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Chuang, Donna Sue. « Magnetic anisotrophy in ultrathin epitaxial films grown on surfaces vicinal to Cu(001) ». Thesis, Massachusetts Institute of Technology, 1994. http://hdl.handle.net/1721.1/32145.
Texte intégralRésumé :
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1994.Vita.
Includes bibliographical references (leaves 115-119).
by Donna Sue Chuang.
Ph.D.
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Mitra, Mukul Kumar. « Determination of order parameters of liquid crystals from birefringence, x-ray and magnetic anisotrophy studies ». Thesis, University of North Bengal, 1987. http://hdl.handle.net/123456789/822.
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Silva, Rafael Emidio da. « Correspondência histológica de parâmetros de imagens de tensores de difusão ». Universidade de São Paulo, 2014. http://www.teses.usp.br/teses/disponiveis/5/5151/tde-04092014-145000/.
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INTRODUÇÃO: Imagens por tensores de difusão (diffusion tensor imaging - DTI) do cérebro são parte fundamental de exames de ressonância magnética (RM) na rotina clínica. Entretanto, não há informação científica suficiente para inferir a base histológica de parâmetros de imagens de DTI. Um dos achados frequentes nos exames de RM são as áreas de alteração de sinal na substância branca (AASSB). Neste estudo, analisamos imagens de RM post-mortem, em particular as AASSB, e seus correlatos histológicos e imuno-histoquímicos. OBJETIVOS: Descrever os parâmetros quantitativos de imagens de DTI imagens obtidas in cranio post-mortem, comparar com parâmetros de relaxometria e transferência de magnetização e avaliar seus correlatos histológicos nas AASSB e AHAN, utilizando técnicas de correspondência espacial ponto-a-ponto. MÉTODOS: Analisamos áreas distribuídas na substância branca de encéfalos de quatro sujeitos submetidos ao exame de RM post-mortem, aproximadamente 12,87 horas (±2,59). A RM foi realizada em sistema com campo de 3,0 T, sendo utilizadas imagens FLAIR; T2 multi-eco, DTI, Densidade de Prótons para cálculo da taxa de transferência de magnetização (magnetization transfer ratio - MTR). Foram delimitadas regiões de interesse (ROI) em 20 áreas contendo AASSB e 20 áreas homólogas aparentemente normais (AHAN). Para cada área, foram obtidos valores de anisotropia fracional (FA), coeficiente de difusão aparente (ADC), MTR e tempo T2. Os encéfalos foram seccionados em fatias de 0,4mm e fotografados em alta resolução. Estas imagens foram co-registradas tridimensionalmente com as imagens de RM, utilizando técnica semi-automatizada para correlação ponto-a-ponto. Os fragmentos analisados foram corados por hematoxilina-eosina (HE), Kluverx Barrera (KB), galocianina (GALL), anti-neurofilamento (anti-NF), antiproteína básica de mielina (anti-MBP) e anti-proteína ácida fibrilar glial (anti- GFAP). RESULTADOS: Os valores de FA foram 0,40 ±0,12 nas AASSB, e 0,52±0,15 nas AHAN. Os valores de ADC foram nas AASSB de 0,129±0,04 x 10-3mm2/s, e 0,108±0,03 x10-3mm2/s nas AHAN. Os valores de MTR foram 51,59±0,88 % nas AASSB, e 52,88±4,09 % nas AHAN. Os valores de RT2 foram 91,79±1,95 % nas AASSB, e 69,94±4,54 % nas AHAN. Os valores de HE foram 162,9±20,0 nas AASSB, e 163,3±20,4 nas AHAN. Os valores de GALL foram de 181,8±4,36 nas AASSB, e 167,11±20,65 nas AHAN. Os valores de KB foram de 168,9±19,08 nas AASSB, e 182.82±20,65 nas AHAN. Os valores de NF médios mensurados foram de 131,6±12,23 nas AASSB, e 128,03±4,74 nas AHAN. Os valores de MBP médios mensurados foram de 153,9 ±13,67 nas AASSB, e 153,18±12,69 nas AHAN. Os valores de GFAP médios mensurados foram de 94.61±7,85 nas AASSB, e 81,23±6,29 nas AHAN. Os testes não paramétricos de correlação (Spearman) entre variáveis de imagens de RM e histologia apresentaram correlação apenas entre FA e GFAP (r=-0,59; p < 0,005). DISCUSSÃO E CONCLUSÕES: De acordo com os resultados obtidos, podemos inferir que a presença de gliose é um processo patológico que pode ser detectado utilizando os parâmetros de FA nas imagens de DTI. Os resultados apresentados demonstram que imagens de RM adquiridas post-mortem podem servir para compreender melhor as alterações estruturais do tecido cerebralBACKGROUND: Diffusion tensor imaging (DTI) is part of magnetic resonance imaging (MRI) acquisition methods available in clinical exams. However, there are not enough scientific basis to infer the histological substract of DTI parameters. White matter hyperintensities (WMH) are frequent findings in clinical MRI routine. In the last years, scientific evidences show that these injuries are not just age-related benign changes, but they could be associated to pathological processes. In this study we analyzed post-mortem MRI DTI data, specifically investigating WMH, their histological and immunohistochemical correlates. OBJECTIVES : At present study, we aimed to analyze quantifiable DTI parameters - Fractional Anisotropy (FA) and Apparent Diffusion Coefficient (ADC) - relaxometry (RT2) and magnetization transfer ratio (MTR) in WMH compared and normal appearing white matter (NAWM) from images obtained postmortem and in situ, and assess their histological substracts at WMH and NAWM using a point-topoint correlation platform. METHODS : We analyzed 20 regions of interest (ROI) encompassing WMH and NAWM selected from four subjects using in situ post-mortem MRI data acquired in 3.0T MR system. The subjects were scanned with a post-mortem interval of approximately 12h54m (±2h36m). The MRI analysis included 3D T1, FLAIR; multi-echo T2, DTI, and proton density to calculate the rate of magnetization transfer (magnetization transfer ratio - MTR). In each ROI we obtained measures of FA, ADC, RT2 and MTR. Brain specimens were posteriorly fixed in celloidin, sectioned into 0.4mm slices and photographed in high resolution. These images were co-registered with three-dimensional MR images using semi-automated technique for correlation using a point-to-point method established in our group. WMH and NAWM ROIs from MRI were submitted to quantitative histological analysis