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This thesis focus on the magnetic behavior, from single atoms to bulk materials. The materials considered in this thesis have been studied by computational methods based on ab initio theory, density functional theory (DFT), including treatment of the spin-orbit coupling, non-collinear magnetism, and methods capable of treating discorded systems. Furthermore strongly correlated materials have been investigated using the dynamical mean field theory (DMFT). The uniaxial magnetic anisotropy energy (MAE) of the Fe2P was investigated using the full-potential linear muffin tin orbital (FP-LMTO) method. Based on a band structure analysis, the microscopical origin of the large magnetic anisotropy found for this system is explained. It is also shown that by straining the crystal structure, the MAE can be enhanced further. This opens up for the possibility of obtaining a room temperature permanent magnet based on the Fe2P. The spectral properties of Fe impurities in a Cs host have been investigated, for both surface and bulk systems, by means of combination of density-functional theory in the local density approximation and the dynamical mean-field theory (LDA+DMFT), using two different impurity solvers, the Hubbard I approximation (HIA) and the Exact Diagonalization (ED) method were used. It is shown that noticeable differences can be seen in the unoccupied part of the spectrum for different positions of Fe atoms inside the host. The calculations show good agreement with the experimental photoemission spectra. The stability of the 12-fold metal-phosphorous coordination, existing in the meteorite mineral melliniite has been investigated trough total energy calculations using the coherent potential approximation (CPA) combined with an analysis of the chemical bonds, performed by balanced crystal overlap population (BCOOP). It was shown that its uniquely high metal–phosphorous coordination is due to a balance between covalent Fe–P binding, configurational entropy and a weaker nickel–phosphorus binding. Supported clusters have drawn a lot of attention as possible building blocks for future data storage applications. This topic was investigated using a real space noncollinear formalism where the exchange interactions between Co atoms were shown to be tuned by varying the substrate surface composition. Furthermore the spin dynamics of small Co clusters an a Cu(111) surface have been investigated and a new kind of dynamics, where magnetization switching can be accelerated by decreasing the switching field, has been found. A method for calculating the electronic structure for both ordered and disordered alloys, the augmented space recursion (ASR) method, have been extended to treat non-collinear magnetic order. The method has been used to investigate the energy stability of non-collinear arrangements of MnPt and Mn3Rh alloys.

This thesis presents a study of magnetic anisotropies of transition metal based thin films investigated by magnetooptic Kerr effect (MOKE) and x-ray radiation techniques. Thin Co films of different thickness deposited on glass are investigated to study the uniaxial magnetic anisotropy (UMA) of these films. The direction of the UMA is determined from the azimuthal dependence of the magnetic remanence and differs with increasing thickness of the Co film investigated by x-ray reflectivity (XRR). Strain in the substrate and possible textures in the film structure can be excluded as the origin of the magnetic behavior as studied by grazing incidence wide angle x-ray scattering (GIWAXS). Furthermore, a new combination of different vectorial magnetometry techniques using MOKE is developed (vector MOKE). The processing of the experimental data contains the separation of linear and quadratic parts of the magnetization curves and determination of all three components of the magnetization vector in units of Kerr rotation. The complete reversal process can be reconstructed and interpreted by monodomain states and domain splitting. The measurement procedure and the processing of the data is demonstrated for ultra-thin epitaxial Fe films on MgO(001), which are structurally characerized by XRR and (grazing incidence) x-ray diffraction (GI(XRD)). Magnetic properties of thin Fe films (of different thickness) and A2-type ordered Co50Fe50 films (prepared with different annealing temperatures) on MgO(001) are studied by this new combined vector MOKE technique. Here, a two-domain switching process can be distinguished from a four-domain switching process generated by the incoherent rotation between magnetization saturation and magnetic remanence. Furthermore, UMA constants and domain wall pinning energies are determined using the Stoner-Wohlfarth model. For the Fe films the domain wall pinning energies obtained from these analyses decrease with increasing lateral crystallite sizes determined by GIXRD. For the Co50Fe50 films the domain wall pinning energies decrease with the annealing temperature due to fewer defects in the film. The magnetic structure of a thin B2 ordered Co50Fe50 film reveal two in-plane magnetic easy axes of different strengths which are not orthogonal. This atypical magnetic structure can be explained by a cubic magnetic anisotropy (CMA) induced by the crystalline film structure superimposed by an additional UMA which is not parallel to one of the magnetic easy axes of the CMA.

