourth chapter are depicted experiments in Ge/Si core shell nanowires. In these nanowire, a quantumdot formnaturally due to contact Schottky barriers and is studied. By the use of electrostatic gates, a double dot system is formed and Pauli spin blockade is revealed.The fifth chapter reports magneto-transport measurements of a two-dimensional holegas in a strained Ge/SiGe heterostructure with the quantum well laying at the surface, revealing weak antilocalization. By fitting quantumcorrection to magneto-conductivity characteristic transport times and spin splitting energy of 2D holes are extracted. Additionally, suppression of weak antilocalization by amagnetic field parallel to the quantum well is reported and this effect is attributed to surface roughness and virtual occupation of unoccupied subbands.Finally, chapter number six reportsmeasurements of quantization of conductance in strained Ge/SiGe heterostructure with a buried quantumwell. First the heterostructure is characterized by means ofmagneto-conductance measurements in a Hall bar device. Then another device engineered specifically as a quantum point contact is measured and displays steps of conductance. Magnetic field dependance of these steps is measured and an estimation of the g-factor for heavy holes in germanium is extracted

L'amélioration des techniques de dépôts et l’évolution de la compréhension de la physique de la matière condensée a conduit à la découverte de phénomènes nouveaux en électronique de spin (spintronique). En particulier, le retournement de l’aimantation par couple de transfert de spin et couple spin-orbite, ainsi que le développement de dispositifs basés sur la propagation d’ondes de spin ont fait de l’amortissement magnétique de Gilbert un paramètre central pour les futures technologies de stockage et de traitement de l’information. Dans cette étude, la prédiction de valeurs très faibles d’amortissement dans les alliages d’Heusler demi métaux magnétiques Co2MnZ est expérimentalement observée et directement corrélée à la structure électronique sous-jacente. En effet, en substituant l’élément Z dans des couches minces monocristallines de haute qualité de Co2MnZ (Z= Al, Si, Ga, Ge, Sn, Sb) faites par épitaxie par jet moléculaire, les propriétés électroniques telles que le gap de spin minoritaire, la position du niveau de Fermi et la polarisation en spin peuvent être accordées et leurs conséquences sur la dynamique de l’aimantation sont analysées. Les résultats expérimentaux nous permettent de comprendre la relation existante entre la structure électronique mesurée et la valeur d’amortissement magnétique, ainsi que de les comparer aux calculs ab initio. Les valeurs d’amortissement entre 4.1 x10-4 et 9 x10-4 pour Co2MnSi, Co2MnGe, Co2MnSn et Co2MnSb sont les plus petites valeurs jamais reportées pour des couches conductrices et constituent une preuve expérimentale qui confirme les prédictions théoriques sur ces alliages d’Heusler demi métaux magnétiques. Ensuite, la relation entre l’amortissement magnétique de Gilbert et le temps de désaimantation ultra-rapide induit par pulse laser dans la série d’alliages quaternaires Co2MnSixAl1-x à polarisation en spin variable est étudiée. Cette partie vise à vérifier des modèles théoriques qui essaient d’unifier ces deux quantités vivant sur des échelles de temps différentes. Finalement, les propriétés structurales et magnétiques de super réseaux Mn3Ga/Co2YZ sont étudiées dans le but de combiner un amortissement de Gilbert très faible, un gap de spin minoritaire ainsi que l’aimantation perpendiculaire aux plans des couches, une caractéristique indispensable pour des dispositifs à faible consommation d’énergie
Improvements in thin film elaboration methods and a deeper understanding of condensed matter physics have led to new exciting phenomena in spin electronics (spintronics). In particular, magnetization reversal by spin-orbit and spin-transfer torque as well as the development of spin waves based devices have placed the Gilbert magnetic damping coefficient as a key parameter for future data storage and information processing technologies. The prediction of ultralow magnetic damping in Co2MnZ Heusler half-metal magnets is explored in this study and the damping response is shown to be linked to the underlying electronic structure. By substitution of the Z element in high quality Co2MnZ (Z=Al, Si, Ga, Ge, Sn and Sb) epitaxial