Academic literature on the topic 'Ferromagnetisme'

Tom Read arrived at Columbia University on the same day that I did in 1948. He was a new professor in the School of Mines and I was a new graduate student. He was more than just a new professor. He was exceptional for that time. His father, T.T. Read, had been famous as an archeological metallurgist and professor at Columbia when the School of Mines was the premier school of its kind in the country. A measure of its eminence is that Irving Langmuir chose to study there rather than in a standard chemistry department.The younger Tom Read had studied physics at Columbia under Shirley Quimby, one of the few solid-state physics professors of the time (pre-transistor). After graduation he worked at the Frankford Arsenal and at the Westinghouse Research Laboratories, where he and Frederick Seitz wrote their definitive review of the mechanisms of the plastic deformation of solids.When he came back to Columbia as a professor, Tom Read's physics background made him almost unique among metallurgy professors. And, he had the zeal of a crusader — he was determined to teach fundamental knowledge rather than recipes. His techniques were often novel.For example, one semester we were to learn about ferromagnetism. But he had trouble finding a good American text, so he announced that we would study both ferromagnetism and German using the famous book by Becker and Doring called Ferromagnetisms. As a result, I have never forgotten the essentials of ferromagnetism.

A series of Y 2 O 3 nanoparticles of average particle size 19–37 nm are synthesized by a glycine-nitrate method. Room temperature ferromagnetism is observed in all samples. The magnetization of these samples decreases with increasing annealing temperature, showing a size-dependent ferromagnetism. Vacuum-heating effect on the ferromagnetism is also investigated, which indicates that the observed ferromagnetism is possibly associated with oxygen defects. Interestingly, an aging behavior of the ferromagnetism is observed when the sample is exposed to air or immersed in ethanol. These results further support the oxygen-vacancy-mediated ferromagnetism mechanism.

From flat band ferromagnetism, we learned that the lowest energy half-filled flat band gives always ferromagnetism if the localized Wannier states on the flat band satisfy the connectivity condition. If the connectivity conditions are not satisfied, ferromagnetism does not appear. We show that this is not always the case namely, we show that ferromagnetism due to flat bands can appear even if the connectivity condition does not hold due to a peculiar behavior of the band situated just above the flat band.

In this paper, α- Mn 2 O 3 thin films were fabricated by plasma-assisted molecular beam epitaxy on SrTiO 3 and Nb : SrTiO 3, respectively. The grown samples showed room temperature ferromagnetism (RFM) properties. All the experimental results manifested that the RFM properties in undoped thin films were induced by oxygen vacancies formed during the growth process. Even more, the ferromagnetism of thin films grown on Nb : SrTiO 3 were enhanced, and these results confirmed the fact that oxygen vacancies induced ferromagnetism. That is to say, more oxygen vacancies result the more unpaired electrons induced prominent abnormal spin causing ferromagnetism.

Ultra-thin ferromagnets, when coupled with magnetoelectric or multiferroic materials, could potentially enable highly energy-efficient electric field control of the magnet for use in nanoelectronic memories. Substitutional doping of magnetic impurities in monolayer transition metal dichalcogenides (TMDs) may be a promising way to create 2D ferromagnets but, according to theoretical calculations, require high doping levels (10-20 %) to achieve above room temperature (RT) Curie temperature. Room-temperature ferromagnetism has been reported for very low doping levels (0.1-1%), in conflict with the theoretical calculations, and always in the presence of high concentrations of defects, making it unclear if the dopants alone are responsible for this ferromagnetism. In this work, we use a combination of molecular beam epitaxy (MBE), plan-view TEM, XRD, XPS, Raman, and magnetometry to study iron and vanadium doping in monolayer WSe2. We show that optimally grown monolayers with up to 35% doping and low defect density show no ferromagnetism. Interestingly, ferromagnetism is observed when these monolayers contain a significant amount of selenium vacancies (Sevac), intentionally created via a post-growth heat treatment process, and magnetism is seen to scale with heating time/vacancy concentration. Moreover, even undoped WSe2 shows similar ferromagnetism for Sevac > 1014 cm-2. This ferromagnetism can later be quenched by filling these vacancies via Se annealing. For films with low Se vacancy concentration, TEM analysis reveals significant clustering of the V and Fe dopants which likely suppresses the ferromagnetism predicted by theory - a problem that has previously plagued III-V based dilute magnetic semiconductors as well. We go so far as to claim that all of the above room-temperature ferromagnetism reports in the literature thus far are due to Se vacancies and not magnetic doping.