based on optical density color-deconvolution technique. Histological section were stained in hematoxylin-eosin (HE) , Kluver-Barrera (KB) , galloccyanin (GALL), anti-neurofilament (anti-NF) , anti-basic myelin protein (anti-MBP) and anti-glial fibrillary acidic protein (anti-GFAP). RESULTS: The mean FA values were 0.40±0.12 in WMH, and 0.52±0.15 in NAWM. ADC in WMH was 0.1290±0.04, x10-3mm2/s and 0.1081± 0.03 x10-3mm2/s in NAWM. MTR was 51.59± 0.88% in WMH, and 52.88±4.09% in NAWM. RT2 was 91.79±1.95% in WMH, and 69.94±4.54% in NAWM. FA, ADC and RT2 values were different between WMH and NAWM in a non-parametric analysis. On the quantitative histological analysis, the HE was 162.9±20.0 in WMH , and 163.3±20.4% in NAWM. GALL values at WMH was 181.8±4.36 and 167.11±20.65 at NAWM. KB was 168.9±19.08 in WMH, and 182.82±20.65 at NAWM. NF mean were 131.6 ±12.23 in WMH, and 128.03 ±14.74 at NAWM. MBP was 153.9±13.67 at WMH, and 153.18±12.69 at NAWM. Only GFAP values were different between WMH and NAWM. Mean GFAP was 94.61±7.85 in WMH, and 81.23±6.29 at NAWM. Non-parametric test (Spearman) between MR images and histology, showed correlation only for FA vs GFAP (r=-0.594, p < 0.005). DISCUSSION AND CONCLUSIONS: DTI FA and ADC parameters (and RT2 values) showed different patterns in WMH compared to NAWM, indicating that MR images acquired post-mortem may serve to understand the structural changes of the brain tissue. The presence of gliosis is a pathological process seems to be related to FA value from DTI
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Ferraro, Filippo Jacopo. « Magnetic anisotropies and exchange bias in ultrathin cobalt layers for the tunnel anisotropic magnetoresistance ». Thesis, Université Grenoble Alpes (ComUE), 2015. http://www.theses.fr/2015GREAY086/document.
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Dans le cadre de l’étude des phénomènes magnétiques et de la spintronique qui sont présents aux échelles nanoscopiques nous avons étudié différents aspects des structures asymétriques de Pt/Co/AlOx. L’un des objectifs de cette thèse est le contrôle de l’oxydation et des propriétés magnétiques de ces multicouches. Nous avons combiné les mesures de structures (réflexion de Rayon-X), transports (Effet Hall anormal), et magnétiques (VSM-SQUID) afin de déterminer les rôles des effets magnétiques et d’interfaces. Un objectif était d’analyser le rôle de quelques monocouches (MCs) de CoO (qui peut se former lors de la sur oxydation de l’Al) sur les propriétés de la multicouche. Nous avons utilisé une technique de déposition avec un gradient d’épaisseur pour contrôler l’oxydation à l’échelle nanométrique. Nous avons établis que quelques monocouches (MCs) de CoO a un impact sur l’anisotropie de a multicouche. Pour approfondir l’effet de la couche de CoO, nous avons construit des bicouches ultrafines de Co(0.6nm)/CoO(0.6nm). Nous avons effectué des mesures refroidi sur champ sur ce système et trouvé un fort effet de couplage d'échange. Ces résultats indiquent que la couche CoO garde une forte anisotropie même en dans la limite des monocouches et permet de réfuter certains modèles sur l’effet d’échange bias et indique que les couches, couramment négligé, de CoO doivent être prises en considération dans le bilan énergétiques du système. Nous avons construits un appareil de mesure perpendiculaire de la magnétorésistance tunnel anisotrope (TAMR) à partir de la structure Pt/Co/AlOx. La TAMR est un effet de spintronique relativement récent dans lequel la rotation d’aimantation dans une électrode magnétique (combiné avec un couplage spin-orbite) peut entrainer un changement de la probabilité de l’effet tunnel, ce qui se manifeste comme un effet de magnétorésistance. Nous avons démontré qu’un contrôle précis de l’état d’oxydation est essentiel pour l’effet TAMR. La forte anisotropie magnétique induite nous permet d’atteindre des valeurs de TAMR plus grande comparée à celle des structures Pt/Co/AlOxIn the context of studying magnetic and spintronics phenomena occurring at the nanoscale, we investigated several aspects of Pt/Co/AlOx asymmetric structures. One of the objectives of this thesis was the control of the oxidation and the tailoring of the magnetic properties of these multilayers. We combined structural (X-Ray Reflectivity), transport (Anomalous Hall Effect) and magnetic measurements (VSM-SQUID), to study the interplay of magnetic and interfacial effects. One objective was to analyze the role that few monolayers (MLs) of CoO (which can form when overoxidizing the Al layer), could have on the properties of the stack. We used a wedge deposition techniques to control the oxidation on a subnanometer scale. We established that few MLs of CoO largely affect the total anisotropy of the stack. To further investigate the impact of the CoO, we engineered ultrathin Co(0.6nm)/CoO(0.6nm) bilayers. We performed field cooled measurements on this system and we found a large exchange bias anisotropy. These results indicate that the CoO keeps a large anisotropy even in the ML regime, help to rule out some of the models proposed to explain the exchange bias effect and imply that the usually neglected CoO presence must be considered in the energy balance of the system. We build perpendicular Tunneling Anisotropic MagnetoResistance (TAMR) devices based on the Pt/Co/AlOx structure. The TAMR is a relatively new spintronics effect in which the rotation of the magnetization in a single magnetic electrode (combined with the Spin-Orbit Coupling) can cause a change of the tunnel probability, which manifests as a magnetoresistance effect. We demonstrated that a careful control of the interface oxidation is crucial for the TAMR effect. The large induced magnetic anisotropy allowed us to achieve enhanced TAMR values compared to similar Pt/Co/AlOx structures
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Martins, Alessandro. « Desenvolvimento do processo de produção e estudo estrutural e magnético de filmes finos ordenados de FePt ». Universidade de São Paulo, 2004. http://www.teses.usp.br/teses/disponiveis/43/43134/tde-24022014-153235/.
