Dissertations / Theses on the topic 'Magnetocrystalline'

The magnetic properties of the (FexNi1-x)2B family of materials are explored using DFT calculations utilizing the FPLO and SPR-KKR code packages. It is found that a uniaxial magnetocrystalline anisotropy exists at around x = 0.8 with a magnetocrystalline anisotropy energy at around 0.3 MJ/m^3. A calculation of the lattice constant for these materials were attempted but failed due to the emergence of local minima and the calculations of magnetic properties were instead done using lattice parameters interpolated between known experimental values.

The ferromagnetic semiconductor gallium manganese arsenide is an important test-bed material for spintronics applications. Whilst a Curie temperature anywhere close to room temperature has yet to be demonstrated, the excellent micromagnetic properties, simple band structure and unusual combination of having both low moment densities and high spin-orbit coupling make this an interesting material to study from both theoretical and experimental perspectives. This thesis reports some experimental studies into the magnetic and magnetoresistive anisotropies in gallium manganese arsenide. In the first main chapter a study of the Anisotropic Magnetoresistance in thin (Ga,Mn)As films is reported, based on transport measurements of micro-scale devices, contributing to the first systematic study in this material. The Anisotropic Magnetoresistance comprises crystalline and non-crystalline components; this study shows that a uniaxial crystalline component can dominate over the whole range of temperatures from 2K up to the Curie temperature, the first time this has been seen in any material system to our knowledge. The following chapter shows that the magnetic anisotropy of gallium manganese arsenide thin films can be engineered by lithographically patterning the material into structures on length scales of a micron or less. Using electron beam lithography to define the structures and SQUID magnetometery to study the resulting magnetic configuration, it is shown that the magnetic anisotropy can be greatly modified, even resulting in a switching of the easy- and hard-axis directions. Finally a new technique based on Anisotropic Magnetoresistance measurements is presented to locate the crossover of competing magnetic anisotropy coefficients in the temperature domain. Conventionally performed by SQUID magnetometry, this new technique is cheaper and simpler whilst qualitatively reproducing the main features of the SQUID measurements.

Nanomagnetic logic, incorporating logic bits in the magnetization orientations of single-domain nanomagnets, has garnered attention as an alternative to transistor-based logic due to its non-volatility and unprecedented energy-efficiency. The energy efficiency of this scheme is determined by the method used to flip the magnetization orientations of the nanomagnets in response to one or more inputs and produce the desired output. Unfortunately, the large dissipative losses that occur when nanomagnets are switched with a magnetic field or spin-transfer-torque inhibit the promised energy-efficiency. Another technique offering superior energy efficiency, “straintronics”, involves the application of a voltage to a piezoelectric layer to generate a strain which is transferred to an elastically coupled magnetrostrictive layer, causing magnetization rotation. The functionality of this scheme can be enhanced further by introducing magnetocrystalline anisotropy in the magnetostrictive layer, thereby generating four stable magnetization states (instead of the two stable directions produced by shape anisotropy in ellipsoidal nanomagnets). Numerical simulations were performed to implement a low-power universal logic gate (NOR) using such 4-state magnetostrictive/piezoelectric nanomagnets (Ni/PZT) by clocking the piezoelectric layer with a small electrostatic potential (~0.2 V) to switch the magnetization of the magnetic layer. Unidirectional and reliable logic propagation in this system was also demonstrated theoretically. Besides doubling the logic density (4-state versus 2-state) for logic applications, these four-state nanomagnets can be exploited for higher order applications such as image reconstruction and recognition in the presence of noise, associative memory and neuromorphic computing. Experimental work in strain-based switching has been limited to magnets that are multi-domain or magnets where strain moves domain walls. In this work, we also demonstrate strain-based switching in 2-state single-domain ellipsoidal magnetostrictive nanomagnets of lateral dimensions ~200 nm fabricated on a piezoelectric substrate (PMN-PT) and studied using Magnetic Force Microscopy (MFM). A nanomagnetic Boolean NOT gate and unidirectional bit information propagation through a finite chain of dipole-coupled nanomagnets are also shown through strain-based "clocking". This is the first experimental demonstration of strain-based switching in nanomagnets and clocking of nanomagnetic logic (Boolean NOT gate), as well as logic propagation in an array of nanomagnets.

In this thesis, two selected topics in magnetism are studied using theoretical modelling and computational methods. The first of these is the magnetocrystalline anisotropy energy (MAE) of transition metal based magnets. In particular, ways of finding 3d transition metal based materials with large MAE are considered. This is motivated by the need for new permanent magnet materials, not containing rare-earth elements, but is also of interest for other technological applications, where the MAE is a key quantity. The mechanisms of the MAE in the relevant materials are reviewed and approaches to increasing this quantity are discussed. Computational methods, largely based on density functional theory (DFT), are applied to guide the search for relevant materials. The computational work suggests that the MAE of Fe1-xCox alloys can be significantly enhanced by introducing a tetragonality with interstitial B or C impurities. This is also experimentally corroborated. Alloying is considered as a method of tuning the electronic structure around the Fermi energy and thus also the MAE, for example in the tetragonal compound (Fe1-xCox)2B. Additionally, it is shown that small amounts (2.5-5 at.%) of various 5d dopants on the Fe/Co-site can enhance the MAE of this material with as much as 70%. The magnetic properties of several technologically interesting, chemically ordered, L10 structured binary compounds, tetragonal Fe5Si1-xPxB2 and Hexagonal Laves phase Fe2Ta1-xWx are also investigated. The second topic studied is that of magnetic effects on the elastic scattering of fast electrons, in the context of transmission electron microscopy (TEM). A multislice solution is implemented for a paraxial version of the Pauli equation. Simulations require the magnetic fields in the sample as input. A realistic description of magnetism in a solid, for this purpose, is derived in a scheme starting from a DFT calculation of the spin density or density matrix. Calculations are performed for electron vortex beams passing through magnetic solids and a magnetic signal, defined as a difference in intensity for opposite orbital angular momentum beams, integrated over a disk in the diffraction plane, is observed. For nanometer sized electron vortex beams carrying orbital angular momentum of a few tens of ħ, a relative magnetic signal of order 10-3 is found. This is considered realistic to be observed in experiments. In addition to electron vortex beams, spin polarised and phase aberrated electron beams are considered and also for these a magnetic signal, albeit weaker than that of the vortex beams, can be obtained.

Felaktigt ISBN i den tryckta versionen: 9789155497149

Magnetic properties of selected compounds and superlattices have been experimentally studied using SQUID (superconducting quantum interference device) and VSM (vibrating sample magnetometer) magnetometry, neutron diffraction and Mössbauer spectroscopy measurements combined with theoretical ab initio calculations.

The magnetic compounds (Fe_1-xM_x)₃P, M=Co or Mn have been studied extensively. It was found that Co can substitute Fe up to x=0.37. Increasing the Co content leads to a reduction of the Curie temperature and the magnetic moment per metal atom. Mn can substitute Fe up to x=0.25 while Fe can be substituted into Mn₃P to 1-x=0.33. On the iron rich side, the drop in Curie temperature and magnetic moment when increasing the Mn content is more rapid than for Co substitution. On the manganese rich side an antiferromagnetic arrangement with small magnetic moments was found.

The interlayer exchange coupling and the magnetocrystalline anisotropy energy of Fe/V superlattices were studied. The coupling strength was found to vary with the thickness of the iron layers. To describe the in-plane four-fold anisotropy, the inclusion of surface terms proved necessary.

The in-plane four fold anisotropy was also studied in a series of Fe/Co superlattices, where the thickness of the Co layers was kept thin so that the bcc structure could be stabilized. Only for samples with a large amount of iron, the easy axis was found to be [100]. The easy axis of bulk bcc Co was therefor suggested to be [111].