thin films grown by molecular beam epitaxy, electronic properties such as the minority-spin band gap, Fermi energy position in the band gap, and spin polarization can be tuned and the consequences for magnetization dynamics analyzed. Experimental results allow us to directly explore the interplay of spin polarization, spin gap and Fermi energy position, with the magnetic damping obtained in these films (together with predictions from ab initio calculations). The ultralow magnetic damping coefficients measured in the range from 4.1 x10-4 to 9 x10-4 for Co2MnSi, Co2MnGe, Co2MnSn and Co2MnSb are the lowest values ever reported in conductive layers and offer a clear experimental demonstration of theoretical predictions on half metal magnetic Heusler compounds. Then, the relation between the Gilbert damping and the ultrafast demagnetization time in quaternary Co2MnSixAl1-x compounds with a tunable spin polarization is analyzed. This way, it is possible to confront theoretical models unifying those two quantities that live in different timescales. Finally, structural and magnetic properties of Mn3Ga/Co2YZ Heusler superlattices are investigated in order to combine ultralow Gilbert damping coefficient, minority spin band gap and perpendicularly magnetized heterostructures, another requirement for low energy consumption devices. Through the present work, we aim to prove that Heusler compounds provide an excellent playground to study fundamental magnetism and offer a pathway for future materials design

Ce travail explore l'utilisation de l'effet magnéto-optique pour étudier la dynamique de spin des électrons de conduction dans les semi-conducteurs non magnétiques lorsqu'ils sont pompés avec des photons polarisés circulairement. En général, les moments magnétiques hors-équilibre induits optiquement dans les semi-conducteurs non magnétiques sont plus petits que ceux des matériaux magnétiques. L'effet magnéto-optique en principe offre une sensibilité suffisante pour détecter ces faibles moments magnétiques via une mesure de rotation Faraday dans la limite de bruit de photons. Nous avons comparés trois méthodes de détection: les polariseurs partiellement croisés, l’interféromètre de Sagnac et le pont optique. L'interféromètre de Sagnac se révèle fonctionnellement équivalent aux polariseurs partiellement croisés, avec une sensibilité diminuée par la perte de photons à chacun des séparateurs de faisceaux nécessairement présents dans cette configuration expérimentale. Par contre, il a été démontré précédemment que les interféromètres de Sagnac permettent de faire la distinction entre les rotations dites réciproques et non réciproques, et cette thèse propose de nouvelles géométries de Sagnac pour distinguer les rotations en fonction de leurs symétries en temps et en parité. La technique du pont optique présente les meilleures performances. Elle permet une mesure de l'angle de rotation de Faraday limitée par le bruit de photons, même avec des puissances lumineuses importantes reçues par les détecteurs, ce qui permet d'obtenir la meilleure figure de mérite possible. Dans les expériences conduites sur des matériaux magnétiques, un bruit de quelques nrad/√Hz a été mesuré pour une puissance de sonde de 10 mW. Une série de mesures de rotation Faraday pompe-sonde à température ambiante a été réalisée sur GaAs pompé optiquement. Les plus grands signaux sont obtenus lorsque le moment magnétique généré et détecté est maximisé en focalisant fortement les faisceaux pompe et sonde et en choisissant une longueur d'onde de la sonde accordée à une résonance optique dans la structure électronique. Les mesures en champ magnétique transversal montrent un champ Hanle de 0.43 T, à partir duquel on déduit la durée de vie de spin de 88 ps