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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.
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Costa, Marcio. « First-principles Studies of Local Structure Effects in Magnetic Materials ». Doctoral thesis, Uppsala universitet, Materialteori, 2012. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-179223.
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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.
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Ouahioune, Nedjma. « MOKE set-upto measure magnetic anisotropy : MOKE set-upto measure magnetic anisotropy ». Thesis, Uppsala universitet, Materialfysik, 2020. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-414388.
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Wack, Michael Richard. « Anisotropy of magnetic remanence ». Diss., lmu, 2012. http://nbn-resolving.de/urn:nbn:de:bvb:19-145717.
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Eisenbach, Markus. « Magnetic anisotropy in nanostructures ». Thesis, University of Bristol, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.364862.
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Edström, Alexander. « Theoretical and Computational Studies on the Physics of Applied Magnetism : Magnetocrystalline Anisotropy of Transition Metal Magnets and Magnetic Effects in Elastic Electron Scattering ». Doctoral thesis, Uppsala universitet, Materialteori, 2016. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-304666.
Texte intégralRésumé :
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
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Boeer, Angelika. « Anisotropy in Molecular Magnetism- Magnetic Exchange Coupling of Octahedral Cobalt (II) Ions ». Thesis, University of Manchester, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.503050.
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Walsh, James Paul Slater. « Anisotropy in molecular magnetism ». Thesis, University of Manchester, 2014. https://www.research.manchester.ac.uk/portal/en/theses/anisotropy-in-molecular-magnetism(11474b91-0d3d-4b0a-97cd-214d1713674e).html.
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A collection of studies are reported that focus on the examination of exchange interactions in complexes containing paramagnetic ions with a large magnetic anisotropy. A number of complementary techniques are used to analyse the complicated systems that arise, including high-field high-frequency electron paramagnetic resonance, inelastic neutron scattering, SQUID magnetometry, and ab initio calculations. The nuclearity of the complexes ranges from dimetallic, to trimetallic, to octametallic. A family of five water- and carboxylate-bridged nickel(II) dimetallics are the focus of a magneto-structural correlation study that succeeds in measuring the magnitude of the exchange interaction despite dominating effects from large zero-field splitting effects. Similar work is reported for four cobalt(II) analogues of these compounds, with the relationship between exchange interactions and geometry also being probed by pressure INS. Charge density studies that combine high resolution X-ray and neutron diffraction studies are reported on cobalt and nickel analogues from the same family of dimetallics, revealing strong evidence for non-direct exchange. A family of four trimetallic triangle complexes containing two nickel(II) ions and one chromium(III) ion bridged by a central fluoride and a total of six carboxylates are reported, and the exchange interactions are elucidated from a global model that accounts for the low-field magnetic, heat capacity, and EPR data. Two new octametallic vanadium(III) wheels—where each pair of adjacent metals are bridged by a fluoride and two carboxylates—are reported along with preliminary results from magnetic measurements and solid state proton NMR spectra, which reveal significant field-dependent effects arising from level crossings at high fields.
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Merabtine, Skander. « Etude expérimentale de la multifissuration et de la décohésion de films minces et nanostructures magnétiques sur substrats flexibles : effet sur l'anisotropie magnétique ». Thesis, Sorbonne Paris Cité, 2017. http://www.theses.fr/2017USPCD058/document.
Texte intégralRésumé :
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 déformablesFlexible 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
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Gueye, Mouhamadou. « Magnetic thin films on flexible substrates : magnetomechanical study by ferromagnetic resonance ». Thesis, Sorbonne Paris Cité, 2017. http://www.theses.fr/2017USPCD007/document.
Texte intégralRésumé :
Les films minces déposés sur des substrats flexibles ont été intensivement étudiés ces dernières années en raison de leur nombreuses applications en électronique flexible. Depuis peu, l'électronique flexible est étendu aux matériaux magnétiques conduisant ainsi au domaine émergeant de la magnéto-électronique flexible actuellement à l'avant garde des sujets de recherche de la spintronique. Ce travail de thèse est dédié à l'étude des propriétés magnéto-mécaniques de films minces magnétiques (Ni, NiFe, Co2FeAl, CoFeB, FeCuNbSi) sur des substrats flexibles. Les analyses structurales ont montré que les films de Ni et de CFA sont polycristallins non-texturés ; le CFB est amorphe. Par conséquent, les propriétés élastiques et magnéto-élastiques de ces films sont considérées comme étant isotropes. Une technique basée sur une utilisation conjointe d'essai mécanique, la résonance ferromagnétique (FMR) et la corrélation d'images numériques (CIN) a été développée pour étudier les propriétés magnéto-mécaniques de films minces sur substrats flexibles. A l'aide de cette méthode, il est possible de suivre l'évolution de l'anisotropie résiduelle omniprésente dans les films magnétiques sur substrats flexibles. Cette anisotropie est liée aux propriétés mécaniques contrastées lorsqu'on dépose un film mince rigide (grand module d'Young) sur un substrat flexible (petit module d'Young). L'effet du recuit sur les propriétés élastiques et magnéto-élastiques a été soigneusement étudié dans le film CFB validant ainsi l'intérêt porté à ses alliages pour des applications en spintronique. Enfin, la résonance ferromagnétique est employé en balayage en fréquence pour suivre la variation de la direction de l'aimantation en fonction des déformations induites par l'application de tension électrique sur l'actionneur piézoélectrique. Un retournement de 90° de la direction de l'aimantation dans le film Co2FeAl sur substrat flexible de Kapton® est observéThin films deposited on flexible substrates have been widely studied in the last decades due to the numerous applications in flexible electronics. Recently, flexible electronics have been extended to magnetic materials leading to the so-called emerging feld of flexible magnetoelectronics which is actually at the cutting-edge of spintronics research topics.This thesis is devoted to the study magnetomechanical properties of magnetic thin films (Ni, NiFe, Co2FeAl, CoFeB, FeCuNbSi) on flexible substrates. Structural analysis have 130 Abstracts hown that the Ni and CFA films are found to be polycrystalline with no strong preferred orientations ; the CFB film is amorphous. Consequently, the elastic and magnetoelastic properties are isotropic. For the study of the magnetomechanical properties, a set-up based on a jointly use of deformation test (bending or piezoactuation), ferromagnetic resonance (FMR) and digital image correlation (DIC) have been developped. Thanks to this method, it is possible to follow the evolution of the inescapable residual anisotropy encountered in magnetic thin films on flexible substrates under deformation and to determine the effective coefficient of magnetostriction of the films (sometimes unknown).This residual anisotropy is ascribed to contrasted mechanical strength when a sti thin film is deposited on a compliant substrate. The effect of the annealing temperature on the elastic and magnetoelastic have been studied carefully in CFB validating then theinterest on such alloys for spintronics applications. Finally, we have employed FMR inits sweep frequency mode to study the effective evolution of magnetization direction as function of the voltage-induced strains. A 90-degree magnetization rotation in Co2FeAl thin film on Kapton® polyimide substrate is observed
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Fanjat, Grégory. « Les fluctuations du champ magnétique terrestre : des variations séculaires récentes aux renversements ». Thesis, Montpellier 2, 2012. http://www.theses.fr/2012MON20154/document.