Dans cette thèse nous avons évalué l'effet de la substitution partielle du cobalt par des éléments non-magnétiques de type p (M) sur les propriétés structurales et magnétiques des composés RCo5. Les échantillons ont été synthétisées par fusion dans un four à induction, puis caractérisés en utilisant des nombreuses techniques expérimentales: diffraction de rayons X et de neutrons, microscopie électronique à balayage, magnétométrie, susceptométrie en courant alternatif et spectroscopie photoélectronique par rayons X.Tous les composés de type RCo5-xMx (R= Pr, Sm, Tb, Er et Tm; M= Si, Ge, Al, Ga; x=0,5 et x=1) étudiés gardent la structure cristalline de type CaCu5 des composés de départ RCo5, mais les paramètres de la maille cristalline sont modifiés par la présence des éléments M. Le domaine de stabilité thermique des phases RCo5-xMx est notablement modifié par la présence d'élément métalloïde en substitution sur les sites du cobalt. Il peut être étendu pour Al et Ga et réduit pour Si et Ge. Les atomes de l'élément M sont localisés préférentiellement sur le site cristallographique Co 3g. La solubilité des éléments Ge et Si dans la structure RCo5 est trouvée être inférieure à celle des métalloïdes ayant un électron de moins tels que Al et Ga.La substitution M/Co a une influence importante sur les propriétés magnétiques des composés RCo5. La température d'ordre et l'aimantation spontanée diminuent significativement après le remplacement partiel du cobalt par l'élément métalloïde. Ces modifications sont induites par l'évolution des interactions d'échange en particulier Co-Co mais aussi par la réduction de l'aimantation du Co liée à sa sensibilité au voisinage atomique et magnétique local. La direction de facile aimantation à la température ambiante est préservée le long de l'axe cristallographique c pour tous les échantillons étudiés. Dans quelques composés RCo5-xMx (R = Sm, Er et Tm) la substitution de M au cobalt change le mécanisme de coercitivité par rapport aux composés de départ RCo5. Une coercitivité élevée a été détectée à basse température, surtout pour les composés SmCo4Al et SmCo4Ga, qui présentent aussi des champs d'anisotropie énormes, bien supérieurs à ceux de phase SmCo5.Les expériences XPS sur les composés de type RCo5-xMx ont détecté la réduction de la densité d'états électroniques au niveau de Fermi par rapport aux composés de type RCo5. La bande 3d du cobalt est remplie lors de la substitution M/Co et conduit à un moment magnétique du cobalt réduit
In this work we have evaluated the effect of the partial substitution of cobalt with non-magnetic p-type elements (M) on the structural and magnetic properties of RCo5 compounds. The samples were prepared by alloying in an induction furnace and were characterized using various experimental techniques: X-ray and neutron diffraction, scanning electron microscopy, magnetometry, AC susceptometry and X-ray photoelectron spectroscopy.All of the studied RCo5-xMx compounds (R=Pr, Sm, Tb, Er and Tm; M=Si, Ge, Al and Ga; x=0.5 and x=1) maintain the CaCu5 crystal structure of the RCo5 compounds, however the lattice parameters are modified due to the presence of M elements. The thermal stability range of the RCo5-xMx phases is modified significantly due to the Co site substitutions. The thermal stability increases for Al and Ga substitutions and decreases when M is Si or Ge. The M atoms were found to preferentially occupy the Co 3g site. The solubility of Ge and Si in the RCo5 structure is inferior to that of metalloid elements with one less electron, such as Al and Ga. The M/Co substitution has an important influence on the magnetic properties of RCo5 compounds. The ordering temperature and the spontaneous magnetization are significantly reduced after the partial substitution of cobalt by the metalloid elements. These changes are induced in particular by the evolutions of the Co-Co exchange interactions and also by the reduction of the Co magnetization due to the sensitivity of Co to the local atomic and magnetic vicinity. The easy magnetization direction at room temperature is preserved along the c-axis for all of the studied samples. In several RCo5-xMx (R = Sm, Er and Tm) compounds the substitution of Co with M atoms changes the coercivity mechanism compared to RCo5. A higher coercivity was found at low temperatures, especially for SmCo4Al and SmCo4Ga compounds, which also show huge values of the anisotropy field, well above the ones found in SmCo5.The XPS measurements on RCo5-xMx compounds show a reduction of the density of states at the Fermi level compared to the RCo5 compounds. There is a filling of the Co 3d band following the M/Co substitution, leading to a lower Co moment

Mit Hilfe der Dichtefunktionaltheorie wurden magnetische Eigenschaften an 3d Elementen und Legierungen und 5f Verbindungen untersucht. Dabei wurde auf die Wichtigkeit von Orbitalpolarisationskorrekturen eingegangen und diese näher erörtert. Im ersten Anwendungsteil wurden magnetische Momente und die Magnetokristalline Anisotropie Energie an 3d Elementen untersucht. Des Weiteren wurden FeCo Legierungen als mögliche Bestandteile in der Festplattenindustrie diskutiert. Im letzten Abschnitt wurden Uranverbindungen in Hinsicht auf Ihre Orbitalpolarisation untersucht.

The large magneto-crystalline anisotropy energy of the L10 phase has pushed the interest to the FePt nanoparticles to get smallest possible not superparamagnetic particles for magnetic data storage media. The DC magnetron sputtering process, in an inert gas atmosphere and subsequently ejection into high vacuum via differential pumping in addition with a newly constructed light furnace, allows us to have a predeposition annealing of FePt nanoparticles. The advantage compared to wet chemical process route is, that we can prevent the growing of particles on a substrate. In order to determine the experimentally hardly accessible temperature of the particles, the thermal history of the particles is rather calculated from the interaction with the light field along the flight path through the light furnace used for the in-flight annealing. The results obtained for the particle temperature are corroborated by experimental findings on the sintering of agglomerated particles and change in magnetic properties due to heating over the L10 stability temperature. The experiments reveal that the effect of the thermal treatment on both the structural and magnetic properties of the FePt nanoparticles strongly depends on the particles’ crystal structure. The magnetic behavior shows a size depending effective uniaxial magnetic anisotropy constant. This behavior is strongly correlated to the structure of the 5 nm to 8 nm L10 FePt particle.

Theoretische Berechnungen sagen für tetragonal gedehntes Fe-Co eine hohe magnetokristalline Anisotropie voraus, wie sie für seltenerdfreie Dauermagnetwerkstoffe vorteilhaft wäre. In dieser experimentellen Arbeit werden epitaktische Fe-Co-Schichten strukturell und magnetisch charakterisiert. Zur Untersuchung der Dehnung in diesen Schichten eignen sich AuxCu100-x-Pufferschichten besonders, da über die Stöchiometrie (x) deren lateraler Gitterparameter eingestellt werden kann. Wird Fe-Co auf einer solchen Pufferschicht abgeschieden, erfolgt aufgrund dessen hoher elastischer Energie schon in den ersten Monolagen eine vollständige Relaxation der pufferinduzierten Dehnung. In ternären Fe-Co-X-Schichten, in denen kleine X-Atome (X = B, C oder N) Oktaederlücken besetzen, wird jedoch eine spontane tetragonale Dehnung c/a bis zu 1,05 beobachtet. Entlang der gedehnten c-Achse tritt eine uniaxiale magnetokristalline Anisotropie auf, die für B- oder C-Zulegierungen von 2 at% eine maximale Anisotropiekonstante von 0,4 MJ/m³ zeigt. Wird der X-Gehalt weiter erhöht, nehmen die Kristallinität der Schichten und die magnetische Anisotropie ab. Neben der magnetokristallinen Anisotropie des Schichtvolumens wird an den Fe-Co(-X)-Schichten eine hohe Grenzflächenanisotropie beobachtet. Der Beitrag der freien Oberfläche übersteigt den der Au-Cu-Grenzfläche dabei deutlich
Theoretical calculations predict a high magnetocrystalline anisotropy for tetragonally strained Fe-Co, which would be beneficial for rare-earth free permanent magnet materials. In this experimental work, epitaxial Fe-Co films are investigated structurally and magnetically. AuxCu100-x buffer layers are very suitable to study the strain in these films since their in-plane lattice parameter can be tailored via the applied stoichiometry (x). However, when Fe-Co is deposited on such a buffer layer, the induced strain of the Fe-Co lattice relaxes completely within the first monolayers, due to its high elastic energy. In ternary Fe-Co-X films, where small atoms X like B, C or N occupy octahedral interstitial sites, a spontaneous strain c/a up to 1.05 is observed. A uniaxial magnetocrystalline anisotropy along the strained c axis appears. Their maximum anisotropy constant is 0.4 MJ/m³ for B or C contents of 2 at%. If the X content is further increased, the crystallinity and thus, the magnetic anisotropy of the films degrade. Together with the magnetocrystalline anisotropy of the films’ volumes, a high interface anisotropy is observed for the Fe-Co(-X) films. The contribution of the free surface clearly exceeds the contribution of the Au-Cu interface