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Les systèmes magnétiques flexibles et étirables sont d’intérêt croissant pour le développement de dispositifs électromagnétiques conformables aux surfaces non planes. Dans ce contexte, l’objectif de cette thèse est d’identifier des relations entre phénomènes mécaniques irréversibles (fissuration et délamination) aux grandes déformations et propriétés magnétiques de films minces d’épaisseurs nanométriques (alliages de CoFeB et NiFe) élaborés sur substrat polymère (Kapton®) par pulvérisation magnétron. Dans un premier temps, des essais de traction couplés in situ à la microscopie à forces atomiques ou des mesures de résistance électrique ont permis d’étudier la multifissuration des films minces et les décollements localisés (cloques) subséquents. Ces mesures ont permis de mettre en évidence des domaines de déformations macroscopiques pour lesquels chaque mécanisme était prépondérant. De plus l’énergie d’adhésion a pu être discutée et estimée à partir du suivi des cloques sous déformation. Dans un second temps, des mesures par résonance ferromagnétique effectuées ex situ ont permis de relier les domaines des déformations identifiés précédemment aux évolutions de l’anisotropie magnétique des films minces. De plus, une corrélation entre hétérogénéités de déformations et coefficient d’amortissement de Gilbert a été trouvée. Enfin, les propriétés magnétomécaniques de réseaux de nanolignes de NiFe ont permis de mettre en évidence l’intérêt de la nanostructuration pour ces systèmes magnétiques déformables. / Flexible and stretchable magnetic systems are of increasing interest for the development of electromagnetic devices conformable to non-planar surfaces. In this context, the objective of this thesis is to identify the relationships between irreversible mechanical phenomena (cracking and delamination) at large strains and magnetic properties of thin films of nanometric thicknesses (CoFeB and NiFe alloys) deposited on polymer substrate (Kapton®) by magnetron sputtering. In a first time, tensile tests coupled in situ with atomic force microscopy or electrical resistance measurements were used to study thin film multifissuration and subsequent localized debonding (buckles). These measurements made it possible to highlight areas of macroscopic strains for which each mechanism was predominant. In addition, the adhesion energy could be discussed and estimated from the monitoring of the buckles under applied strain. In a second step, ferromagnetic resonance measurements carried out ex situ made it possible to link the previously identified areas of strains to the evolutions of the magnetic anisotropy of thin films. In addition, a correlation between strain heterogeneities and Gilbert damping coefficient was found. Finally, the magnetomechanical properties of NiFe nanowires arrays have made it possible to highlight the advantage of nanopatterning for these deformable magnetic systems.