Increasingly impressive demonstrations of voltage-controlled magnetism have been achieved recently, highlighting potential for low-power data processing and storage. Magnetoionic approaches appear particularly promising, electrolytes and ionic conductors being capable of on/off control of ferromagnetism and tuning of magnetic anisotropy. A clear limitation, however, is that these devices either electrically tune a known ferromagnet or electrically induce ferromagnetism from another magnetic state, e.g., antiferromagnetic. Here, we demonstrate that ferromagnetism can be voltage-induced even from a diamagnetic (zero-spin) state suggesting that useful magnetic phases could be electrically induced in “nonmagnetic” materials. We use ionic liquid–gated diamagnetic FeS2 as a model system, showing that as little as 1 V induces a reversible insulator-metal transition by electrostatic surface inversion. Anomalous Hall measurements then reveal electrically tunable surface ferromagnetism at up to 25 K. Density functional theory–based modeling explains this in terms of Stoner ferromagnetism induced via filling of a narrow eg band.

In recent years, ferromagnetism induced by natural defects of nonmagnetic semiconductors has been widely investigated and expected to be applied in spintronics. On this basis, we report the ferromagnetic behavior of copper (I) nitride (Cu3N) nanoparticles. A robust room temperature ferromagnetism is found in Cu3N nanoparticles with the saturated magnetization of 4 memu/g (300 K). Based on the element-specific X-ray magnetic circular dichroism (XMCD) and the density functional theory (DFT) analysis, it is concluded that the ferromagnetism of Cu3N nanoparticles originate from the surface Cu vacancies. Moreover, by increasing the surface area of Cu3N, the variation of magnetism is realized, and the surface states related to ferromagnetism is further revealed.

Despite our traditional concept-based understanding of ferromagnetism, an investigation of this phenomenon has revealed several other facts. Ferromagnetism was previously supposed to be exhibited by only a few elements. Subsequently, it was realized that specific elements with d- or f- orbitals demonstrated this phenomenon. When elements without these orbitals exhibited ferromagnetism, intrinsic origin-based and structural defect-based theories were introduced. At present, nonmagnetic oxides, hexaborides of alkaline-earth metals, carbon structures, and nonmetallic non-oxide compounds are gaining significant attention owing to their potential applications in spintronics, electronics, biomedicine, etc. Therefore, herein, previous work, recent trends, and the applications of these materials and studies based on relevant topics, ranging from the traditional understanding of ferromagnetism to the most recent two-element-based systems, are reviewed.