Le domaine de l’enregistrement magnétique est en constante évolution pour repousser davantage les limites de stockage de l’information, une approche prometteuse étant l’enregistrement perpendiculaire. Le matériau faisant l’objet de ce manuscrit est le ferrite de cobalt CoFe2O4 (= CFO). Ses propriétés font de lui un candidat prometteur pour la réalisation de dispositif à enregistrement perpendiculaire, cela passant par le contrôle de sa direction de facile aimantation.Ce travail de thèse traite ainsi de la modification de l’anisotropie magnétocristalline du CFO en couche mince par dopage aux éléments de terres rares. Nous avons démontré la possibilité de moduler la direction de facile aimantation du CFO non dopé, en modifiant la pression partielle en O2/N2 lors de l’élaboration. Nous avons également mis en évidence l’insertion des éléments lanthanides dans la structure du CFO ainsi que l’impact de l’anisotropie de la terre rare sur les propriétés magnétiques du matériau
The field of magnetic storage is in constant progress to constantly push further the storage capacity of the device. A promising approach is the perpendicular magnetic recording of datas. The material presented in this manuscript is cobalt ferrite. It is an excellent candidate for the realization of perpendicular storage device due to its properties. The present work deals with the modification of the magnetic anisotropy by doping the ferrite cobalt thin films with rare earth elements. We have demonstrated the possibility to modulate the easy magnetization axis of undoped cobalt ferrite by changing the partial pressure of O2/N2 during the elaboration of the thin films. We have also highlighted the insertion of rare earth elements into the structure of the cobalt ferrite although their important ionic radii. The impact of the rare earth anisotropy on the magnetic properties of the ferrite cobalt has also been observed

La thèse porte sur la synthèse et l'étude des propriétés magnétiques de borates de fer-gallium,FexGa1-xBO3 avec 0 supérieur ou égal à x supérieur ou égal à 1. Ces matériaux sont prometteurs pour les applications; en plus, grâce à la présence, en fonction de x, de différents types d’ordre magnétique, ils sont bien adaptés au traitement de nombreux problèmes du magnétisme des solides.Le borate de fer, FeBO3 est un antiferromagnétique possédant un plan de facile aimantation et un faible ferromagnétisme. Les caractéristiques du borate de fer sont radicalement modifiées par substitution isomorphe fer – gallium diamagnétique.Nous avons mis au point une route de synthèse de monocristaux FexGa1-xBO3 de haute qualité. Comme principales techniques expérimentales, nous avons choisi les résonances magnétiques électronique (RME) et nucléaire (RMN). Selon le contenu du fer, nous avons observé:(i) la résonance antiferromagnétique, (ii) la résonance de clusters magnétiques et (iii) la résonance paramagnétique électronique (RPE). Les différents états magnétiques ont été identifiés et leurs caractéristiques – la température de Néel, le champ de Dzyaloshinskii-Moriya; les paramètres de l’hamiltonien de spin de Fe3+, etc.– ont été déterminées. La coordinence et la symétrie de sites de 11B et 71Ga ont été précisées par RMN à rotation sous l’angle « magique » (MAS). Moyennant la simulation des spectres de RPE et de MAS RMN, à l’aide de codes mis au point ad hoc, les distributions de paramètres dues au désordre local ont été déterminées. L’analyse théorique, tenant compte de contributions du champ cristallin et de l’interaction dipôle-dipôle, permet d’expliquer l’anisotropie magnétocristalline de volume et de surface
The thesis is concerned with synthesis and studying magnetic properties of iron-galliumborates, FexGa1-xBO3 with [0 supérieur ou égal à x supérieur ou égal à 1]. These materials are promising candidates for applications;besides, occurrence of different types of magnetic ordering, depending on x, makes them suitablefor treating a number of fundamental problems in solid state magnetism.Iron borate, FeBO3 is a two-sublattice easy-plane antiferromagnet with weakferromagnetism. Physical characteristics of iron borate are radically modified by isomorphoussubstitution of a part of iron by diamagnetic gallium.We have started with developing a synthesis route for growing high-quality FexGa1-xBO3single crystals. As main experimental techniques, we have chosen Electron and Nuclear MagneticResonances (EMR, NMR). Depending on iron contents and temperature, we have observed:(i) Antiferromagnetic, (ii) Cluster Magnetic and (iii) Electron Paramagnetic Resonance (EPR).Different magnetic states have been identified and their characteristics: Néel temperature,Dzyaloshinskii-Moriya field; spin Hamiltonian parameters of isolated Fe3+ ion, etc., have beendetermined. Coordination and site symmetry of 11B and 71Ga nuclei have been specified by meansof Magic Angle Spininng (MAS) NMR. Carrying out computer simulations of EPR and MASNMR spectra with laboratory-developed codes, the parameter distributions caused by localdisorder have been determined. Theoretical analysis taking into account crystal field and dipoledipolecontributions allow interpreting volume and surface magnetocrystalline anisotropy of thecrystals

Die vorliegende Arbeit präsentiert die ersten detaillierten Untersuchungen des Spin-Reorientierungs-Übergangs in epitaktischen NdCo5-Schichten. Die Proben, die mit gepulster Laserdeposition hergestellt wurden, konnten sowohl als in-plane- als auch als out-of-plane-texturierte Schichten präpariert werden. Für beide Wachstumsvarianten ergaben Röntgendiffraktometrie- und Texturmessungen eine sehr gute Texturierung mit einer nahezu einheitlichen Orientierung der c-Achse, die eine Untersuchung der magnetischen Eigenschaften entlang ausgewählter kristallografischer Richtungen ermöglichte. Die globalen Magnetisierungsmessungen der In-plane-Proben zeigten einen Spin-Reorientierungs-Übergang von einer magnetisch leichten c-Achse für Temperaturen oberhalb von 310 K über einen magnetisch leichten Kegel hin zu einer magnetisch leichten Ebene (a-Achse) unterhalb von 255 K. Die Übergangstemperaturen liegen damit geringfügig über den bisher an Massivproben gemessenen Werten. Aus den magnetischen Hysteresemessungen wurden die magnetokristallinen Anisotropiekonstanten erster und zweiter Ordnung für den Temperaturbereich der magnetisch leichten c-Achse und der magnetisch leichten Ebene ermittelt. Die Untersuchungen der Out-of-plane-Proben wiesen die Existenz einer magnetokristallinen Anisotropie höherer als zweiter Ordnung nach. Sie bewirkt ein unterschiedliches Schaltverhalten der parallel zur a- bzw. b-Achse gemessenen magnetischen Hysteresekurven im Temperaturregime der magnetisch leichten Ebene. Für die in-plane-texturierten Schichten wurde das Domänenmuster und dessen Änderung mit der Temperatur im gesamten Spin-Reorientierungs-Bereich analysiert. Diese Untersuchungen basieren auf in Kooperation mit der Universität Hamburg durchgeführten SEMPA-Messungen. Oberhalb von 318 K liegt eine Zweidomänenkonfiguration mit einer Ausrichtung der Magnetisierung parallel zur c-Achse vor, die beim Abkühlen in das Regime des magnetisch leichten Kegels in einen Vierdomänenzustand übergeht. Unterhalb von 252 K bildet sich eine Zweidomänenkonfiguration mit parallel zur a-Achse orientierter Magnetisierung. Diese lokalen Messungen bestätigten den Spin-Reorientierungs-Übergang mit zu den globalen Magnetisierungsmessungen vergleichbaren Übergangstemperaturen. Für charakteristisch orientierte Domänenwände erfolgten genauere Analysen der Magnetisierungsprozesse in den angrenzenden Domänen. Um ein erweitertes Verständnis der Domänenkonfiguration, deren Temperaturabhängigkeit und der vorhandenen Domänenwände zu erarbeiten, erfolgten mikromagnetische Simulationsrechnungen für ausgewählte Temperaturen. Die Berechnungen wurden sowohl für homogene Systeme als auch für Geometrien mit verschiedenen Pinningzentren durchgeführt. Die Analyse der Domänenwände ergab, dass ihr Bloch- oder Néel-Charakter und die Domänenwandweite von der Temperatur sowie ihrer Ausrichtung parallel zur c- oder a-Achse abhängt
This thesis presents the first detailed investigation of the spin-reorientation-transition in epitaxial NdCo5 thin films. The samples were prepared by pulsed laser deposition as in-plane or out-of-plane textured films. For both kinds of samples X-ray diffraction and texture measurements revealed a high degree of texture with one common orientation of the c-axis within the film, which allowed an investigation of the magnetic properties along distinct crystallographic directions. Global magnetization measurements of the in-plane textured films showed a spin-reorientation from a magnetic easy axis (c-axis) at temperatures above 310 K via a magnetic easy cone to a magnetic easy plane (a-axis) at temperatures below 255 K. The transition temperatures are slightly higher than values reported for bulk samples. The magnetocrystalline anisotropy constants of first and second order were determined for the regime of the magnetic easy axis and plane. Measurements of the out-of-plane textured films verified the existence of a magnetocrystalline anisotropy of order larger than two, which becomes obvious from a different magnetic switching behavior along the a- and b-axis in the temperature regime of the magnetic easy plane. The domain structure and its changes with temperature were investigated for the in-plane textured films. There exists a two domain state at temperatures above 318 K with an orientation of the magnetization parallel to the c-axis from which a four domain state evolves when cooling down the sample to the easy cone state. Finally, a two domain state exists in the regime of the magnetic easy plane (easy a-axis) with an orientation of the magnetization parallel to the a-axis at temperatures below 252 K. The local measurements confirm the spin-reorientation-transition with transition temperatures comparable to those derived from global magnetization measurements. In addition, a detailed analysis of the magnetization processes for some characteristically oriented domain walls was performed. Micromagnetic simulations were carried out for selected temperatures to achieve a deeper understanding of the temperature dependence of the domain configuration and of the domain walls. The simulations considered homogeneous systems as well as systems with pinning centers. An analysis of the domain walls showed that their character and width depend on temperature and the orientation parallel to the a- or c-axis