This work explores the use of the magneto-optical Kerr effect to study conduction electron spin dynamics in non-magnetic semiconductors when pumped with circularly polarized photons. Typically, non-equilibrium, optically-induced magnetic moments in non-magnetic semiconductors are orders of magnitude smaller than those of magnetized materials, including both magnetic and non-magnetic materials in an external magnetic field. The magneto-optical Kerr effect in principal offers sufficient sensitivity to detect such small magnetic moment via a measurement of the Faraday rotation angle of a probe beam in the photon shot noise limit. Three detection configurations have been experimentally compared: partially crossed polarizers, a Sagnac interferometer and an optical bridge. The Sagnac interferometer is shown to be functionally equivalent to partially crossed polarizers, although its sensitivity is compromised by lost photons at each of the obligatory beam splitters present in such a geometry. On the other hand, it has previously been shown that Sagnac interferometers can distinguish between so-called reciprocal and non-reciprocal rotations, and this thesis proposes novel Sagnac geometries to distinguish rotations according to their time and parity symmetries. The optical bridge technique allows for a photon-shot noise limited measurement of the Faraday rotation angle, even with large photon intensities on the detectors, thereby yielding the best possible figure-of-merit. In demonstrations on magnetic materials, a noise floor of a few nrad//√Hz was measured for a probe power of 10 mW. A series of room-temperature, pump-probe Faraday rotation measurements is performed on optically pumped GaAs to compare and contrast this method with standard polarized photo-luminescence techniques. The largest signals are found when the locally probed moment is maximized by strongly focusing the pump and probe beams, and by choosing a probe wavelength tuned to an optical resonance in the electronic structure. Measurements in transverse magnetic field show a Hanle field of 0.43 T, from which the spin lifetime of 88 ps is deduced

Aquesta tesi s’ha basat en la síntesi i el processament del grafè per tal d’obtenir les condicions òptimes per a la seva utilització en aplicacions d’espintrònica. La tesi està emmarcada en dos camps de recerca punters: el món del grafè amb la seva riquesa de propietats úniques i el camp de l’espintrònica que explora el grau de llibertat de l’espí de l’electró de cara a noves aplicacions en informàtica i tecnologia de comunicacions (com és ara els dispositius de lògica i d’emmagatzematge d’informació). En aquest context, el grafè és un material prometedor de cara a transportar l’espí amb grans longituds de difusió. Per aconseguir-ho, és clau que el grafè sigui d’alta qualitat amb el mínim de centres de dispersió, tant en el moment de la seva producció com en el processat. Per tant, en aquesta tesi s’ha fet un gran esforç per optimitzar els paràmetres de creixement pel mètode de deposició química per vapor (CVD). S’han aconseguit vàries contribucions rellevants en aquest camp: -S’ha demostrat la importància de la reacció inversa del grafè (“gravat”) durant el creixement, la qual comença a dominar per a temps llarg de creixement, degut a un augment de la concentració d’hidrogen in-situ. Aquest és un fenomen que ha estat ignorat anteriorment però que és de gran rellevància degut al seu efecte sobre l’estructura i la morfologia del grafè. S’ha aconseguit una caracterització completa de l’evolució de la forma dels dominis de grafè ajustant el temps de creixement, el flux dels gasos precursors i el confinament del catalitzador, fet que permet identificar millor l’inici del procés de “gravat”. Controlar aquest efecte és molt important per minimitzar els defectes estructurals induïts per la reacció inversa ja que poden afectar el transport d’electrons/spins. -S’ha introduït un nou tractament previ del catalitzador de coure per reduir els punts de nucleació per al creixement del grafè. La supressió dels punts de nucleació és molt important per tal de promoure un creixement més monocristal•lí del grafè i així minimitzar la dispersió dels electrons en la frontera dels grans de cristall del grafè. El procediment es basa en un procés de curat tèrmic assistit per fotocatàlisi, que elimina eficaçment els contaminants de carboni que són punts actius per a la nucleació del grafè. -S’ha demostrat una distància de propagació d’espí rècord, superior als 30 micròmetres, en el canal de grafè. Aquest resultat s’ha obtingut utilitzant grafè CVD d’alta qualitat crescut sobre platí i una tècnica de fabricació de dispositius recentment desenvolupada que minimitza la contaminació/ els defectes estructurals durant el processament del grafè. La vida útil de l’espí i les longituds de relaxació obtingudes han resultat ser els valors més alts aconseguits a temperatura ambient en comparació amb previs resultats obtinguts en condicions similars, és a dir, amb grafè CVD sobre substrat estàndard de SiO2/Si.