The goal of this work is to investigate the magnetic deflagration phenomena in two very different magnetic systems, using two very different experimental techniques. In the first chapter of this dissertation I introduce the concept of magnetic deflagra-tion, together with a description of the state of the art of the field. In the second chapter of this thesis I present the study of magnetic deflagration in single crystals of the prototypical single molecule magnet Mn12—ac, using the magneto-optical imaging method; never done before. In the chapter the reader will find out that, effectively, the deflagration process can be explored using this technique. The second part of the thesis include the other five chapters. Such a difference in length between both parts is due two factors. The first factor is that the Mn12—ac is a very well known system, with few new experiments to study; while on the contrary, the Nd5Ge3 intermetallic compound has been barely studied in its single crystalline form, and therefore there are many experiments to conduct to understand its dynamics. The second factor is the rich phenomena that this intermetallic compound presents directly related with the abrupt magnetic changes that possesses. In this system sudden changes occur in every studied physical property when the magnetization changes in a stepped manner. Two fundamental properties of the magnetic dynamics of the Nd5Ge3 system make it so remarkable. First, it is one of few systems with a large irreversible AFM-+FM transition induced by the magnetic field, and second, the magnetic changes during its magnetization processes occur in a stepped manner. Therefore, the motivation to investigate the possibility of finding for the first time magnetic deflagration phenomena in a ferromagnet, and also for the first time in two different magnetic phases of a given material, was very high. In the first two chapters dedicated to the Nd5Ge3 I explore the magnetic properties, the heat capacity and the electric resistivity. Some of the measurements were not previously reported in a single crystal. In the fifth chapter I explore explicitly the spon-taneous field-induced avalanches in magnetism, heat capacity and resistivity. The reason of this three chapters is to have as much ingredients as possible to be able to answer the questions that the possible magnetic deflagrations would raise. I say possible because it is not until the fifth chapter of this thesis when we get the experimental confirmation that the field-induced spontaneous avalanches that the compound present correspond to magnetic deflagration phenomena. In this chapter the magnetic deflagrations were studied in detail in the AFM-+FM transition and in the FM reversal, as a function of the magnetic field and the temperature. The spontaneous and induced deflagrations are explored, obtaining good estimations of the propagation speed and the temperature of the deflagration flame. Using the theoretical framework of the magnetic deflagrations we fit the speed propagation of the front to the experimental data using only one pa-rameter, the heat diffusivity of the material; obtaining a value within the range of heat diffusivities found in other intermetallic compounds.
El objetivo que persigue esta tesis es impulsar el estudio de las deflagraciones magnéti-cas gracias, por una parte al descubrimiento del fenómeno en un sistema nuevo y pro-metedor como es el compuesto intermetálico Nd5Ge3, y por otra a la presentación de un método nuevo de medición de las dependencias espacio-temporales de las mismas utilizando técnicas magneto-ópticas. Manteniendo el hilo conductor del fenómeno de las deflagraciones magnéticas, esta tesis doctoral se divide en dos partes. En la primera parte presento mis investigaciones en el estudio del sistema Mn12—ac. A partir de los tratamientos de los videos obtenidos se confirma la presencia de deflagraciones magnéticas. La segunda parte de la tesis está dedicada al compuesto intermetálico Nd5Ge3. Éste compuesto se trata de uno de los pocos sistemas en los que mediante un campo magnéti-co externo se induce espontáneamente un estado ferromagnético (FM) con gran irre-versibilidad proviniendo de un estado antiferromagnético (AFM). Además, los cambios magnéticos que experimenta el sistema, tanto dicha transición AFM—>FM como la in-versión de la magnetización en el estado FM, ocurren de forma muy abrupta, siendo también uno de los escasos sistemas que presenta esta propiedad. Dedico tres capítulos al estudio de sus propiedades magnéticas, térmicas y eléctri-cas, tanto estáticas como dinámicas. En esas medidas encuentro fenómenos interesantes, desde generación espontánea de voltaje durante las deflagraciones magnéticas, hasta la aparición de saltos espontáneos de la magnetización con el tiempo (manteniendo la tem-peratura y el campo magnético constantes), pasando por la obtención de términos de origen antiferromagnético en la dependencia térmica de la capacidad calorífica del estado ferromagnético saturado, o una magnetorresistencia gigante entre ambos estados, entre otros. En el sexto capítulo, las medidas experimentales confirman la existencia del fenómeno de la deflagración magnética en ambas fases, AFM y FM. La velocidad de propagación del frente obtenida en la teoría de deflagraciones se ajusta bien a los datos experimen-tales. Utilizando la bondad del ajuste, extrapolamos la velocidad teórica hacia campos magnéticos elevados y encontramos la posibilidad de que ésta iguale o supere la velocidad del sonido en el material. Lo más remarcable es que esta posible transición se observa en la extrapolación para campos menores de 50 kOe. Por lo que, en principio, reduciendo la temperatura podríamos ser capaces de obtener medidas de dicha transición. Sin em-bargo, el estudio de las deflagraciones espontáneas en función de la temperatura llevado a cabo en un criostato de dilución resultó un claro ejemplo de serendipia. En vez de alcanzar velocidades supersónicas, lo que encontré fueron unas discontinuidades de salto en los campos de deflagración espontánea no predichas. Por lo tanto, el capítulo pasa a enfocarse en su estudio, concluyendo que su origen está relacionado con propiedades intrínsecas del Nd5Ge3.