Les nanofils magnétiques constituent un domaine de recherche en plein essor. De section cylindrique, ils permettent la propagation des parois de domaines magnétiques à très grandes vitesses et des interactions fortes avec les ondes de spin, ce qui les rend particulièrement intéressants pour le développement de futurs composants de la spintronique. L'objectif de ce travail de thèse est de fournir une analyse quantitative et qualitative complète de la configuration magnétique locale dans des nanofils magnétiques cylindriques d'alliage CoNi à anisotropie magnétocristalline perpendiculaire en utilisant les techniques d'imagerie magnétique avancées de la microscopie électronique à transmission (MET), principalement axées sur l'holographie électronique (HE). Une étude corrélative entre les propriétés structurales, les variations locales de composition et les configurations magnétiques de ces nanofils a été réalisée. De plus, les configurations tridimensionnelles (3D) complexes des domaines et des parois magnétiques ont été analysées par tomographie holographique de champ vectoriel (THCV) afin d'obtenir les trois composantes de l'induction magnétique. Enfin, un protocole a été développé pour étudier in situ par microscopie de Lorentz la configuration magnétique de ces nanofils lors de l'injection d'impulsions de courant. La première partie de ce travail est focalisée sur la corrélation des configurations magnétiques de nanofils individuels de CoNi avec les propriétés structurales et chimiques locales. L'orientation de la phase cristalline a été cartographiée en diffraction électronique par précession et combinée à des mesures de composition par spectroscopie de perte d'énergie des électrons. Les résultats révèlent une coexistence de grains de phase cfc et de phase hcp, cette dernière présente sa direction cristallographique c orientée presque perpendiculairement à l'axe du nanofil. Cette coexistence de phases cristallographiques est à l'origine de variations localisées et abruptes de la configuration magnétique. Deux nanofil configurations principales ont été observées : une chaîne d'états transversaux par rapport à l'axe du, de type vortex, et un état longitudinal. Nous avons observé que les états transversaux sont liés à la phase hcp possédant une forte anisotropie magnétocristalline perpendiculaire, ce que confirment les simulations micromagnétiques. Une autre partie de ce travail concerne l'étude de la structure magnétique 3D des domaines et des parois de domaines dans la phase hcp. Cette étude a été menée pour des états rémanents différents en fonction de l'application d'un champ de saturation perpendiculaire et parallèle à l'axe du nanofil. Les mesures ont été réalisées par la méthode THCV afin d'extraire les trois composantes de l'induction magnétique et reconstruire en 3D la configuration magnétique locale du nanofil. Les résultats montrent une stabilisation d'une chaîne de vortex dans le cas d'une saturation perpendiculaire, et des états d'enroulement longitudinaux séparés par des parois de domaine transversales après l'application d'un champ externe parallèle à l'axe du fils. La dernière partie du manuscrit présente les résultats obtenus en microscopie de Lorentz in situ démontrant la possibilité de manipuler les parois des domaines magnétiques d'un nanofil de CoNi par injection d'impulsions électriques. Cette preuve de concept est considérée comme le précurseur des observations in situ de la dynamique des parois de domaines en EH. Un protocole précis, axé sur les étapes cruciales de préparation des échantillons et les développements à poursuivre pour réaliser ces expériences délicates, est détaillé
Cylindrical magnetic nanowires (NWs) are currently subjects of high interest due to fast domain wall velocities and interaction with spin-waves, which are considered interesting qualities for developing future spintronic devices. This thesis aims to provide a wholesome quantitative and qualitative analysis of the local magnetic configuration in cylindrical Co-rich CoNi NWs with perpendicular magnetocrystalline anisotropy using state-of-the-art transmission electron microscopy (TEM) magnetic imaging techniques, mainly focused on two-dimensional (2D) and three-dimensional (3D) electron holography (EH). A correlative study between the NW's texture, modulation in composition, and magnetic configuration has been conducted. Further, the complex 3D nature of the domain and domain wall configurations have been analyzed using holographic vector field electron tomography (VFET) to retrieve all three components of the magnetic induction. Finally, I have successfully manipulated the magnetic configuration observed by Lorentz microscopy in Fresnel mode by the in situ injection of a current pulse. A TEM study comparing the magnetic configuration to the local NW structure was performed on single NWs. The crystal phase analysis was done by precession electron diffraction assisted automated crystal orientation mapping in the TEM combined with compositional analysis by scanning-TEM (STEM) electron energy loss spectroscopy (EELS) for a detailed correlation with the sample's magnetic configuration. The results reveal a coexistence of fcc grains and hcp phase with its c-axis oriented close to perpendicular to the wire axis in the same NW, which is identified as the origin of drastic local changes in the magnetic configuration. Two main configurations are observed in the NW region: a chain of transversal vortex-like states and a longitudinal curling state. The chain or vortices are linked to the hcp grain with the perpendicular magnetocrystalline anisotropy, as confirmed by micromagnetic simulations. The 3D magnetic structure of the domains and domain walls observed in the hcp grain of the NWs has been studied for two different remnant states: after the application of a saturation field perpendicular (i) and parallel (ii) to the NW axis. The measurements were done using state-of-the-art holographic VFET to extract all three components of the magnetic induction in the sample, as well as a 3D reconstruction of the volume from the measured electric potentials, giving insight into the local morphology of the NW. The results show a stabilization of a vortex chain in the case of perpendicular saturation, but longitudinal curling states separated by transversal domain walls after applying a parallel external field. Finally, preliminary Lorentz microscopy results are presented, documenting the manipulation of magnetic domain walls by the in situ injection of electrical pulses on a single cylindrical CoNi nanowire contacted by focused ion beam induced deposition. This is believed to be the forerunner for quantitative electrical measurements and in situ observations of domain wall dynamics using EH at the CEMES. A detailed protocol focusing on the crucial steps and challenges ahead for such a delicate experiment is presented, together with suggestions for future work to continue the developments