Esta tesis se ha basado en ajustar la síntesis y el procesamiento de grafeno para el desarrollo de dispositivos espintrónicos optimizados. La tesis está enmarcada en dos temáticas punteras: el mundo del grafeno con su riqueza de propiedades únicas y el campo de la espintrónica que explora el grado de libertad del espín de los electrones para nuevas aplicaciones en tecnología de la información y la comunicación (por ejemplo, dispositivos de lógica y almacenamiento de información). En este contexto, el grafeno es un material muy prometedor para transportar el espín con longitud de difusión alta. Para lograr esto, un grafeno de alta calidad con mínimos centros de dispersión de electrones es un parámetro clave y debe asegurarse estas propiedades desde el momento de su crecimiento y durante su procesamiento. Por lo tanto, en esta tesis, se han invertido muchos esfuerzos para ajustar los parámetros de crecimiento del grafeno mediante la deposición química por vapor (CVD). Se han logrado varias contribuciones relevantes en el campo: - Se ha demostrado la importancia de la reacción inversa del grafeno (“etching”) durante el crecimiento, la cual comienza a dominar a tiempos largos de crecimiento debido a un aumento in-situ de la concentración de hidrógeno. Este es un fenómeno que se ha ignorado anteriormente pero que es muy importante a tener en cuenta ya que afecta la estructura y morfologia del grafeno. Se ha logrado una caracterización completa de la evolución de la forma de los dominios del grafeno ajustando el tiempo de crecimiento, el flujo de precursores de gases y el confinamiento del catalizador lo cual permite identificar mejor el inicio del proceso de “etching”. Controlar este efecto es muy relevante para minimizar los defectos estructurales inducidos por la reacción inversa ya que pueden afectar el transporte de electrones / espines. - Se ha introducido un nuevo pretratamiento del catalizador de cobre para reducir los sitios de nucleación para el crecimiento de grafeno. La supresión de los sitios de nucleación es muy importante para promover un crecimiento más monocristalino de grafeno y minimizar así la dispersión de electrones en las fronteras de los granos de cristal de grafeno. El procedimiento se basa en un proceso de curado térmico asistido por fotocatalisis que elimina eficientemente los contaminantes carbono que son sitios activos para la nucleación de grafeno. - Se ha demostrado una propagación récord de espín de más de 30 micrómetros en el canal de grafeno. Dicho resultado se logró utilizando grafeno CVD de alta calidad crecido sobre platino y una técnica de fabricación de dispositivos recientemente desarrollada que minimiza la contaminación / defectos estructurales durante el procesamiento de grafeno. La vida útil del espín y las longitudes de relajación resultaron ser los valores más altos en contrados a temperatura ambiente en comparación a resultados previos obtenidos en condiciones similares, es decir grafeno CVD sobre sustrato estándar de SiO2 / Si.
“This thesis has focused on tuning the synthesis and processing of graphene to achieve optimized spintronic applications. Thus the thesis is framed in two cut-edging topics: the graphene world with its richness of unique properties and the field of spintronics which explores the spin degree of freedom of the electrons for novel applications in information and communication technology (e.g. information storage and logic devices). Under this context graphene is a very promising spin channel material to transport spin with long spin diffusion lengths. To accomplish that, a high quality-graphene with minimum electron scattering centers is a key parameter and must be ensured from the moment of its production and during its processing. Accordingly, in this thesis, a lot of efforts have been invested to fine-tune the growth parameters of graphene by chemical vapor deposition (CVD). Several relevant contributions in the field have been achieved: -THE DEMONSTRATION of the importance of the graphene etching backreaction during growth which begins to dominate at long growth times due to an in-situ increase of hydrogen concentration. That is a phenomenon that has been previously ignored but very important to consider since it impacts on the graphene domain reshaping. A thorough characterization of the graphene domain shape evolution has been accomplished by tuning the growth time, the flow of gas precursors and the catalyst confinement which allows better identifying the onset of the etching process. Controlling this effect is very relevant to minimize structural defects induced by etching which can impact the electron/spin transport. -THE INTRODUCTION of a novel pretreatment of the copper catalyst to reduce nucleation sites for graphene growth. The suppression of nucleation sites is very important to promote a more single-crystalline growth of graphene and thus minimize electron scattering at the domain boundaries of the graphene crystal grains. The procedure is based on a photocatalyst-assisted thermal annealing process that efficiently removes carbonaceous contaminants which are active sites for graphene nucleation. -THE DEMONESTRATION of record-long propagation of spins over 30 micrometers at the graphene channel. Such output was achieved using high-quality CVD graphene grown on platinum foil and a newly developed device fabrication technique which minimizes contamination/structural defects during graphene processing. The spin lifetimes and relaxation lengths were the highest values reported at room temperature in CVD grown graphene on a standard substrate, SiO2/Si.