Texte intégralRésumé :
Le champ magnétique terrestre présente une vaste gamme de variations temporelles, de l'année à plusieurs millions d'années. J'ai étudié au cours de ma thèse divers aspects de ces fluctuations, des variations séculaires récentes aux renversements.La première partie de ma thèse porte sur l'archéomagnétisme, discipline qui permet de retracer l'évolution temporelle du champ magnétique terrestre au cours des derniers millénaires, principalement à partir des matériaux archéologiques. J'ai étudié deux jeux d'échantillons provenant pour l'un de la Grèce (période néolithique, 6800-3200 avant J.C.) et pour l'autre du Mexique (Palenque, période Maya Classique 320-840 après J.C.), dans le but d'acquérir de nouvelles données d'archéointensité et de mieux contraindre la variation séculaire du champ magnétique terrestre. La comparaison de mes données avec celles disponibles dans la bibliographie et avec les différents modèles globaux et régionaux a mis en évidence que les variations séculaires au Néolithique en Grèce et au cours du premier millénaire en Amérique Centrale sont très mal définies. Mes données suggèrent que des composantes locales, non prisent en compte dans les modèles globaux peuvent exister au niveau ces régions. Elles renforcent l'intérêt de développer des modèles régionaux précis, qui nécessitent un développement en harmoniques sphériques à un degré plus élevé. Pour ce faire, l'acquisition de nouvelles données de haute qualité est un élément majeur.La deuxième partie traite de la description d'un renversement du champ magnétique terrestre. Cette étude a été basée sur deux points précis: étudier les directions transitionnelles afin d'apporter de nouvelles contraintes sur le possible confinement longitudinal des pôles géomagnétiques virtuels (PGV) d'une part, et d'autre part vérifier des paléointensités transitionnelles obtenues sur la séquence volcanique d'Akaroa (Nouvelle Zélande), dont l'intensité est significativement supérieure à celles des intensités avant et après le renversement. Nous avons ré-échantillonné cette séquence, et l'évolution directionnelle obtenue pour ce renversement est une succession complexe de polarités N-T-R-T-N-T-R. Les PGV obtenus semblent se regrouper sous deux bandes longitudinales sous l'Australie et l'Amérique, ce qui renforce l'hypothèse d'une interaction entre le manteau et le noyau sur plusieurs millions d'années. Suite à une étude de minéralogie magnétique, j'ai sélectionné les échantillons susceptibles de fournir une valeur de paléointensité par les méthodes de Thellier et du multispecimen. Les paléointensités obtenues sont relativement faibles (environ 20 microT) au cours du changement de polarité et forte à la fin de la séquence. Mon interprétation, basée à la fois sur les valeurs de l'intensité du champ et sur les données radiochronologiques montrant que la séquence s'est mise en place très rapidement, est de considérer que seul le renversement C4Ar.1n-C4Ar.1r a été enregistré dans cette séquence. Dans cette hypothèse, le renversement montre un cheminement complexe comparable à d'autres renversements enregistrés dans l'hémisphère nord (Steens Mountain par exemple), incluant un phénomène de rebond avant de se stabiliser. La troisième partie de ma thèse est consacrée au développement d'une nouvelle méthodologie et d'un nouvel appareillage pour déterminer des paléointensités. Le faible taux de réussite des expériences de paléointensité de l'étude précédente m'a poussé à m'intéresser au protocole multispecimen, qui peut s'appliquer aux échantillons possédant un comportement polydomaine. L'inconvénient technique majeur de cette méthode réside dans l'application du champ le long de l'aimantation naturelle, difficile à réaliser avec précision dans les fours standards. Pour ce faire, nous avons décidé de développer des porte-échantillons permettant d'orienter les échantillons dans l'espace pour le four standard et un prototype de four à chauffage ultra-rapide particulièrement bieThe Earth's magnetic field shows a large range of temporal variations from the year to several million years. I studied during my PhD thesis several aspects of these fluctuations, from recent secular variations to reversals.The first part of my manuscript deals with archeomagnetism, a discipline that allows to track the temporal variations of the Earth's magnetic field through millennia, mainly from archeological materials. I studied two sets of samples, one from Greece (Neolithic period 6800-3200 B.C.) and the other from Mexico (Palenque, Maya Classic period 320-840 A.D.), to acquire new archeointensity data in order to better constrain the secular variation of the geomagnetic field. By comparing my data with those available in the literature and with the various global and regional models, I showed that the secular variations during the Neolithic in Greece and during the first millennium in Central America are poorly defined. My data suggest that local components, not described by global models, may exist in these regions. They reinforce the importance of developing specific regional models, which require development in higher spherical harmonic degree. As a consequence, the acquisition of new high quality data is of main importance. The second part presents the description of a geomagnetic field reversal. This work was based on two points: first by studying transitional directions to provide new constraints on the possible preferred longitudinal paths of virtual geomagnetic poles (VGPs) and second by checking transitional paleointensities