In our pursuit of ever smaller transistors, with greater computational throughput, many questions arise about how material properties change with size, and how these properties may be modelled more accurately. Metallic nanocontacts, especially those for which magnetic properties are important, are of great interest due to their potential spintronic applications. Yet, serious challenges remain from the standpoint of theoretical and computational modelling, particularly with respect to the coupling of the spin and lattice degrees of freedom in ferromagnetic nanocontacts in emerging spintronic technologies. In this thesis, an extended method is developed, and applied for the first time, to model the interplay between magnetism and atomic structure in transition metal nanocontacts. The dynamic evolution of the model contacts emulates the experimental approaches used in scanning tunnelling microscopy and mechanically controllable break junctions, and is realised in this work by classical molecular dynamics and, for the first time, spin-lattice dynamics. The electronic structure of the model contacts is calculated via plane-wave and local-atomic orbital density functional theory, at the scalar- and vector-relativistic level of sophistication. The effects of scalar-relativistic and/or spin-orbit coupling on a number of emergent properties exhibited by transition metal nanocontacts, in experimental measurements of conductance, are elucidated by non-equilibrium Green’s Function quantum transport calculations. The impact of relativistic effects during contact formation in non-magnetic gold is quantified, and it is found that scalar-relativistic effects enhance the force of attraction between gold atoms much more than between between atoms which do not have significant relativistic effects, such as silver atoms. The role of non-collinear magnetism in the electronic transport of iron and nickel nanocontacts is clarified, and it is found that the most-likely conductance values reported for these metals, at first- and lastcontact, are determined by geometrical factors, such as the degree of covalent bonding in iron, and the preference of a certain crystallographic orientation in nickel.
Physics
Ph. D. (Physics)

The magnetocrystalline anisotropy energy (MAE) for fcc Ni and bcc Fe is calculated as the difference of single particle energy eigenvalue sums using a tight-binding model. For nickel we predict a MAE of -0.15 eV and the wrong easy axis, for iron we find a MAE of -0.7 eV with the easy axis in agreement with experiment. Our results compare favorably with previously reported first-principles calculations based on density functional theory and the local spin density approximation. The inclusion of an orbital polarization correction improves the magnitude of the MAE for iron, but fails to bring the result for nickel closer to the experimental value. The outstanding feature of our calculations is the careful handling of the necessary Brillouin zone integrals. Linear interpolation schemes and methods based on Fermi surface smearing were used and analyzed. An alternative method of calculating the MAE based on the torque on a magnetic moment centered on an atom is found to be equivalent to the calculation of the MAE in terms of energy differences.
Graduation date: 1999

A large amount of research has been done in which the magnetocrystalline anisotropy energy for fcc Ni and bcc Fe was calculated based on the electronic structure of these elements. Unfortunately; the results of these studies don't agree with each other and also differ from the experimental observation. In a previous thesis the effects of numerical errors in the Brillouin zone integrations were investigated. The results of that work explain why different calculations give different results, but do not explain the difference with experiment. The conclusion was that the underlying bandstructure, which was calculated using standard approximations, was not correct. The bandstructure of these elements will be different when improved prescriptions for the exchange-correlation energy are used. There is, however, no clear indication along which lines this approximation should be improved. Here we have taken a different approach to change the bandstructure. We suspected that some important interactions between different atomic orbitals are either ignored or miscounted. In this work, we examined the sensitivity of the energy on the interaction between those orbitals and studied in detail the consequences of changes in some interaction parameters which gave rise to a large energy change. The main result of this work is a better understanding of the relation between changes in the electronic structure in k-space and the resultant change in the magnetocrystalline anisotropy energy. In addition, this work takes another step in trying to find a better understanding how the magnetocrystalline anisotropy energy relates to interactions between neighboring atoms.
Graduation date: 2000

The permanent magnet candidate Fe8N is a metastable bct phase that has an enhanced magnetocrystalline anisotropy compared to its parent bcc phase Fe and a much discussed potential giant magnetic moment. In the first part of this work, the frame conditions to obtain Fe8N are established for thin films grown by molecular beam epitaxy (MBE). It is found that temperatures of approximately 373 K, in combination with MgO (100) or MgAl2O4 (100) substrates, are required to form the phase of interest. Either FexN or Fe4N, depending on the substrate utilized, form at higher nitrogen concentrations. For higher temperatures of 473 and 623 K, the thermodynamic gamma phase becomes more pronounced and occupies a broader stability window in terms of temperature and nitrogen content compared to equilibrium conditions. From these results, a low temperature thin film phase diagram is established. The optimization of growth parameters for MBE and sputter grown Fe8N films and the subsequent determination of their magnetic properties as a function of the degree of tetragonalization is used in order to clarify the results found in the literature. While no evidence of a giant magnetic moment beyond 2.5µB per iron atom could be found, the magnetocrystalline anisotropy was increased up to 1.18x10e5 J/m3 (1.18x10e6 erg/cm3) for MBE grown and 2.05x10e5 J/m3 (2.05x10e6 erg/cm3) for sputtered films as a function of the tetragonal distortion while the magnetic easy axis aligns parallel to the c-axis. The Curie temperature was extrapolated from M vs. T plots and is reduced to (770+-73) K compared to an Fe sample with TC = (1056+-85) K. By annealing samples in vacuum at 353 and 423 K, the decomposition temperature of the phase was investigated, revealing that the onset of decomposition lies significantly lower than the previously established 453 K. An attempt to increase the decomposition temperature by introducing Co into the phase was performed for (Fe100-xCox)8N with x = 6.4, 8.5, 12.7, 14.8, and 20. However, Co is found to inhibit the incorporation of nitrogen, leading to inhomogeneous samples with a wide distribution of c-axis lattice constants most likely due to a mix- ture of FeCo and Fe-N phases. Furthermore, boron instead of nitrogen interstitials are introduced into the Fe lattice as a potential method to increase the decomposition temperature. A deposition temperature of 573 K is required in order to obtain crystalline samples for the maximum boron content of 13 at.% in the films, determined by X-ray photoelectron spectroscopy. Qualitatively identical to the evolution observed for Fe8N, the c-axis lattice constants of the Fe-B samples increase as a function of boron content while the a-axis lattice constants shrink. Compared to the Fe-N case with a c=a ratio of 1.11, the maximum c/a observed for the Fe-B samples is 1.05. In addition, the magnetic easy axis remains in-plane while the absolute value of magnetic anisotropy increases to -5.1x10e5 J/m3 (-5.1x10e6 erg/cm3). Density functional theory confirms these results and shows that the local boron ordering is found to be the reason for the easy-plane configuration.

碩士
明志科技大學
材料工程研究所
101
Abstract The HCP CoxPt100-x films of 50 nm thick with Pt contents(x) of 81-92at.% are deposited on Ru(30 nm)/Ni(100 nm)/glass substrate at ambient temperature by magnetron co-sputtering. The effect of Co content on the microstructure and magnetic properties of CoxPt100-x films are investigated. Experimental results show that Single-layered CoxPt100-x film tended towards in-plane magnetic anisotropy. When Co content is 81at.% ,the in –plane coercivity and in-plane squareness are 4699 Oe and 0.71 ,respectively. The magnetic properties degrades when the films are post-annealed at 300℃ for 30 min. However ,when Ru(30 nm)/Ni(100 nm) underlayers is introduced ,the magnetic anisotropy of Co81Pt19 film is changed to random orientation. The CoPt /Ru/Ni/Glass film with perpendicular magnetic anisotropy is obtained as Co content is increased to 85at.%. Its perpendicular coercivity ,in-plane coercivity , perpendicular squareness and in-plane squareness are 2750 Oe ,1500 Oe ,0.6 and 0.3 ,respectively. High power impulse magnetron sputtering (HiPIMS) has proven to be capable of substantial improvement of the quality of deposited coatings. The effect of Pt content on the microstructure and magnetic properties of Fe100-xPtx films are investigated. Experimental results show that Single-layered FePt film with post-annealed at 300℃ for 30 min tended towards out of plane magnetic anisotropy. When Pt content is 49at.% ,the out of plane coercivity and squareness are 12 kOe and 0.92 ,respectively Keywords：. Fe100-xPtx films , Magnetic properties.