The goal of this thesis is to explore and control the magnetization dynamics on magnetic multilayered thin films through two different techniques: the application of strain and spin- polarized currents, which represent lower-power consumption approaches to the control of magnetization dynamics compared with conventional techniques. The ferromagnetic materials with nanometric thickness used in this thesis are magnetic materials widely used in research. Aside the purely scientific interest, these materials are potentially applicable in telecommunications or technologies for storing and transmitting information at high speeds. 1. Magnetization Dynamics Induced by the Application of Oscillating Strain The first part of the thesis studies the magnetization dynamics induced by the application of dynamic strain on the magnetic material. The strain deforms the magnetic material and induces a change in the direction and intensity of the magnetic anisotropy. Therefore, the magnetic states are affected by this variation and align with the new direction of magnetic anisotropy inducing dynamics in the magnetization. The main result of the first part of the thesis is the simultaneous time- and space-resolved observation of both the piezoelectric voltage wave associated to the SAW and the induced magnetization excitations on the ferromagnetic thin film of Nickel (Ni). We have found that manipulation of magnetization states in ferromagnetic thin films with SAWs is possible at the picosecond scale with efficiencies as high as for the static case. In Chapter 3 we have studied Ni nanostructures whose magnetization dynamics are governed by the intrinsic configuration of the magnetic domains and by their orientation with respect to the SAW- induced strain resulting in considerable delays between strain and magnetization. In Chapter 4 we have studied extended Ni thin film, on which SAWs induce spin waves that propagate millimeter distances and have a rotation amplitude of about 25 deg. 2. Magnetization Dynamics Induced by the Spin-Polarized Current The second part of the thesis studies the magnetization dynamics induced by the application of spin-polarized current through the magnetic material that exchanges magnetic moment with the magnetic spins of the electrons in the current. The current density has to be high to induce dynamics on the magnetization (~106-107 A/cm2) and this results in a reduction of the diameter of the electrical contact 50-200 nm. The main results of the second part of the thesis are related with the stability and the nucleation process of magnetic solitons. On the one hand, we have showed that magnetic solitons can exhibit multiple stable states, which are tunable with current or magnetic field. We also have correlated the existence of unstable states with an increment of low- frequency noise. Using simulations, we have identified the low-frequency spectra with the existence of drift resonances and we have observed that any asymmetry on the effective magnetic field suffered by the magnetic soliton can leads to drift resonances. On the other hand, we have experimentally observed that the processes of nucleation and annihilation of magnetic solitons have different intrinsic times, and using simulations we have identified a waiting time associated with the creation process, which make it a longer than annihilation. We also have studied, using micromagnetic simulations, the initial magnetization states that lead to the nucleation of topological and non-topological magnetic solitons.
La tesis gira en torno al estudio de la dinámica de la magnetización en capas y multicapas delgadas ferromagnéticas. Sin embargo, los sistemas estudiados son diversos y pueden clasificarse por la técnica utilizada para la excitación de la dinámica de la magnetización. Este hecho queda plasmado en la estructura de la tesis que consta de una introducción general, Capítulo 1, y luego de dos partes independientes y separadas, a su vez, en varios capítulos. El orden en la exposición de los resultados pretende seguir una linea lógica para su compresión. Como contrapartida, los resultados son presentados sin seguir un orden cronológico. La primera parte de la tesis estudia la dinámica de la magnetización inducida por la aplicación de tensión dinámicamente sobre el material magnético, que al deformarlo induce en él un cambio en la dirección e intensidad de la anisotropía magnética. Por lo tanto, los estados magnéticos se ven afectados por esta variación y cambian para alinearse con la nueva dirección de anisotropía magnética induciendo dinámica en la magnetización. La segunda parte de la tesis estudia la dinámica de la magnetización inducida por la aplicación de corriente polarizada a través del material magnético que intercambia momento magnético con los espines magnéticos de los electrones de la corriente. Para que esta transferencia de momento magnético sea efectiva la densidad de corriente ha de ser elevada (~106-107 A/cm2) y para conseguirla se reduce hasta los 50-200 nm el diámetro del contacto eléctrico. Los materiales ferromagnéticos con grosor nanométrico usados en esta tesis son materiales magnéticos usados ampliamente en la investigación. Aparte del interés puramente científico, estos materiales son potencialmente aplicables en telecomunicaciones o tecnologías del almacenaje y transmisión de información a altas velocidades.

Los trabajos de esta memoria persiguen un doble objetivo. Por un lado se ha pretendido ahondar en modelos de campo cristalino para cationes 3d(n) y su correlación con los datos experimentales, de sustancias de interés catalítico y básico. En segundo lugar, se trataba de iniciar un estudio amplio y ambicioso sobre sistemas desordenados. En esta Tesis hemos estudiado sistemas ferromagnéticos diluídos y se han utilizado modelos de campo medio y de simulación Monte Carlo para determinar diferentes magnitudes macroscópicas, a la vez que contestar a ciertas preguntas fundamentales, referidas al comportamiento crítico de estos sistemas. En el capítulo primero de este trabajo se desarrolla un formalismo matemático general que permite determinar la estructura electrónica de cationes 3d(n) situados en el seno de un campo cristalino, a partir de las propiedades magnéticas de la molécula considerada. En el caso particular de iones 3d(5) sometidos a un campo cristalino de elevado caracter tetragonal, hemos obtenido expresiones analíticas que describen la dependencia con la temperatura de la susceptibilidad y los momentos eficaces según las direcciones principales de la molecula. En el capitulo segundo se aplican los modelos citados al estudio de las propiedades magnéticas y, en consecuencia, de la estructura electrónica de algunos compuestos sintéticos análogos a las porfirinas naturales (ftalocianinas). En el Apéndice 2.I se comprueba la validez del modelo particular para cationes 3d(5), comparando las predicciones teóricas con los resultados experimentales para algunas mioglobinas férricas. El capitulo tercero está dedicado al estudio de tres teorías de campo medio de tipo Bethe-Peierls que describen dos situaciones particulares que se dan con frecuencia en sistemas magnéticos "desordenados": i) sistema ferromagnetico Ising con dilución de impurezas no-magnéticas, ii) sistema ferromagnético Ising con dilución de enlace. La validez de estas teorías se pone de manifiesto comparando con algunos resultados experimentales y de simulación Monte Carlo, asi como con las predicciones de teorias mas sofisticadas. Por último, en el capitulo cuarto presentamos un estudio Monte Carlo de un sistema Ising tridimensional diluido con impurezas no-magnéticas. De la simulación Monte Carlo se obtienen las dependencias con la temperatura de las principales magnitudes termodinámicas que caracterizan el sistema magnético. El estudio se restringe a concentraciones de impurezas relativamente pequeñas (X menor o igual que 0.2). Un análisis detallado de los resultados nos ha permitido deducir importantes consecuencias sobre el comportamiento crítico de este sistema.