obtained on a volcanic sequence in Akaroa volcano (New Zealand). Indeed the transitional field intensity is significantly higher than the field intensity before and after the reversal. We re-sampled this sequence, and the directional results show a complex sequence of N-T-R-T-N-T-R polarity. The transitional VGPs obtained are clustered in two longitudinal bands through Australia and America. This observation seems to reinforce the assumption of a core-mantle interaction over several million years. Following a rock magnetic study, I selected samples that could provide a value for the Thellier and multispecimen paleointensity methods. The obtained paleointensity are relatively low (about 20 microT) during the polarity change and strong at the end of the sequence.Based both on the field strength values and on the radiochronological ages, showing that the sequence was erupted in a very short time, I suggest that only the C4Ar.1n-C4Ar.1r reversal was recorded in this sequence. In this assumption, the reversal shows a complex path comparable to other reversals recorded in the northern hemisphere (for example the Steens Mountain), including a rebound before stabilizing.Finally the last part is devoted to the development of a new methodology and a new apparatus to determine absolute paleointensity. Following the low success rate of paleointensity experiments from the previous study, I decided to test the multispecimen protocol, which can be applied to samples yielding a predominant multidomaine behavior. The main technical drawback of this method lies in the application of the laboratory field along the natural remanent magnetization, a difficult task to perform accurately in standard paleointensity ovens. Thus, we decided to adapt sample holders from our standard oven in order to allow the sample orientation in space and to develop an ultra-fast heating oven prototype particularly well-suited for this method, allowing to apply the laboratory field in the 3 dimensions. I checked the different multispecimen protocols on historical lavas from Reunion and Etna volcano, yielding very different magnetic mineralogies. For all flows, I obtained paleointensities very close from the expected values, regardless from the magnetic mineralogy, revealing the feasibility of our apparatus and the promising interest of the method. The application of various corrections on the statistical estimation o
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MUSCAS, GIUSEPPE. « Tuning the magnetic anisotropy in nanostructured magnetic oxides ». Doctoral thesis, Università degli Studi di Cagliari, 2015. http://hdl.handle.net/11584/266794.
Texte intégralRésumé :
Among nanostructured magnetic materials, nanoparticles (NPs) are unique complex physical objects: in these systems a multidomain organization is energetically unfavorable and single-magnetic-domain particles are formed, each one with a huge magnetic moment with comparison to that of single atoms, thus they are often named “supermoment”. The attractive performance of magnetic NPs based materials are appealing for several technological fields ranging from nanomedicine to high-density magneto recording. Thus, understanding the physics of magnetic nanoparticles and controlling their magnetic properties represent hot topics not only for fundamental studies but also for technological applications. The magnetic behavior of such entities is related to the reversal of their magnetization; this can be a thermal or a field activated transition, which is characterized by an energy barrier defined as a magnetic anisotropy energy (MAE), which is influenced by several parameters. Thus, the tuning of the magnetic properties of nanoparticles means control of the MAE.
In this work it will be discussed how to tune the MAE at the nanoscale showing the main parameters that can influence the anisotropy itself. It will be investigated the role of particle volume in the effective anisotropy, and its correlation with the surface contribution, exploring its strong effect with particle size below 10 nm. In this framework it will be investigated the role of organic coating, underlining its ability to reduce the magnetic disorder arising from the broken symmetry at particles surface. In addition, in nanoparticle ensemble, the MAE may differ from one particle to another due to particles size and shape distributions. Thus it will be defined a detailed statistical analysis of particles’ morphology, leading to the development of a new instrument to analyze particles morphology, called “aspect maps”.
The relation between the physical chemical structures of nanoparticles will be investigated on nickel doped cobalt ferrite samples, demonstrating how to tune the MAE by chemical composition, i.e., controlling magnetocrystalline anisotropy. Furthermore it will be analyzed the evolution of interparticles interactions with respect single particle magnetic anisotropy by means of a modified random anisotropy model.
The last part of this work will deal with the design of novel nanostructured composites. La0.67Ca0.33MnO3 and CoFe2O4 will be combined using two different structures, which can be easily extent to other materials, to improve their magnetic interactions in order to obtain tunable magnetotrasport proprieties of the final composites.
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Carvalho, Peterson Grandini de 1985. « Propriedades estruturais e magnéticas de nanofios de Co eletrodepositados ». [s.n.], 2013. http://repositorio.unicamp.br/jspui/handle/REPOSIP/276989.