The magnetic and physical properties of oxide-free, ligand passivated, iron nanoparticles were studied using superconducting quantum interference device (SQUID) magnetometry and synchrotron based X-ray radiation. Particles used for this study ranged in diameter between 2 and 10 nm, which made it possible to distinguish between bulk and surface effects in the nanoparticles’ properties. Additionally, the effects of two different weakly interacting ligands (2,4-pentanedione and hexaethylene glycol monododecylether) on the nanoparticles’ behavior were studied. The results of this study were compared to theoretical predictions of magnetic transition metal behavior in both thin films and nanoparticles, as well as experimental results from measurements of transition metal clusters formed in an inert carrier gas and measured with a Stern-Gerlach magnet. Magnetometry revealed that the iron nanoparticles have a magnetocrystalline anisotropy an order of magnitude greater than bulk iron. At the same time, these particles exhibit a saturation mass magnetization up to 209 Am2/kg, which is only slightly lower than bulk iron. The structural properties of these particles were characterized using high energy X-ray diffraction analyzed using the atomic pair distribution function method (PDF). The PDF analysis indicates that the Fe particles have a distorted and expanded form of the bcc lattice, which could, at least in part, explain the magnetocrystalline anisotropy of these nanoparticles. X-ray absorption fine structure (XAFS) was used to study the surface properties of the iron nanoparticles and further characterize their structural properties. XAFS showed that oxidized species of iron exist at the nanoparticles’ surface and can be attributed to iron/ligand interactions. The percentage of oxidized species scales with the surface to volume ratio of the nanoparticles, and therefore appears limited to the nanoparticle surface. The layer of Fe(II) species present at the nanoparticles’ surface accounts for the reduction in saturation mass magnetization values (when compared to bulk iron) observed in these particles. XAFS analysis also provided further confirmation of the nanoparticles’ expanded crystalline lattice.
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碩士
國立中正大學
物理系研究所
105
The power computational resources, together with recent improvements in electronic structure calculation algorithms, are providing a dispensable tool for researchers in the fields of condensed materials physics. Magnetic materials with large perpendicular magnetic anisotropy demonstrate high potential for next-generation high-density non-volatile memories and magnetic recording with high thermal stability. Experiments confirmed that Ta/FeCoB/MgO/FeCoB/Ta perpendicular magnetic tunnel junctions exhibit they have high magnetocrystalline anisotropy energy (MAE). The origin of the high MAEs of FeCo/Pd、Fe/Pt、Fe/Ag and Co/Pd multilayers has been widely investigated. In this project, we will perform first-principles density functional calculations to study the MAE in metal/Fe layer structures. The substrate materials are Ag, Au, and Pd. We will also study the effect of substrate layer and field on MAE. In the framework of first- and second-order perturbation, we can decompose the MAE into k- and atomic-resolved MAE contributions. This will provide a better understanding of the physical origin of the effect of field and substrate layers on MAE of these materials.

In this thesis the influence of different growth conditions on the structural and the magnetic properties of magnetite were analyzed. Therefore, ultrathin Fe3O4 films were grown on MgO(001) substrates, on NiO, and on Fe pre-covered MgO(001) substrates. In the first part of this thesis magnetite films with different film thicknesses were deposited directly on MgO by RMBE to investigate the thickness dependence of the anomalous strain and the in-plane magnetic anisotropy. Surface sensitive methods like XPS and LEED have shown that all films in the investigated thickness range are stoichiometric and epitactic magnetite. Bulk sensitive XRD experiments at the specular rod point to well-ordered films with homogenous film thickness indicated by the distinct Laue oscillations. However, the vertical layer distances are smaller than expected even for strained magnetite. Raman measurements were carried out to clarify this contradiction between surface sensitive and bulk sensitive measurements. While the 20 nm and 30 nm films exhibit the typical bands for magnetite, no distinct bands can be observed for the 7.6 nm film. Due to this results we first assume a partial formation of a thin maghemite layer on top of the uncapped magnetite film under ambient conditions. Nevertheless, additional XPS measurement exclude the complete oxidation of magnetite to maghemite since there is no significantly increased Fe3+-signal visible. Thus, the low vertical layer distance can be attributed to the presence of APBs causing an anomalous strain relaxation as reported in literature. Although all films feature ferromagnetic behavior there are differences in the characteristic of the magnetic in-plane anisotropy. The 7.6 nm film has an in-plane magnetic isotropy while the 20 nm and 30 nm film have an in-plane fourfold magnetic anisotropy. Here, the fourfold magnetic anisotropy is stronger for the 20 nm magnetite film than for the 30 nm film. The critical film thickness for the transition from magnetic isotropy to magnetic fourfold anisotropy may be influenced by film thickness and lattice strain induced by the substrate. The second part of this thesis features the thickness dependence of the structural quality of Fe3O4/NiO bilayers. Each film of the Fe3O4/NiO bilayer on MgO(001) have been successfully grown by RMBE. LEED and XPS experiments have proven that the surface near regions of the distinct films have high structural and stoichiometric properties. Here, too, the detailed ’bulk’ structural characterization of Fe3O4/NiO bilayers were carried out using XRD. It was shown that the Fe3O4 films grow homogeneously and smoothly on NiO films if the NiO film thickness is below 24 nm. Above this NiO film thickness the structural quality of the magnetite films gets distinctly worse. This behavior can be attributed to the fact that the interface roughness between NiO and Fe3O4 depends on the NiO film thickness. The roughness of the 3 nm NiO film is rather small and it is rising obviously with increasing NiO film thickness. Thus, the structural quality of the magnetite films grown on 30 nm NiO films is constantly reduced with increasing magnetite film thickness since the quality of the Fe3O4 films is influenced by the quality of the Fe3O4/NiO interface. In the third study the influence of initial iron buffer layers on the magnetic properties of magnetite grown on MgO(001) substrates has been investigated. In situ XPS and LEED indicate that the structural and stoichiometric properties of the surface near region of the magnetite films are not influenced by the iron buffer layer. However, the structural and magnetic properties of the whole film have changed compared to magnetite grown directly on MgO as shown by XRD and MOKE. The crystalline quality is poor and the magnetic easy axis of the magnetic in-plane anisotropy is rotated by 45◦ compared to magnetite films grown directly on MgO. Both crystalline quality and the rotation of the magnetic in-plane anisotropy show no dependence on the film thickness. However, the strength of the magnetocrystalline anisotropy decreases with increasing film thickness. XPS and XRD measurements have indicated that the iron buffer layer is completely oxidized during the second growth stage of the magnetite. The small Kerr rotation in the MOKE experiments of the samples with film thicknesses up to 28 nm confirms this result since a remaining iron film would cause a higher Kerr rotation. In the last part of this thesis the structural and magnetic properties of a partially oxidized, a completely oxidized and a metallic iron film are analyzed. The partially oxidized iron film is a bilayer with a metallic iron film and an iron oxide film. The surface near stoichiometry of both oxidized iron films correspond to magnetite as proven by XPS. The structural analysis by XRD reveals that though these magnetite films are crystalline, they have an inhomogeneous thickness. The magnetization curves of the partially oxidized film (bilayer) measured by MOKE exhibit a magnetic saturation which is comparable to the magnetic saturation of the single metallic iron film. However, the coercive field is higher compared to the single metallic iron film due to the high interface roughnesses. Considering the coercive field as a function of the sample rotation α the bilayer exhibits a fourfold anisotropy with eight maxima. However, the angular dependence of the magnetic remanence features a simple fourfold anisotropy with easy axes in h110i directions of MgO(001). Vector MOKE analysis displays that the magnetic reversal processes of the bilayer are similar to single metallic iron films. Thus, the bilayer exhibits mostly the magnetic properties of a single iron film. The bilayer has the same magnetic easy axis and a similar magnetic saturation. The magnetic easy axis of the magnetic in-plane anisotropy of the completely oxidized iron film is rotated by 45◦ compared to magnetite films grown directly on MgO as already observed in Ref. [148]. The completely oxidized iron film exhibits also a significantly increased coercive field due to high surface roughness. A magneto-dynamic investigation of the exchange-coupling of the Fe3O4/Fe bilayer system was carried out to yield all relevant magnetic parameters, such as anisotropies, as well as the coupling constant J1. Here, also a complex fourfold anisotropy was observed, which might be due to a coupling of a perpendicular spin-wave mode in the magnetite layer with the acoustical coupling mode. We have also successfully calculated the angular dependence of the resonance field of all films using the eighth order of the magnetocrystalline anisotropy. Although we have applied many measurement methods, we have found no explanation for the complex fourfold angular dependence of the coercive field of the bilayer. Nevertheless, this bilayer provides interesting properties for application in MTJs due to its enhanced magnetic properties like complex fourfold magnetic anisotropy and higher coercive field and remanence. All in all, we have shown that the structural and magnetic properties of magnetite films are strongly influenced by interlayers between film and substrate. While magnetite films directly deposited on MgO exhibit a homogeneous film thickness, both NiO interlayers with a thickness above 24 nm and initially grown iron films deteriorate the structural quality of the on top grown magnetite films. In addition, the magnetic fourfold anisotropy is rotated by 45◦ in comparison to magnetite films grown directly on MgO for the structurally disturbed magnetite on iron pre-covered MgO substrates. Here, further investigations are necessary to understand why this anisotropy rotation occurs and how we can improve the structural quality of magnetite on iron and NiO. The growth of magnetite by oxidizing previously deposited iron films leads to crystalline magnetite films but with inhomogeneous film thicknesses. In case of a Fe3O4/Fe bilayer, the magnetic properties are drastically changed due to the magnetic coupling between the iron and the magnetite film.