Cette thèse est porté sur l'étude du transport électronique dans différents matériaux organiques semi-conducteurs, considérés comme candidats potentiels pour des applications en Electronique de Spin Organique. Pour rendre possible la diffusion d'un courant polarisée en spin à l'intérieur d'un semi-conducteur (injection-transport-détection), le mécanisme de transport et la mobilité des porteurs de charge, ainsi que la nature et la valeur de la résistance de contact de l'interface séparant matériau organique et électrodes métalliques ferromagnétiques, doivent répondre à des critères très stricts. Tous les dispositifs sont en géométrie latérale. Nous étudions trois matériaux organiques différents: des fibres supramoléculaires auto-assemblées, une encre de nana-flocons de graphene exfolié en phase liquide et un polymère semi-conducteur fortement dopé en forme de couche mince. Nos résultats montrent que les conditions sont partiellement respectées, mais que des défis demeurent
In this thesis, we study the electronic charge transport properties in different high mobility organic semiconductors considered as possible candidates for applications in Organic Spintronics. Stringent conditions are needed to make possible the diffusive transport of a spin-polarized current through an organic spacer (injection-transport-detection): the mechanism of charge transport and the carriers mobility, as well as the interface between the organic semiconductor and the ferromagnetic metallic electrodes, should meet special criteria. Our devices are in lateral geometry. We investigate three organic materials, all compatible with wet processing of organic electronics: supramolecular fibers self-assembled by light irradiation, an ink of liquid-phase exfoliated graphene nano-sheets and a conjugated polymer semiconductor thin film exposed to strong electrochemical doping. We observe that the criteria are partially matched, but some challenges are still present

Organic semiconductors have a major impact on our daily life, as they are increasingly implemented in commercial devices like organic light emitting diodes (OLED), organic field effect transistors (OFET) or solar cells. Recently, this class of materials became of interest for spintronic applications, which make use of the spin in addition to the charge of the electrons. Due to a high spin life-time and an easy manipulation of the chemical and physical properties, organic molecules become a promising alternative to transition metals and metal oxides for both spin transport and polarisation media. In order to achieve the desired device properties like room temperature operation, it is essential to investigate new materials and fabrication methods. This thesis focuses on vacuum processed organic semiconductor thin films, which further exhibit remarkable magnetic properties, making them suitable for spintronic applications. The materials of choice, iron phthalocyanine (FePc) and manganese phthalocyanine (MnPc), show ferromagnetic behaviour with Curie temperatures of up to 40K and the properties can be tuned by blending with other organic semiconductors. In particular, mixing n-type semiconducting materials such as fluorinated cobalt phthalocyanine (F16CoPc) and tetracyanoquinodimethane (TCNQ) with the p-type magnetic metal phthalocyanines has great potential for both molecular electronics and spintronics. In this work, it was found that the paramagnetic F16CoPc becomes ferromagnetic in a blend with FePc and the coercivity of the compound can be tuned by the concentration. Furthermore, ferromagnetic MnPc films with significant coercivity were obtained by annealing of MnPc:TCNQ blends, which exhibit charge transfer from MnPc to TCNQ in the as-deposited state. Finally, the first spin valves based on FePc were produced and exceptionally large magnetoresistive effects were observed.