Texte intégralRésumé :
Orientador: Kleber Roberto PirotaDissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin
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Resumo: Redes de nanofios (NF) magnéticos têm atraído muita atenção devido suas possíveis aplicações tecnológicas. Do ponto de vista tecnológico tais redes poderiam ser utilizadas em diversas aplicações tais como dispositivos spintrônicos, sistemas de armazenamento de memória magnética, elementos sensores de campo magnético e em diversas aplicações biológicas. Por outro lado, do ponto de vista da ciência de base é fundamental compreender o efeito da baixa dimensionalidade nas propriedades magnéticas dos materiais. Muitas das propriedades magnéticas dos NF decorrem simplesmente de sua geometria. Neles, há uma forte anisotropia magnética de forma que tende a manter os momentos magnéticos alinhados ao eixo principal dos NF. No cobalto com estrutura hexagonal compacta (hcp), os momentos magnéticos tendem a se alinhar em uma direção cristalina preferencial (eixo c), ou seja, há uma anisotropia magnetocristalina. Tal anisotropia é da mesma ordem de grandeza da anisotropia de forma de NF ideias (cujo comprimento é muito maior que o diâmetro). Nanofios de cobalto hcp são interessantes basicamente porque suas propriedades magnéticas dependerão fortemente da estrutura cristalina (tamanho do grão e orientação do cristal em relação ao eixo principal dos nanofios). Como as duas contribuições de anisotropia são da mesma ordem de grandeza, pode-se controlar a anisotropia efetiva ao longo do NF através do controle da direção cristalina. Neste trabalho foram produzidas redes de NF de Co via eletrodeposição em moldes de membrana nanoporosa de alumínio anodizado (MNPAA). Alterou-se a estrutura cristalina dos NF através do pH de eletrodeposição (1,9; 3,9 e 5,5). Os NF possuem diâmetro de 35 nm, comprimento de ~10 ?m e a distância entre eles na rede é de 110 nm . A estrutura cristalina foi estudada através de técnicas de microscopia eletrônica (de varredura e de transmissão) e por difração de raios-X. As propriedades magnéticas foram inferidas através de técnicas de magnetometria. Foram obtidas curvas de histerese em função da temperatura e do ângulo entre o campo magnético aplicado e o eixo principal dos NF. Também foram obtidas curvas de magnetização inicial com o campo aplicado paralelo e perpendicular aos NF, e após diferentes ciclos de desmagnetização. Também foram feitos alguns breves estudos sobre o efeito do tratamento térmico nas propriedades magnéticas dos NF; sobre a dieletroforese, que é uma técnica de separação dos NF removidos da MNPAA e dispersos em solução, e sobre as alterações morfológicas e grau de oxidação dos nanofios quando retirados da MNPAA. Observou-se que para pH¿s baixos o eixo c é perpendicular ao eixo principal e os grãos são pequenos. O aumento do pH tende a tornar o eixo c paralelo ao eixo principal dos nanofios, o que aumenta a anisotropia magnética efetiva da rede. O tamanho dos grãos também aumenta com o pH, tornando o meio cristalino local mais expressivo no processo de inversão da magnetização. Também se observou que, dependendo da direção do eixo c, a diminuição da temperatura pode inverter a direção fácil de anisotropia efetiva dos nanofios, devido ao aumento da anisotropia magnetocristalina
Abstract: Magnetic nanowire (NW) arrays have been attracting great attention due to their possible technological applications. Such arrays are promising candidates for different applications, such as spintronic devices, magnetic memory storage systems, magnetic field sensors and in several biological systems. On the other hand, from a basic science point of view, it is fundamental to understand the low dimensionality effect on materials magnetic properties. Many of the NW magnetic properties simply occur due to its elongated geometry. It creates a strong magnetic anisotropy which tends to maintain the magnetic moments aligned to the NW¿s main axis. In bulk cobalt with a hexagonal compact structure (hcp), the magnetic moments tend to align in a preferential crystalline direction, i.e. there is a magnetocrystalline anisotropy. Such anisotropy is of the same order of magnitude than NW¿s shape anisotropy (when the length is much bigger than the diameter). Hcp cobalt nanowires are mainly interesting because their magnetic properties strongly depend on their crystalline structure (grain size and crystal orientation with respect to the NW main axis). Since the two anisotropy contributions are of the same order of magnitude, we can control the effective anisotropy along the NW by controlling the crystalline structure orientation. In this study were produced Co NW arrays through eletrodeposition in aluminum nanoporous templates. The NW crystalline structure was altered by the pH during the eletrodeposition process. The NW present a diameter of 35 nm, length of ~10 ?m and an interwire distance of 110 nm. The crystalline structure was investigated through electronic microscopy techniques (scanning and transmission) and X-ray diffraction. The magnetic properties were inferred through magnetometry techniques. Hysteresis curves were obtained as a function of temperature and of the angle between the applied magnetic field and the NW¿s main axis. Initial magnetization curves with the applied field parallel and perpendicular to the NW, and after different demagnetization cycles, were also obtained. Finally, some brief studies have also been made about the effect of heat treatment on the NW magnetic properties; about dielectrophoresis, which is a separation technique for the NW removed from the alumina template and dispersed in solution; and about the morphological changes and the NW oxidation degree when removed from the alumina template
Mestrado
Física
Mestre em Física
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Stiller, Markus, Jose Barzola-Quiquia, Pablo Esquinazi, Daniel Spemann, Jan Meijer, Michael Lorenz et Marius Grundmann. « Strong out-of-plane magnetic anisotropy in ion irradiated anatase TiO2 thin films ». Universitätsbibliothek Leipzig, 2016. http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-216180.
Texte intégralRésumé :
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.
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Gottwald, Matthias. « Nouveaux systèmes modèles à aimantation perpendiculaire pour l'étude des effets de transfert de spin ». Thesis, Nancy 1, 2011. http://www.theses.fr/2011NAN10053/document.
Texte intégralRésumé :
Les effets de transfert de spin sont devenus un sujet de recherche majeur ces quinze dernières années. Cependant, un manque de vérifications expérimentales pour beaucoup de modèles décrivant les effets de transfert de spin peut être constaté. Ceci est surtout lié à un manque de systèmes magnétiques modèles permettant un contrôle précis des paramètres pertinents utilisés dans les modèles théoriques. Dans ce travail deux systèmes magnétiques à aimantation perpendiculaire ont été analysés : les alliages amorphes de Co1-xTbx élaborés par pulvérisation cathodique et les super-réseaux [Co/Ni](111) élaborés par épitaxie par jets moléculaires. L'anisotropie et l'aimantation, qui sont des paramètres pertinents dans beaucoup de modèles sur le transfert de spin, sont variables dans une large gamme. L'origine de cette anisotropie est discutée. La structure des domaines magnétiques est analysée et les résultats des mesures de transport sont interprétés. Pour les super-réseaux [Co/Ni](111) une forte polarisation en spin au niveau de Fermi est démontrée grâce à des expériences de photo émission résolue en spin et un coefficient d'amortissement intrinsèque [alpha] très faible est trouvé. Il est conclu que les alliages amorphes de Co1-xTbx et les super-réseaux [Co/Ni](111) sont des systèmes modèles pour le transfert de spinSpin transfer torque effects have become a research subject of high interest during the last 15 years. However, in order to probe the fundamental physics of spin transfer torque model systems are needed. For a model system it must be as simple as possible to tune the significant parameters (magnetic and structural). In this work we analyze the suitability of two materials for this need. The studied materials are amorphous Co1-xTbx alloys elaborated by sputtering and MBE grown [Co/Ni](111) superlattices. Both systems show perpendicular magnetic anisotropy (PMA), which provides a uniaxial anisotropy to the system. This anisotropy and the magnetization, which are significant parameters for many models on spin transfer torque, can be tuned in a large range of values. The origin of this PMA is discussed. The domain structure is analyzed and transport measurements are interpreted. In addition we show a strong spin polarization of the electrons close to the Fermi level by doing photoemission experiments. A small intrinsic Gilbert damping parameter [alpha] is found by FMR spectroscopy. We conclude that both materials are good candidates to be used as model systems for spin transfer torque
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Silva, Elvis Lira da. « Propriedades estruturais e magnéticas de nanofios de Ni e Co ». [s.n.], 2006. http://repositorio.unicamp.br/jspui/handle/REPOSIP/278291.