Les propriétés chimiques de nanoparticules de Co supportées dépendent de leur taille, de leur morphologie, de leur structure cristalline et de la force de l'interaction avec le support. Ces caractéristiques se reflètent dans les propriétés magnétiques des nanoparticules, notamment dans la distribution du champ magnétique hyperfin à travers la particule et donc dans leur ferromagnétisme et dans la fréquence de résonance des noyaux de 59Co dans une expérience de RMN à champ interne (IF NMR). Cette technique de RMN non conventionnelle est un outil puissant pour étudier la structure cristalline et magnétique du cobalt métallique ainsi que l'environnement local des noyaux de Co dans des matériaux ferromagnétiques. Cependant, l'application de cette technique est entravée par la nature complexe des spectres expérimentaux qui sont soumis à diverses contributions. Ainsi, l'objectif principal de ce travail était d'établir les effets sur les spectres IF NMR du 59Co de deux aspects particulièrement importants dans l'application à la catalyse hétérogène : la distribution de la taille des particules et l'interaction avec la surface du support.L'effet de la distribution de la taille des particules a été démontré sur un échantillon modèle de petites nanoparticules de Co supportées sur des nanotubes de carbone multi-parois où la transition des particules de l'état superparamagnétique à l'état ferromagnétique a été observée en utilisant la spectroscopie IF NMR du 59Co. La température d'une telle transition pour une particule individuelle est liée à son volume, ce qui rend possible l'utilisation de la spectroscopie IF NMR pour la caractérisation de la distribution de la taille des particules dans un échantillon.L'influence de la surface du support sur la structure des nanoparticules de Co a été étudiée en utilisant des échantillons supportés sur alumine. Selon les mesures de 59Co IF NMR, la surface de χ-Al2O3 a favorisé la formation de plus grandes particules de Co de structure principalement hcp. En utilisant des calculs semi-empiriques, nous avons montré qu'un tel effet du support sur la structure des nanoparticules de Co était dû à la couverture hydroxyle de l'alumine
The chemical properties of supported Co nanoparticles depend on their size, morphology, crystal structure, and the strength of interaction with the support. These characteristics reflect themselves in the magnetic properties of ferromagnetic nanoparticles, namely in the distribution of the hyperfine magnetic field across the particle and thus in the resonant frequency of 59Co nuclei in a 59Co Internal Field NMR (IF NMR) experiment. This unconventional NMR technique is a powerful tool for studying the crystal and magnetic structure of ferromagnetic metallic cobalt as well the local environment of Co nuclei. However, the application of this technique is hindered by the complicated nature of the experimental spectra that are subject to contributions from various sources. Thus, the main aim of this work was to establish the effects on 59Co IF NMR spectra of two aspects particularly important in heterogeneous catalysis – particle size distribution and metal-support interaction.The effect of particle size distribution was demonstrated on a model sample of small Co nanoparticles supported on multi-walled carbon nanotubes where the transition of the particles from superparamagnetic to ferromagnetic state was observed using 59Co IF NMR spectroscopy. The temperature of such a transition for an individual particle depends on its volume, making it possible to use 59Co IF NMR to characterize particle size distribution in a sample.The influence of the support surface on the structure of the Co nanoparticles was investigated using samples supported on metastable alumina phases. According to the 59Co IF NMR, the surface of χ-Al2O3 promoted formation of larger Co particles and favored the hcp crystal structure. Using semi-empirical calculations, we suggest that the hydroxyl coverage of the alumina is the main support characteristic determining the Co nanoparticles structure