Les mémoires magnétiques actuelles commencent à atteindre leurs limites physiques en terme de stabilité, vitesse et consommation énergétique, alors que la course à la miniaturisation s'intensifie. Le champ émergeant de la spintronique étudie le comportement collectif des spins dans la matière ainsi que leurs interactions aux interfaces, afin de trouver une solution en termes de matériaux, architectures et sources excitatrices. En particulier, les matériaux antiferromagnétiques sont particulièrement prometteurs. Ces matériaux ordonnées sont abondants, naturellement stables, robustes, ultra rapides et compatibles avec l'électronique des isolants. En effet, la plupart des oxydes à base de métaux de transition sont des isolants antiferromagnétiques ayant leur fréquence de résonance dans le terahertz et un champ de flop de quelques dizaines de teslas. Ils peuvent aussi être semi-métalliques, métalliques, semiconducteurs, supraconducteurs ou multiferroïques. Cette thèse s'intéresse aux deux antiferromagnétiques: oxyde de nickel (NiO) et ferrite de bismuth (BiFeO₃). NiO est un antiferromagnétique type à température ambiante, avec une structure cristalline simple. Une étude basée sur des simulations dynamiques atomiques montre que des courants de spin atteignables peuvent réaliser une mémoire à trois états avec ce composé, avec un temps de réponse de l'ordre de la picoseconde. La simulation explique aussi la formation de structures chirales dans BiFeO₃, un antiferromagnétique également ferroélectrique, présentant un couplage magnétoélectrique entre ses deux ordres. Dans une deuxième partie, les domaines antiferromagnétiques dans BiFeO₃ sont observés expérimentalement par génération de seconde harmonique optique, avec une résolution spatiale de un micron. Les domaines antiferromagnétiques de BiFeO₃ sont ensuite excités par une impulsion laser intense, et la dynamique des deux ordres couplés (antiferromagnétisme et ferroélectricité) est étudiée dans le régime picoseconde. Enfin, l'injection d'impulsions de spins dans dans un antiferromagnétique, tel que BiFeO₃ ou NiO est envisagée en utilisant la génération de courant de spin induite par la désaimantation ultrarapide de couches adjacentes magnétiques par des impulsions laser
Current magnetic memory devices are reaching their physical limits in terms of stability, speed and power consumption as the race to miniaturization intensifies. The emergent research field of spintronics studies the collective behavior of spins in matter and their interplay at interfaces, to find new avenues in terms of materials, architectures and stimulation sources. A particularly promising group of materials are the antiferromagnets. These abundant magnetically ordered materials are naturally stable, robust, ultra-fast and compatible with insulator electronics. Indeed, most transition metal oxide compounds are antiferromagnetic insulators, have resonance in the terahertz range and flop fields of tens of teslas. They can also be semi-metals, metals, semiconductors, superconductors or multiferroics. This thesis focuses on two antiferromagnets: nickel oxide (NiO) and bismuth ferrite (BiFeO₃). NiO is the archetypical antiferromagnet at ambient temperature with a simple crystalline structure. Using dynamical atomistic simulations, I show that this compound can be the elemental brick of a three state memory device controlled by currently available pulses of spin currents, with a picosecond response time. The simulations also explain the formation of chiral structures in BiFeO₃, a ferroelectric antiferromagnet with magnetoelectric coupling between the two orders. In a second part, antiferromagnetic domains in BiFeO₃ are experimentally observed using second harmonic generation of light, with a sub-micron spatial resolution. Antiferromagnetic domains of BiFeO₃ are then excited by an intense femtosecond laser pulse, and the dynamics of the two coupled orders (antiferromagnetism and ferroelectricity) is studied with a sub-picosecond time resolution. Finally, the injection of spin current in an antiferromagnet such as BiFeO₃ or NiO is envisioned by characterizing the spin bursts generated by ultrafast laser-induced demagnetization of adjacent ferromagnetic layers

Spintronics is an emergent interdisciplinary topic for the studies of spin-based, other than or in addition to charge-only-based physical phenomena, which promises not only new capabilities of electronic devices, but also abundant science. For applied materials, the spin ordering has long been investigated within the context of conventional ferromagnetic materials (FMs), while the study of spin generation, relaxation, and spin-orbit coupling (SOC) in semiconductors (SCs) took off only recently with the advent of spintronics and it is here that many novel materials and FM/SC hybrid structures can find their greatest potential in both science and technology. In the pursuit for such goals, the intrinsic material properties are important indicators and the artificially synthetized hybrid systems (layered FM/SC structures and FM-doped SCs) are valuable models for studying spindependent phenomena and could potentially be used as actual components for an eventual spintronic device. These results are expected to contribute to some of the most fundamental questions of the contemporary spintronic materials research, such as the FM/SC interfacial hybridization and magnetism, the spin and orbital ordering of ferrites, and the fundamental magnetism of doped TIs, and the proximity effects in FM/DMS and FM/doped TI heterostructures.