Texte intégralRésumé :
Orientadores: Marcelo Knobel, Daniela ZanchetDissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin
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Resumo: Os arranjos de nanofios magnéticos tem atraído um interesse considerável da comunidade científica, motivado principalmente pela sua utilização como sistemas-modelo e na possível aplicação em mídias magnéticas de alta densidade de informação. O comportamento magnético macroscópico desses sistemas é fortemente dependente das anisotropias magnéticas efetivas (determinadas principalmente pelas contribuições das anisotropias de forma, magnetocristalina e magnetoelástica). Neste trabalho realizamos um estudo completo das propriedades magnéticas de nanofios magnéticos de Ni e Co, variando o comprimento dos nanofios e a temperatura das amostras. Os nanofios são obtidos por eletrodeposição em nanoporos de membranas de alumina preparadas por um duplo processo de anodização em substratos de alumínio. A caracterização estrutural dos nanofios foi feita por microscopia eletrônica de varredura de alta resolução e microscopia de força magnética e revela que os arranjos de nanofios estão organizados em uma rede hexagonal onde possuem diametros de aproximadamente 35 nm e a distância entre os nanofios de aproximadamente 105 nm. O comprimento dos nanofios varia de aproximadamente 560 nm até 2250 nm. Observamos uma mudança do eixo fácil de magnetização da direção paralela ao eixo dos nanofios na temperatura ambiente para direção perpendicular ao eixo dos nanofios em baixas temperaturas. Analisamos a dependência da remanência reduzida e da coercividade em relação à temperatura e verificamos que as amostras apresentam uma temperatura de cruzamento entre as remanências reduzidas com campo aplicado paralela e perpendicularmente ao eixo dos nanofios, que varia de acordo com o comprimento dos nanofios. Interpretamos nossos resultados em termos de uma competição entre a anisotropia de forma, que tende orientar a magnetização na direção paralela ao eixo dos nanofios, e uma anisotropia dependente da temperatura, que tenta alinhar a magnetizaçao na direção perpendicular ao eixo dos nanofios. O mecanismo utilizado para tentar explicar de maneira qualitativa os resultados que observamos, ao diminuirmos a temperatura da amostra, origina-se da tensão provocada pela alumina sobre os nanofios, em decorrência dos diferentes coeficientes de expansão térmica desses materiais, que induz uma anisotropia magnetoelástica perpendicular ao eixo dos nanofios
Abstract: Arrays of magnetic nanowires have attracted considerable interest, mainly motivated by their use as model systems and by possible applications in high-density magnetic information storage. The macroscopic magnetic behavior of such systems is strongly dependent on the effective magnetic anisotropy (mainly determined by shape and crystalline contributions). In this work, we carry out a systematic study of the magnetic properties on highly-ordered magnetic arrays of Co and Ni nanowires as functions of length of the nanowires and temperature. Nanowires were obtained by electrodeposition into nanopores of alumina membranes prepared by a two-step anodization process from pure aluminium. Structural studies were performed by high resolution scanning electron microscopy and magnetic force microscopy. The images revealed uniform arrays of nanowires with diameter of 35 nm, and with hexagonal symmetry arrangement with lattice constant (or inter-nanowire distance) of 105 nm. The nanowires length varies between 560 nm and 2250 nm. We observed a change in the magnetic easy axis from parallel to the axis wires at room temperature to transverse to the wire axis at low temperatures. We analysed the temperature dependence of the reduced remanence and coercive field we verified that the samples present a crossover temperature of reduced remanence with magnetic field applied both perpendicular and parallel to the nanowires axis that varies with nanowires length. We interpreted our results in terms of a competition between the shape anisotropy of the wires, which tends to align the magnetization along the wires axis and the temperature dependent magnetic anisotropy, which tends to orient the magnetization transverse to the wires axis. The mechanism which can qualitatively explain the observed results as a function of temperature is an induced anisotropy of magnetoelastic origin transversal to the nanowires axis, caused by strains and stresses, due to the different thermal expansion coefficient of nanowires and the alumina matrix, respectively
Mestrado
Materiais Magneticos e Propriedades Magneticas
Mestre em Física
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Maurer, Thomas. « Magnetism of anisotropic nano-objects : magnetic and neutron studies of Co1-xNix nanowires ». Paris 11, 2009. http://www.theses.fr/2009PA112340.