Multiferroic materials, in which two or more ferroic ordering take place in the same phase, have driven major interest in the last few years, not only due to the possibility of exploring novel physical properties in those materials, but also the implications that such properties show in novel technological applications. From those materials, the especially interesting are those in which the ferromagnetic (FM) and ferroelectric (FE) ordering take place, due to their direct application in magnetodielectric devices. In the field of multiferroic materials such materials could play an important role in a new generation of none volatile magnetic random access memories (M RAM), in which a sufficiently strong magnetodielectric coupling could allow for the modification of the magnetic state, not only with a magnetic field, but with an electric field. This fact would allow for a dramatic reduction in energy consumption and would promote the further technological integration (the major commercial drawback of MRAMs), due to the fact that an electric field, contrary to the magnetic field, can be applied locally. Additionally, such multiferroic materials could prove useful in magnetic tunnel junctions, in which the ferroelectric and ferromagnetic nature would allow them to codify four resistive states, instead of the traditional two states of ferroelectric or ferromagnetic junctions, allowing for the implementation of a generation of four state memories. The materials with perovskite structure, ABB"03 (A=Rare Earth, Bismuth, Lead and Yttrium), bring a broad spectrum of possibilities when it comes to design of multifunctional materials. This is due to the wide variety of A, B, B" cations that are compatible with such structure. However, in the case of R(NiMn)03, such oxides have been poorly studied and many detailed studies, both in bulk and thin films are needed. The cation selection of B and B' seems to transform the paramagnetic ordering (PM) into FM below room temperature. The multiferroicity of these materials is typically provided by the A cation of the perovskite formula, which can be Bi or Pd, in order to create a Type 1 multiferroic. In this type of materials, i.e: Bi2NiM n06, the ferroelectricity and ferromagnetism arose by separate mechanisms, the FE is provided by the A cation, with so called long pair electrons, which are free electrons in the valence band that do not participate in any chemical reaction in the compound, while the Ni2+(d8) and (M n4+) (d3) provides the FM. However, even though the materials are multiferroic, their magnetodielectric coupling, crucial for future industrial applications, is weak, due to the different mechanisms that provide their FM and FE ordering. On the other hand, the FE induction by geometrical distortion of the perovskite lattice, for example in YM n03, is an interesting case since rotations of the M nO6 octahedrons promote an important structural change, in which the oxygen atoms move closer to the Y and, due to a large dipole interaction, generate a stable FE state. Moreover, the deformation of the unit cell generates a weak spin canting on the Mn cations, that can be promoted by Li doping or lattice distortions. This behavior could prove useful in the R(NiM n)06 family, which shows strong FM . This thesis is devoted to the study of R(Ni0.5M n0.5)03 (Y,Sm, Nd and Pr) and Bi(Fe0.5M n0.5)06 grown in thin films by pulsed laser deposition technique. Firstly, this thesis focuses on the growth and characterization of thin films of Y(Ni0.5M n0.5)03 (YNM 0) on strontium titanate substrates SrTiO3(001) (STO). The influence of the deposition parameters, such as temperature, fluence and ablation frequency, on the morphology and crystalline quality of the films is investigated. The study reveals that the YNMO films grown on STO(001,011 and 111) substrates are epitaxial and that their crystalline quality and epitaxial relationship are similar to those of the YMO compound. In particular, it is observed that a single out of plane domain is the norm for all the substrate orientations, while there are various in-plane domains. Moreover, chemical composition studies reveal Ti diffusion from the substrate to the YNMO film when STO(111) substrates are used. Once the growth conditions of YNMO are optimized, the magnetic and dielectric properties are studied. All the films show a paramagnetic to ferromagnetic transition at a temperature around 95K, with a magnetic moment of YNMO(001) = 4.35µB/f.u, YNMO(100) = 4,4 µB/f.u and YNMO(101) = 3,7µB/f.u, confirming the ferromagnetic nature of the samples. The dielectric characterization reveal a FE ordering on the YNMO films, and what is more, the existence of a dielectric anisotropy on the films, that is characterized by the absence of ferroelectric response on YNMO samples deposited on STO(001), while YNMO samples on STO(111) show a strong FE response. This anisotropy could be explained, according to recent theoretical studies, in the improper origin of the observed ferroelectriciy. The coexistence of FM and FE response shows in a conclusive manner the multiferroic nature of the YNMO compound. Secondly, studies similar to those previously presented are performed for thin films of R(Ni0.5Mn0.5)O3 (Sm, Nd and Pr) compounds grown on STO(001). In this case the deposition temperature turns out to play a crucial role on the epitaxial growth of all the studied compounds. It is shown that the ratio between the b/a lattice parameters influences the epitaxial growth of the films, being the decisive factor between single or multi domain films. All the samples show a PM to FM transition at temperatures around 190K Finally, films of Bi(Fe0.5Mn0.5)O6 have been grown on STO(001) substrates. The films are epitaxial and grow under epitaxial strain. Samples show a FM behavior at room temperature with a weak signal of 7,42 emu/cm3 and 0,4 µB/f.u(Fe-Mn). The dielectrical characterization shows the influence of external magnetic fields on the dielectric properties of the film above room temperature.