Future computing will require significant development in new computing device paradigms. This is motivated by CMOS devices reaching their technological limits, the need for non-Von Neumann architectures as well as the energy constraints of wearable technologies and embedded processors. The first device proposal, an energy-efficient voltage-controlled domain wall device for implementing an artificial neuron and synapse is analyzed using micromagnetic modeling. By controlling the domain wall motion utilizing spin transfer or spin orbit torques in association with voltage generated strain control of perpendicular magnetic anisotropy in the presence of Dzyaloshinskii-Moriya interaction (DMI), different positions of the domain wall are realized in the free layer of a magnetic tunnel junction to program different synaptic weights. Additionally, an artificial neuron can be realized by combining this DW device with a CMOS buffer. The second neuromorphic device proposal is inspired by the brain. Membrane potential of many neurons oscillate in a subthreshold damped fashion and fire when excited by an input frequency that nearly equals their Eigen frequency. We investigate theoretical implementation of such “resonate-and-fire” neurons by utilizing the magnetization dynamics of a fixed magnetic skyrmion based free layer of a magnetic tunnel junction (MTJ). Voltage control of magnetic anisotropy or voltage generated strain results in expansion and shrinking of a skyrmion core that mimics the subthreshold oscillation. Finally, we show that such resonate and fire neurons have potential application in coupled nanomagnetic oscillator based associative memory arrays.

La presente tesis doctoral se centra en el campo de la electrónica molecular, en particular se ocupa del desarrollo de nuevos dispositivos moleculares y del estudio de los fenómenos de transferencias electrónica asociados a ellos. Las propiedad de los policloro trifenilmetil radicales (PTM) han sido utilizadas en este trabajo para investigar los diferentes mecanismos de transferencia de carga asociados a varios sistemas en los cuales los PTM están involucrados. En la primera parte de la tesis se ha descrito el estudio de los procesos de transferencia de carga a través de dos diferentes familias de hilos moleculares, una de vinilo tiofene y la otra de oligo-p-fenilenos vinilenos fusionado conectados a dos moléculas de PTM las cuales actúan como dadores y aceptores de electrones en sistemas de valencia mixta D-B-A. Estos sistemas han sido completamente caracterizados por diferentes técnicas espectroscópicas en sus diferentes estados de oxidación: neutro, de valencia mixta y oxidada. Además, los mecanismos para la transferencia electrónica intermolecular a través de estos hilos moleculares han sido elucidados. En la segunda parte de la tesis ha sido reportada la síntesis de una familia de derivados de PTM con grupos tiol conectados al PTM a través de un cadena alquílica de diferentes longitudes, capaces de formar monocapas auotensambladas (SAM) sobre substratos de oro. Asimismo, se ha estudiado los mecanismos de transferencia electrónica a través de las SAMs de PTM en sus diferentes estados de oxidación, contactadas estas por el electrodo de eutéctico galio-indio y por el microscopio de efecto túnel. Para finalizar, en la última parte de la tesis, se ha reportado el estudio de las propiedades magnéticas y eléctricas de dos derivados de PTM en break-juction unimoleculares de oro y HOPG. Interesante fue el hecho de que en las break-juction unimoleculares de oro, se detectó un pico Kondo lo que indica que el momento magnético del radical PTM interactúa con los electrones de conducción.