碩士
國立中正大學
物理系研究所
106
The power computational resources, together with recent improvements in electronic structure calculation algorithms, are providing a dispensable tool for researchers in the fields of condensed materials physics. Magnetic materials with large perpendicular magnetic anisotropy demonstrate high potential for next-generation high-density non-volatile memories and magnetic recording with high thermal stability. Experiments confirmed that Ta/FeCoB/MgO/FeCoB/Ta perpendicular magnetic tunnel junctions exhibit they have high magnetocrystalline anisotropy energy (MAE). The origin of the high MAEs of FeCo/Pd、Fe/Pt、Fe/Ag and Co/Pd multilayers has been widely investigated. We will also consider the effect of QWs on MAE. In this thesis, we will perform first-principles density functional calculations to study the MAE in metal/Fe layer structures. The substrate materials are Ag and Au. We will study the effect of relaxation of structure on MAE. Finally, the Quantum confinement effect on the MAE of the system will be presented.

Mit Hilfe der Dichtefunktionaltheorie wurden magnetische Eigenschaften an 3d Elementen und Legierungen und 5f Verbindungen untersucht. Dabei wurde auf die Wichtigkeit von Orbitalpolarisationskorrekturen eingegangen und diese näher erörtert. Im ersten Anwendungsteil wurden magnetische Momente und die Magnetokristalline Anisotropie Energie an 3d Elementen untersucht. Des Weiteren wurden FeCo Legierungen als mögliche Bestandteile in der Festplattenindustrie diskutiert. Im letzten Abschnitt wurden Uranverbindungen in Hinsicht auf Ihre Orbitalpolarisation untersucht.:1 Introduction 1 2 Theoretical Considerations 5 2.1 Quantum Mechanics Applied to Solids . . . . . . . . . . . . . . . 6 2.2 Density Functional Theory . . . . . . . . . . . . . . . . . . . . . 7 2.2.1 non-Relativistic DFT . . . . . . . . . . . . . . . . . . . . 7 2.2.1.1 Hohenberg and Kohn . . . . . . . . . . . . . . . 7 2.2.1.2 Kohn-Sham Equations . . . . . . . . . . . . . . 10 2.2.1.3 Local Density Approximation and More . . . . 12 2.2.2 Relativistic DFT . . . . . . . . . . . . . . . . . . . . . . . 13 2.3 FPLO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 2.4 Magneto-Crystalline Anisotropy Energy . . . . . . . . . . . . . . 18 2.5 Disorder within DFT . . . . . . . . . . . . . . . . . . . . . . . . 20 3 Orbital Polarisation in DFT 23 3.1 Hund’s Rules in DFT . . . . . . . . . . . . . . . . . . . . . . . . 24 3.2 An Introduction to OPC and DFT . . . . . . . . . . . . . . . . . 25 3.2.1 OPC Brooks . . . . . . . . . . . . . . . . . . . . . . . . . 25 3.2.2 OPC Eschrig . . . . . . . . . . . . . . . . . . . . . . . . . 26 4 Transition Metals 39 4.1 Fe, Co, and Ni . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 4.1.1 Calculational Details . . . . . . . . . . . . . . . . . . . . 40 4.1.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 4.1.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 4.2 Fe1−xCox . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 4.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 50 4.2.2 Fixed Spin Moment Calculations . . . . . . . . . . . . . . 50 4.2.3 Epitaxial Bain Path . . . . . . . . . . . . . . . . . . . . . 51 4.2.4 Calculational Details . . . . . . . . . . . . . . . . . . . . 54 4.2.5 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 4.2.6 LSDA vs. GGA . . . . . . . . . . . . . . . . . . . . . . . 69 4.2.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 5 Uranium Compounds 75 5.1 UX, with X = (N, P, As, Sb, O, S, Se, and Te) . . . . . . . . 77 5.1.1 UN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 5.1.2 UX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 5.1.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 5.2 UM2, with M = (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) 90 5.2.1 Calculational Details . . . . . . . . . . . . . . . . . . . . 90 5.2.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 5.2.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 5.3 UAsSe, USb2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 5.3.1 Calculational Details . . . . . . . . . . . . . . . . . . . . 97 5.3.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 5.3.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 6 Summary and Outlook 101 A Definitions i A.1 Spherical Harmonics . . . . . . . . . . . . . . . . . . . . . . . . . i A.2 Other Definitions Used in Text . . . . . . . . . . . . . . . . . . . ii B Input Parameters for the Racah Parameter iii B.1 d-Shells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii B.2 f-Shells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v Biblography vi Acknowledgement xxiv Versicherung xxvii

Title: Ground state investigations of Ce and U intermetallic compounds Author: Attila Bartha Department: Department of Condensed Matter Physics Supervisor: RNDr. Jan Prokleška, Ph.D., Department of Condensed Matter Physics Abstract: Rare earth and actinide intermetallic compounds offer a plethora of interesting physical properties due to the varied behavior of f -electrons together with numerous interactions these electrons are exposed to. In this thesis we address a broad spectrum of ground state investigations on CePd2X3 (X=Zn, Ga) and (Ce,U)nTIn3n+2 (T=Rh, Ir) compounds. Single crystals of CePd2Zn3 and CePd2Ga3 compounds were synthesized for the first time using Bridgman method. CePd2Ga3 revealed a ferromagnetic transition with TC = 6.7 K with a strong magnetocrystalline anisotropy. CePd2Zn3 orders antiferromagnetically below TN = 1.9 K. Results of magnetization measurements on Ce2IrIn8 revealed effective magnetic moment µeff = 2.45µB/Ce3+ and a paramagnetic Curie temperature θP = −31 K. Decomposition of Hall resistivity ρxy(B) into NHE and AHE revealed a predom- inance of AHE in the temperature range from 60 K up to 100 K. Ce2RhIn8 was studied by means of magnetic field and angle dependent magnetization and heat capacity measurements. The resulting phase diagram reveals a complete unfold- ing and...

Magnetic nanoparticles have received extensive attention in the biomedical research, e.g. as prospective contrast agents for T2-weighted magnetic resonance imaging. The ability of a contrast agent to enhance the relaxation rate of 1 H in its vicinity is quantified by relaxivity. The main aim of this thesis is to evaluate the transversal re- laxivity of ε-Fe2−x Alx O3 nanoparticles coated with amorphous silica or citrate - its dependence on external magnetic field, temperature and thickness of silica coating - by means of nuclear magnetic resonance. The aluminium content x = 0.23(1) was determined from XRF, the material was further characterised by XRPD, Möss- bauer spectroscopy, DLS, TEM and magnetic measurements. The size of magnetic cores was ∼ 21 nm, the thickness of silica coating ∼ 6,10,17 and 21 nm. Magne- tization of the ε-Fe2−x Alx O3 nanoparticles increased by ∼ 30 % when compared to ε-Fe2O3. The saturating dependence of relaxivity on external magnetic field and on the linear decrease with increase of thickness of silica coating contravene the theo- retical model of motional averaging regime (MAR); nevertheless, the temperature dependence acquired in 0.47 T and 11.75 T may be explained by MAR. In compari- son to ε-Fe2O3 nanoparticles, the relaxivity of examined samples was higher for par-...