Researches in spintronics, especially those remarkably classified in the current induced spin-transfer torque (STT) framework, circumvent challenges with different materials and geometries. Perpendicular magnetic anisotropy (PMA) materials are showing capability of holding promise to be employed in STT based spintronics elements, e.g. spin-torque oscillators (STOs), STT-magnetoresistive random access memories (STT-MRAMs) and current induced domain wall motion elements. This dissertation presents experimental investigations into developing sputter deposited Co/Ni multilayers (MLs) with PMA and employs these materials in nano-contact STOs (NC-STOs) based on giant magnetoresistance (GMR) effect and in pseudo-spin-valve (PSV) structures. The magnetostatic stray field coupling plays an important role in perpendicular PSVs. The temperature dependent coupling mechanism recommends that this coupling can be tailored, by i) the saturation magnetization and coercivity of the individual layers, ii) the coercivity difference in layers, and iii) the GMR spacer thickness, to get a well decoupled and distinguishable switching response. Moreover, this thesis focused on the implementation and detailed characterization of NC-STOs with strong PMA Co/Ni ML free layers and in-plane Co reference layers as orthogonal (Ortho) magnetic geometry in so-called Ortho-NC-STOs. The primary target of reaching record high STO frequencies, 12 GHz, at close to zero field, 0.02 Tesla, was achieved. However, in large external fields, &gt;0.4 Tesla, an entirely new magnetodynamic object, a “magnetic droplet”, theoretically predicted in 1977, was discovered experimentally. Detailed experiments, combined with micromagnetic simulations, demonstrate the formation of a magnetic droplet with a partially reversed magnetization direction underneath the NC, and a zone of large amplitude precession in a region bounding the reversed magnetization. The magnetic droplet exhibits a very rich dynamics, including i) auto-modulation as a combine of droplet frequency with a slow time evolution (few GHz) of un-centering the droplet mode under the NC, ii) droplet breathing as reversible deformation of droplet mode with ½ droplet frequency. All observation of droplet opens a new mechanism of excitation for future fundamental studies as well as experiments especially for domain wall electronics and nano-scopic magnetism. / <p>QC 20121119</p>

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We present a study on perpendicular magnetic tunnel junctions with W as buffer and capping layers. A tunneling magnetoresistance of 138% and an interfacial magnetic anisotropy of 1.67 erg/cm(2) were obtained in optimally annealed samples. However, after extended annealing at 420 degrees C, junctions with W layers showed extremely small resistance due to interdiffusion of W into the MgO barrier. In contrast, in Ta-based junctions, the MgO barrier remained structurally stable despite disappearance of magnetoresistance after extended annealing due to loss of perpendicular magnetic anisotropy. Compared with conventional tunnel junctions with in-plane magnetic anisotropy, the evolution of tunneling conductance suggests that the relatively low magnetoresistance in perpendicular tunnel junctions is related to the lack of highly polarized Delta(1) conducting channel developed in the initial stage of annealing. Published by AIP Publishing.

The temperature and field dependence of the magnetization of epitaxial, undoped anatase TiO2 thin films on SrTiO3 substrates was investigated. Low-energy ion irradiation was used to modify the surface of the films within a few nanometers, yet with high enough energy to produce oxygen and titanium vacancies. The as-prepared thin film shows ferromagnetism which increases after irradiation with low-energy ions. An optimal and clear magnetic anisotropy was observed after the first irradiation, opposite to the expected form anisotropy. Taking into account the experimental parameters, titanium vacancies as di-Frenkel pairs appear to be responsible for the enhanced ferromagnetism and the strong anisotropy observed in our films. The magnetic impurities concentrations was measured by particle-induced X-ray emission with ppm resolution. They are ruled out as a source of the observed ferromagnetism before and after irradiation.