Los materiales multiferroicos, en los que dos o más ordenes ferroicos tienen lugar en la misma fase, ha despertado gran interés en los últimos años debido, no solo al hecho de explorar nuevas propiedades físicas en los materiales, sino también a las implicaciones de las nuevas propiedades funcionales en las aplicaciones tecnológicas. De dichos materiales resultan especialmente interesantes aquellos que presentan un orden ferroeléctrico (FE) y ferromagnético (FM) debido a su aplicación directa en dispositivos magnetoelectrónicos. En este ámbito los materiales multiferroicos podrían tener una gran relevancia en una nueva generación de memorias magnéticas RAM (MRAM) de control eléctrico, no volátiles, en las que, si el acoplamiento magnetoeléctrico es suficientemente grande, se podría modificar el estado magnético no con un campo magnético sino con un campo eléctrico. Este hecho permitiría una reducción radical en el consumo de potencia y favorecería a su vez una mayor integración (la principal desventaja de las MRAMs para competir en el mercado), ya que el campo eléctrico, a diferencia del campo magnético, puede aplicarse de forma muy localizada. Por otro lado, dichos materiales multiferroicos podrían emplearse en una nueva generación de uniones túnel, en las que el carácter ferroeléctrico y ferromagnético permitiría codificar información en cuatro estados resistivos en lugar de en dos, como viene siendo hasta ahora en las convencionales uniones túnel magnéticas o ferroeléctricas, dando lugar a una nueva generación de memorias de cuatro estados. Los materiales con estructura perovskita, ABB '03, (A=Tierra Rara, Bismuto, Plomo e Ytrio) ofrecen una gran versatilidad a la hora de diseñar materiales funcionales debido a la gran variedad de cationes A, B y B' compatibles con tal estructura. Sin embargo en el caso de R(NiMn)03, estos óxidos han sido poco estudiados y muchos carecen de estudios detallados tanto en forma másica como en capa fina. Esta selección de cationes en la posición B y B' parece transformar la estructura perovskita la cual típicamente presenta un ordenamiento paramagnético (PM) en FM a temperaturas inferiores a la ambiente. El carácter multiferroico de estos materiales es típicamente aportado por el catión A en la formula perovskita, el cual puede ser un átomo de Bi, o Pb, para crear un multiferroico tipo 1. En los materiales de este tipo, por ejemplo el Bi2NiMnO6, la ferroelectricidad y el ferromagnetismo provienen de fuentes diferentes, el carácter FE es aportado por el catión A con -lone pairs electrons-, los cuales son electrones libres en la banda de valencia que no participan en las reacciones químicas del compuesto, mientras la combinación Ni2+ (d8) and Mn4+ (d3) aporta el FM. Pese al carácter multiferroico de estos materiales su acoplamiento magnetoelectrico, indispensable para sus aplicaciones industriales futuras, es débil, puesto que su FE y FM provienen de efectos independientes. Por otra parte la inducción de FE por distorsiones geométricas de la celda perovskitas, como es el caso de YMnO3 (YMO), es un caso interesante de considerar ya que la rotación de los octaedros Mn05 genera un cambio estructural importante, en el cual los oxígenos se desplazan a una posición más cercana al Y, esto sumado a una larga interacción de los dipolos conduce al material a un estado FE estable. Además la deformación de la celda genera un débil FM en este material, el cual proviene un pequeño giro en los espines del Mn ya sea debido a un dopaje con Li o por la deformación de la celda. Este comportamiento podría resultar interesante en la familia de perovskitas R(NiMn)03 las cuales presentan un fuerte FM. Esta tesis está dedicada al estudio de la perovskitas R(Ni0.5Mn0.5)O3 (Y, Sm, Nd y Pr) y Bi(Fe0.5Mn0.5)O6 crecidas en capa fina usando la técnica de depósito mediante ablación por láser pulsado. En primer lugar, esta tesis se centra en el crecimiento y caracterización de capas finas del compuesto Y(Ni0.5Mn0.5)O3 (YNMO) sobre substratos de titanato de estroncio, SrTiO3(001) (STO). Se estudia la influencia de los parámetros de depósito tales como temperatura, fluencia y frecuencia de ablación sobre la morfología y la calidad cristalina de las capas obtenidas. El estudio pone de manifiesto que las capas de YNMO crecidas sobre substratos de STO(001,011 y 111) son epitaxiales de YNMO y que la calidad cristalina y las relaciones epitaxiales entre la capa y el substrato son semejantes a las obtenidas en el compuesto YMO. En particular se observa un único dominio cristalino fuera del plano independientemente de la orientación del sustrato, mientras que dentro del plano se presentan varios dominios cristalinos. Por otra parte, los estudios de composición química revelan una difusión de Ti desde el sustrato hacía la capa de YMNO cuando se utilizan substratos STO(111).. Una vez optimizadas las condiciones de crecimiento del compuesto YNMO, se estudian sus propiedades magnéticas y dieléctricas. Todas las capas presentan una transición de fase paramagnetica a ferromagnética a una temperatura alrededor de 95K con un momento magnético de YNMO(001)= 4.35µB/f.u, YNMO(100) = 4,4 µB/f.u and YNMO(101) = 3,7µB/f.u, confirmando el carácter ferromagnético de las muestras. La caracterización dieléctrica revela el carácter FE de las capas de YNMO y lo que es más interesante, la existencia de anisotropía dieléctrica en las capas, ésta se pone de manifiesto en la ausencia de respuesta FE en capas YNMO sobre STO(001) que contrasta con la fuerte respuesta de las capas de YNMO sobre STO(111). Esta anisotropía puede tener su origen, a la luz de los recientes estudios teóricos, en el carácter impropio de la ferroelectricidad observada, a la luz de recientes estudios teóricos. La coexistencia de FM y FE muestra de manera conclusiva el carácter multiferroico del compuesto YNMO. En segundo lugar se han realizado estudios similares a los anteriores para el caso de capas finas de los compuestos del tipo R(Ni0.5Mn0.5)O3 (Sm, Nd y Pr) crecidas en STO(001). En este caso la influencia de la temperatura de depósito resulta ser un factor importante para la obtención, en todos los compuestos estudiados, de crecimiento epitaxial. Se observa que el cociente b/a entre las constantes red juega un factor importante en la epitaxia de las capas, siendo este cociente un factor determinante en el crecimiento mono-dominio o multi-dominio de las capas. Todas las muestras presentan transiciones PM a FM a temperaturas alrededor de 190K. Por último, se han crecido y estudiado capas finas del compuesto Bi(Fe0.5Mn0.5)O6 depositadas sobre STO(001). Las capas obtenidas son epitaxiales y crecen sometidas a estrés inducido por el substrato. Presentan comportamiento FM a temperatura ambiente pero con una débil señal de 7,42 emu/cm3 y 0,4 µB/f.u(Fe-Mn). La caracterización dieléctrica pone de manifiesto la influencia, a temperaturas superiores a la ambiente, de la presencia de campo magnético sobre las propiedades dieléctricas.