Theoretische Berechnungen sagen für tetragonal gedehntes Fe-Co eine hohe magnetokristalline Anisotropie voraus, wie sie für seltenerdfreie Dauermagnetwerkstoffe vorteilhaft wäre. In dieser experimentellen Arbeit werden epitaktische Fe-Co-Schichten strukturell und magnetisch charakterisiert. Zur Untersuchung der Dehnung in diesen Schichten eignen sich AuxCu100-x-Pufferschichten besonders, da über die Stöchiometrie (x) deren lateraler Gitterparameter eingestellt werden kann. Wird Fe-Co auf einer solchen Pufferschicht abgeschieden, erfolgt aufgrund dessen hoher elastischer Energie schon in den ersten Monolagen eine vollständige Relaxation der pufferinduzierten Dehnung. In ternären Fe-Co-X-Schichten, in denen kleine X-Atome (X = B, C oder N) Oktaederlücken besetzen, wird jedoch eine spontane tetragonale Dehnung c/a bis zu 1,05 beobachtet. Entlang der gedehnten c-Achse tritt eine uniaxiale magnetokristalline Anisotropie auf, die für B- oder C-Zulegierungen von 2 at% eine maximale Anisotropiekonstante von 0,4 MJ/m³ zeigt. Wird der X-Gehalt weiter erhöht, nehmen die Kristallinität der Schichten und die magnetische Anisotropie ab. Neben der magnetokristallinen Anisotropie des Schichtvolumens wird an den Fe-Co(-X)-Schichten eine hohe Grenzflächenanisotropie beobachtet. Der Beitrag der freien Oberfläche übersteigt den der Au-Cu-Grenzfläche dabei deutlich.
Theoretical calculations predict a high magnetocrystalline anisotropy for tetragonally strained Fe-Co, which would be beneficial for rare-earth free permanent magnet materials. In this experimental work, epitaxial Fe-Co films are investigated structurally and magnetically. AuxCu100-x buffer layers are very suitable to study the strain in these films since their in-plane lattice parameter can be tailored via the applied stoichiometry (x). However, when Fe-Co is deposited on such a buffer layer, the induced strain of the Fe-Co lattice relaxes completely within the first monolayers, due to its high elastic energy. In ternary Fe-Co-X films, where small atoms X like B, C or N occupy octahedral interstitial sites, a spontaneous strain c/a up to 1.05 is observed. A uniaxial magnetocrystalline anisotropy along the strained c axis appears. Their maximum anisotropy constant is 0.4 MJ/m³ for B or C contents of 2 at%. If the X content is further increased, the crystallinity and thus, the magnetic anisotropy of the films degrade. Together with the magnetocrystalline anisotropy of the films’ volumes, a high interface anisotropy is observed for the Fe-Co(-X) films. The contribution of the free surface clearly exceeds the contribution of the Au-Cu interface.

Die vorliegende Arbeit präsentiert die ersten detaillierten Untersuchungen des Spin-Reorientierungs-Übergangs in epitaktischen NdCo5-Schichten. Die Proben, die mit gepulster Laserdeposition hergestellt wurden, konnten sowohl als in-plane- als auch als out-of-plane-texturierte Schichten präpariert werden. Für beide Wachstumsvarianten ergaben Röntgendiffraktometrie- und Texturmessungen eine sehr gute Texturierung mit einer nahezu einheitlichen Orientierung der c-Achse, die eine Untersuchung der magnetischen Eigenschaften entlang ausgewählter kristallografischer Richtungen ermöglichte. Die globalen Magnetisierungsmessungen der In-plane-Proben zeigten einen Spin-Reorientierungs-Übergang von einer magnetisch leichten c-Achse für Temperaturen oberhalb von 310 K über einen magnetisch leichten Kegel hin zu einer magnetisch leichten Ebene (a-Achse) unterhalb von 255 K. Die Übergangstemperaturen liegen damit geringfügig über den bisher an Massivproben gemessenen Werten. Aus den magnetischen Hysteresemessungen wurden die magnetokristallinen Anisotropiekonstanten erster und zweiter Ordnung für den Temperaturbereich der magnetisch leichten c-Achse und der magnetisch leichten Ebene ermittelt. Die Untersuchungen der Out-of-plane-Proben wiesen die Existenz einer magnetokristallinen Anisotropie höherer als zweiter Ordnung nach. Sie bewirkt ein unterschiedliches Schaltverhalten der parallel zur a- bzw. b-Achse gemessenen magnetischen Hysteresekurven im Temperaturregime der magnetisch leichten Ebene. Für die in-plane-texturierten Schichten wurde das Domänenmuster und dessen Änderung mit der Temperatur im gesamten Spin-Reorientierungs-Bereich analysiert. Diese Untersuchungen basieren auf in Kooperation mit der Universität Hamburg durchgeführten SEMPA-Messungen. Oberhalb von 318 K liegt eine Zweidomänenkonfiguration mit einer Ausrichtung der Magnetisierung parallel zur c-Achse vor, die beim Abkühlen in das Regime des magnetisch leichten Kegels in einen Vierdomänenzustand übergeht. Unterhalb von 252 K bildet sich eine Zweidomänenkonfiguration mit parallel zur a-Achse orientierter Magnetisierung. Diese lokalen Messungen bestätigten den Spin-Reorientierungs-Übergang mit zu den globalen Magnetisierungsmessungen vergleichbaren Übergangstemperaturen. Für charakteristisch orientierte Domänenwände erfolgten genauere Analysen der Magnetisierungsprozesse in den angrenzenden Domänen. Um ein erweitertes Verständnis der Domänenkonfiguration, deren Temperaturabhängigkeit und der vorhandenen Domänenwände zu erarbeiten, erfolgten mikromagnetische Simulationsrechnungen für ausgewählte Temperaturen. Die Berechnungen wurden sowohl für homogene Systeme als auch für Geometrien mit verschiedenen Pinningzentren durchgeführt. Die Analyse der Domänenwände ergab, dass ihr Bloch- oder Néel-Charakter und die Domänenwandweite von der Temperatur sowie ihrer Ausrichtung parallel zur c- oder a-Achse abhängt.
This thesis presents the first detailed investigation of the spin-reorientation-transition in epitaxial NdCo5 thin films. The samples were prepared by pulsed laser deposition as in-plane or out-of-plane textured films. For both kinds of samples X-ray diffraction and texture measurements revealed a high degree of texture with one common orientation of the c-axis within the film, which allowed an investigation of the magnetic properties along distinct crystallographic directions. Global magnetization measurements of the in-plane textured films showed a spin-reorientation from a magnetic easy axis (c-axis) at temperatures above 310 K via a magnetic easy cone to a magnetic easy plane (a-axis) at temperatures below 255 K. The transition temperatures are slightly higher than values reported for bulk samples. The magnetocrystalline anisotropy constants of first and second order were determined for the regime of the magnetic easy axis and plane. Measurements of the out-of-plane textured films verified the existence of a magnetocrystalline anisotropy of order larger than two, which becomes obvious from a different magnetic switching behavior along the a- and b-axis in the temperature regime of the magnetic easy plane. The domain structure and its changes with temperature were investigated for the in-plane textured films. There exists a two domain state at temperatures above 318 K with an orientation of the magnetization parallel to the c-axis from which a four domain state evolves when cooling down the sample to the easy cone state. Finally, a two domain state exists in the regime of the magnetic easy plane (easy a-axis) with an orientation of the magnetization parallel to the a-axis at temperatures below 252 K. The local measurements confirm the spin-reorientation-transition with transition temperatures comparable to those derived from global magnetization measurements. In addition, a detailed analysis of the magnetization processes for some characteristically oriented domain walls was performed. Micromagnetic simulations were carried out for selected temperatures to achieve a deeper understanding of the temperature dependence of the domain configuration and of the domain walls. The simulations considered homogeneous systems as well as systems with pinning centers. An analysis of the domain walls showed that their character and width depend on temperature and the orientation parallel to the a- or c-axis.