Filmes de ligas FePt com uma estrutura quimicamente ordenada Tetragonal de Face Centrada FCT (tipo L1 IND.0) e textura [001] apresentam anisotropia magnética perpendicular, alta coercividade e grande efeito Kerr polar magnetoóptico. Entretanto, filmes completamente ordenados são normalmente obtidos através de um processo de \"sputtering\" com deposições em altas temperaturas do substrato(T IND. S > OU =600 ºC) ou por pós-tratamento térmico em temperaturas maiores que 500 ºC. As pesquisas atualmente em desenvolvimento visam a obtenção dessa estrutura ordenada a temperaturas mais baixas, o que seria mais adequado para aplicações práticas dos filmes. Neste trabalho, procuramos as melhores condições para a preparação de filmes de FePt com a fase ordenada FCT(001) em reduzidas temperaturas. Investigamos a influência do método de deposição, temperatura do substrato, espessura do filme e tipo de substrato sobre o grau de ordem química e textura dos filmes de FePt e, consequentemente, sobre suas propriedades magnéticas. Os filmes finos de ligas Fe IND.xPt IND. x-1(com x = 50 at%) foram preparados por \"Sputtering Magnetron DC\", através de dois diferentes métodos de deposição: pelo usual de codeposição e pelo método de deposiçãO alternada monocamadas atômicas (DAM). Os filmes foram crescidos sobre substratos de SiO IND.2/ Si(100), Si(100) e MgO(100), com e sem uma camada \"buffer\" de Pt ( com espessuras de 50 1 766 nm), sob T IND. s variando de temperatura ambiente a 600 ºC. O \"buffer\" de Pt foi usado para permitir melhores condições para a indução de um crescimento \"epitaxial\" da ordenada fase FCT(001) nos filmes de FePt. Os resultados da caracterização estrutural e magnética realizados através das técnicas de Difração de RAios-X (XRD) e Magnetometria de Amostra Vibrante (VSM) mostram que os substratos de MgO São adequados para induzir um crescimento \"epitaxial\" de filmes de FePt ) com uma estrutura FCT e textura [001]. Entretanto, com o uso de um \"buffer\" de Pt otimizado (com predominante fase FCC(100)), pré-depositado sobre MgO, foram obtifos melhores resultados em termos de \"epitaxia\" conveniente para o ordenamento da estrutura cristalina, para os filmes de FePt preparados por ambos os métodos de deposição. Os resultados mostram também que o efeito de um crescimento \"epitaxial\" induzido varia com a espessura do filme. Em relação ao método de deposição, os resultados revelam a maior eficiência de método DAM em comparação com o métodos de codeposição para a preparação de filmes de FePt com a fase FCT(001), em temperaturas reduzidas (T IND.s < OU = 400ºC). Com o uso do método DAM, foi possível a obtenção da fase ordenanda FCT(001) em filmes de FePt crescidos diretamente sobre o MgO em T IND. s = 400ºC. Para filmes de FePt crescidos sobre Pt/MgO, a formação da fase FCT(001) foi verificada em T IND.s = 200ºC e um alto grau de ordenamento químico (S = 0,88) em T IND. s = 400ºC. Estudos de Espectroscopia de Absorção de Rios-X (XAS) também foram realizados. / FePt alloy films, with a chemically ordered face-centered-tetragonal FCT (type LI IND.0) structure and [001] texture, present perpendicular magnetic anisotropy, high coactivity and large polar magneto-optical Kerr effect. However, completely ordered films are usually obtained either by sputtering process at high substrate temperatures (T IND.s MAIOR IGUAL A 600°C) or by postanneling treatment at temperatures higher than 500°C. For technological purposes the preparation temperatures has to be as low as possible. In this work we have searched the more adequate conditions to preparation of FePt films with ordered FCT(001) phase at reduced temperatures. We have investigated the influence of deposition methods, substrate temperature, thickness of film, and type of substrate on the degree of the chemical ordering and preferred texture of FePt films and, consequently, on their magnetic properties. The Fe IND.xPt IND.1-x alloy thin films (with x SEMELHANTE A 50 at%) were prepared by DC Magnetron Sputtering, via two different deposition methods: a conventional code position method and an alternate monatomic layer (DAM) deposition method. The films were grown on SiO IND.2/Si (100), Si (100) and MgO (100) substrates, with and without a Pt buffer layer (thickness ranging from 50 to 76 nm), at T IND.s varying from room temperature to 600°C. The Pt buffer layer was used for obtaining better conditions to the induction of an epitaxial grown of FePt films with the ordered FCT(001) phase. The results of structural and magnetic characterization performed by X-ray diffraction (XRD) and vibrating sample magnetometry (VSM) show that the MgO substrates are adequate to induce an epitaxial growth of FePt films with a FCT structure and [001] texture. However, with the use of an optimized Pt FCC(100) buffer layer pre-deposited on MgO, the best conditions to an epitaxial induced growth were obtained, by both deposition methods. The results show also that the quality of the epitaxial induced growth depends on the thickness of the film. In relation to the deposition method in comparison to the code position method for preparation of FePt FCT(001) films, at reduced temperatures (T IND.s MENOR IGUAL A 400°C). With the use of the DAM method it was possible to obtain the ordered FCT(001) phase in the FePt film grown directly on MgO, at T IND.s = 400°C. For FePt film grown on Pt/MgO, the formation of the FCT(001) phase was verified at T IND.s = 200°C, and high degree of chemical order (S=0,88) was obtained at T IND.s = 400°C. X-ray Absorption Spectroscopy (XAS) studies also were done.