The present Doctoral Thesis is framed in the field of molecular electronics, specifically is focused on the development of new molecular electronic devices and on the study of the electron transfer phenomena associated to them. We exploit the properties of polychloro thriphenylmethyl radical (PTM) molecules to explore the charge transfer mechanisms involved in many different systems containing PTM derivatives. In the first part of the Thesis, we have described the study of the charge transfer process through two different families of molecular wires, oligo vinylene-thiophne (nTV) and fused oligo-p-phenylene vinylene (nCOPV), connecting two PTM moieties acting as electron donor/acceptor in mixed valence systems D-B-A. These systems were fully characterized by different spectroscopic techniques in their neutral, mixed valence and oxidized states. The mechanism for the intramolecular charge transfer through these wires was elucidated. In the second part of Thesis we have reported the synthesis of a family of PTM derivatives containing a thiol terminal group connected to the PTM through an alkyl chain with different length, able to form self-assembled monolayers (SAM) on gold substrates. We have studied the charge transport mechanisms through PTM SAMs contacted by eutectic gallium-indium electrode and scanning tunneling microcopy, in their different redox states. Finally, in last part of the thesis we have reported the study of the electric and magnetic properties of two PTM derivatives in gold and HOPG single molecule break-junctions. On gold PTM break-junctions, a Kondo peak was detected indicating that the localized magnetic moment of PTM radical interacts with conducting electrons.

The properties of dilute magnetic semiconductors have been studied by combined ab initio, Monte Carlo, and experimental techniques. This class of materials could be very important for future spintronic devices, that offer enriched functionality by making use of both the spin and the charge of the electrons. The main part of the thesis concerns the transition metal doped ZnO. The role of defects on the magnetic interactions in Mn-doped ZnO was investigated. In the presence of acceptor defects such as zinc vacancies and oxygen substitution by nitrogen, the magnetic interactions are ferromagnetic. For dilute concentrations of Mn (~ 5%) the ordering temperature of the system is low, due to the short ranged character of the exchange interactions and disorder effects. The clustering tendency of the Co atoms in a ZnO matrix was also studied. The electronic structure, and in turn the magnetic interactions among the Co atoms, is strongly dependent on the exchange-correlation functional used. It is found that Co impurities tend to form nanoclusters and that the interactions among these atoms are antiferromagnetic within the local spin density approximation + Hubbard U approach. The electronic structure, as well as the chemical and magnetic interactions in Co and (Co,Al)-doped ZnO, was investigated by joined experimental and theoretical techniques. For a good agreement between the two, approximations beyond the local density approximation must be used. It is found that the Co atoms prefer to cluster within the semiconducting matrix, a tendency which is increased with Al co-doping. We envision that it is best to describe the system as superparamagnetic due to the formation of  Co nanoclusters within which the interactions are antiferromagnetic. The magnetic anisotropy and evolution of magnetic domains in Fe81Ni19/Co(001) superlattices were investigated both experimentally, as well as using model spin dynamics. A magnetic reorientation transition was found.

Materials exhibiting new functionalities due to interdependent electric (e.g. conductivity) and magnetic properties are potentially interesting for spintronics applications. We have investigated electronic and magnetic properties by means of x-ray spectroscopies and SQUID magnetometry in several magnetic materials, often in the form of thin films, which have shown promising properties for applications. One of the main subjects has been studies of inter-diffusion between layers in multilayer structures, which is an important factor for spin-dependent transport and magnetic properties. These studies have been performed by high kinetic (HIKE) photoemission spectroscopy where high photon energies increase the bulk sensitivity in comparison to soft x-ray photoemission spectroscopy. Cu/Ni multilayers were studied mainly as a model system and revealed a diffusion process that was dependent on layer thicknesses and capping materials. CoFeB/MgO/CoFeB, which is used as a magnetic field sensor in hard drives, has recently been shown to exhibit a perpendicular magnetic anisotropy (PMA) switchable by electric fields. We have studied both the interface quality and magnetic properties of thin CoFeB layers exhibiting PMA. Layered structures of full Heusler alloys Co2MnGe/Rh2CuSn have been proposed as a promising candidate for current-perpendicular-to-plane giant magneto-resistance sensors. Using HIKE,we have shown that diffusion of atoms, mainly Mn, occurs at temperatures lower than what is used in device fabrication, which likely contributes to the limited magneto-resistance values obtained. Lately, a large body of research has been performed on semiconductors doped with transition metal elements with the hope to find a ferromagnetic semiconductor at room temperature, a foundation for new devices combining spin and charge in their functionality. We have investigated Co and Fe doping in ZnO for different concentrations of the dopants and different annealing temperatures. The Co and Fe atoms are shown to forms clusters for which antiferromagnetic interactions are dominating.