Deux series de composes de basse dimensionnalite a base de terre-rare, d'yttrium, d'indium et de scandium ont ete synthetises et caracterises, les oxocuprates d'une part et les bisphtalocyanines d'autre part. Dans le premier chapitre est presente un rappel theorique des mecanismes d'echange de systemes magnetiques de basse dimensionnalite. Une partie de ce chapitre concerne les techniques de calcul utilisees. Le second traite du ferromagnetisme moleculaire. Nous y faisons un bref historique, lequel est loin d'etre exhaustif. Le troisieme porte sur la caracterisation structurale et l'analyse des donnees magnetiques des oxocuprates de terres rares. Les interactions magnetiques de oxocuprates d'indium de lutetium et de scandium y sont determinees a l'aide de l'hamiltonien d'heisenberg. Les constantes ainsi obtenues evoluent de maniere logique avec les donnees cristallographiques. Le dernier chapitre est consacre aux bisphtalocyanines, a leurs proprietes physiques. Du ferromagnetisme moleculaire unidimensionnel a basse temperature est mis ici en evidence pour la premiere fois. L'interpretation de ce phenomene a fait appel a un modele a deux sous-reseaux, l'un de spin s=1/2, le second orbital l=1/2, du fait de la delocalisation du radical sur les deux macrocycles de la molecule sandwich. Le formalisme d'anderson a ete utilise pour determiner l'expression de l'hamiltonien ainsi que les proprietes magnetiques

L'etude des proprietes magnetiques et de transport de differentes series de solutions solides autour des alliages d'heusler tico#2sn et demi-heusler ticosn montre des caracteristiques classiques du ferromagnetisme itinerant. Dans la serie tico#2sn-tini#2sn, la densite locale est fortement reduite et le critere de stoner n'est plus maintenu pour tini#2sn. Le ferromagnetisme disparait au profit d'un etat paramagnetique de pauli. Vers la limite magnetique-non magnetique, la resistivite et la chaleur specifique montrent de fortes fluctuations de spin. Dans la serie tico#xsn avec 1