Texte intégralRésumé :
Le nanomagnétisme est actuellement un champ d'investigation très actif grâce aux développements de méthodes de synthèse et d'investigation originales. Cette thèse s'attache à sonder le magnétisme de nanofils magnétiques synthétisés par un procédé polyol. Ce procédé présente l'avantage de fournir un large éventail d'objets magnétiques anisotropes présentant une très bonne qualité cristalline. Les nanofils ainsi synthétisés présentent des diamètres variant de 7nm à 20nm, leur conférant un mode de renversement de l'aimantation cohérent. Cette thèse montre que les nanofils ainsi synthétisés ont des coercivités élevées comparées à celles de nanofils synthétisés par d'autres voies. Par ailleurs, les effets de l'oxydation de ces nanofils sur leurs propriétés magnétiques ont été étudiés. Les mesures magnétiques ont révélé une dépendance en température des champs d'échange et coercitif inhabituelle. Cela a permis de mettre en lumière le rôle prépondérant des fluctuations superparamagnétiques des grains antiferromagnétiques d'oxyde de cobalt dans le phénomène d'Exchange Bias. Enfin, cette thèse a aussi pour objectif de développer la technique de Diffusion de Neutrons Polarisés aux Petits Angles pour sonder le magnétisme de nanofils. Jusqu'à présent, cette technique a été surtout réservée à l'étude d'objets magnétiques isotropes. En effet, l'alignement des nanofils est crucial pour extraire des informations quantitatives d'une telle étude. C'est pour cela, qu'outre les nanofils synthétisés par procédé polyol, des nanofils inclus dans des matrices d'alumine poreuse ont aussi été étudiés par cette techniqueMagnetism of individual nano-objects is a very active research field thanks to the development of original synthesis routes and investigation tools. This thesis aims at probing the magnetism of nanowires synthesized via a pure chemical route, the polyol process. This process provides a large variety and an excellent crystallinity of the synthesized nanowires. This process allows to tune the diameter of the nanowires from 7nm to 20nm leading to coherent magnetization reversaI in the nanowires. I show how the large shape and magnetocrystalline anisotropies provide large coercivities compared to magnetic nanowires synthesized via other routes. Furthermore, the oxidation of such objects has also been investigated. Magnetic measurements has revealed unsual temperature dependencies of both the coercive and exchange fields, emphasizing the role of the superparamagnetic fluctuations of the CoO antiferromagnetic grains in the Exchange Bias effect. Finally, this thesis also aims at developing Polarized Small Angle Neutron Scattering to probe magnetism in complex nano-objects. Such a technique has been ignored until now to study magnetic anisotropic nano-objects despite being well adapted. The key ingredient to carry through such a study is the perfect alignment of the nanowires. This is why, besides the nanowires synthesized via the polyol process, Polarized Small Angle neutron Scattering measurements have been performed on arrays of magnetic nanowires included in porous alumina membranes
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Bali, R. « Exchange anisotropy in magnetic thin films ». Thesis, University of Cambridge, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.596311.
Texte intégralRésumé :
I have studied Exchange Bias in epitaxial α-Fe2O3 and polycrystalline FeMn and CoMn antiferromagnets coupled to the ferromagnet Permalloy (Ni81Fe19). Despite the nominal magnetocrystalline anisotropy of Permalloy, surface features such as vicinal steps and roughness induced strong uniaxial anisotropy. Exchange coupling with epitaxial α-Fe2O3 led to a biaxial anisotropy component in the ferromagnet. Anisotropy axes of the biaxial component remained fixed despite applying magnetic field at elevated temperatures. The magnitude of biaxial anisotropy varied with every temperature cycle, providing implicit evidence of domains in the antiferromagnet. In polycrystalline systems, uniaxial anisotropy of the ferromagnet set-up during growth competes with exchange anisotropy. By setting up an AF/F/AF trilayer, magnetic field cooling allowed control of the net anisotropy. These findings demonstrate that the two crucial factors that impact exchange biased thin films are (1) uniaxial anisotropies in the ferromagnet from surface and growth conditions and (2) exchange coupling with domains in the antiferromagnet. Competing effect of these two factors is the key to understanding any exchange coupled system.
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Chilton, Nicholas Frederick. « Magnetic anisotropy of transition metal complexes ». Thesis, University of Manchester, 2015. https://www.research.manchester.ac.uk/portal/en/theses/magnetic-anisotropy-of-transition-metal-complexes(64b34057-8a7a-44db-a89a-22a233fdefb5).html.
Texte intégralRésumé :
The study of magnetic anisotropy in molecular systems permeates the physical sciences and finds application in areas as diverse as biomedical imaging and quantum information processing. The ability to understand and subsequently to design improved agents requires a detailed knowledge of their fundamental operation. This work outlines the background theory of the electronic structure of magnetic molecules and provides examples, for elements across the Periodic Table, of how it may be employed to aid in the understanding of magnetically anisotropic molecules. The magnetic anisotropies of a series of dimetallic NiII2 complexes and a RuIII2MnII triangle are determined through multi-frequency Electron Paramagnetic Resonance (EPR) spectroscopy and ab initio calculations. The magnetic anisotropy of the former is found to be on the same order of magnitude as the isotropic exchange interactions, while that of the latter is found to be caused by large antisymmetric exchange interactions involving the RuIII ions. An intuitive electrostatic strategy for the prediction of the magnetic anisotropy of DyIII complexes is presented, allowing facile determination of magnetic anisotropy for low symmetry molecules. Through the presentation of the first near-linear pseudo-two-coordinate 4f-block complex, a new family of DyIII complexes with unprecedented Single Molecule Magnet (SMM) properties is proposed. Design criteria for such species are elucidated and show that in general any two-coordinate complex of DyIII is an attractive synthetic target. The exchange interaction between two DyIII ions is directly measured with multi-frequency EPR spectroscopy, explaining the quenching of the slow magnetic relaxation in the pure species compared to the SMM properties of the diluted form. The interpretation of this complex system was achieved with supporting ab initio calculations.
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Rafique, Sadia. « Magnetic Anisotropy of Fe1-xGax Alloys ». College Park, Md. : University of Maryland, 2003. http://hdl.handle.net/1903/133.
Texte intégralRésumé :
Thesis (M.S.) -- University of Maryland, College Park, 2003.Thesis research directed by: Dept. of Materials Science and Nuclear Engineering. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.
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Roučka, Václav. « Šíření spinových vln ve strukturách s lokálně modifikovanou magnetickou anizotropií ». Master's thesis, Vysoké učení technické v Brně. Fakulta strojního inženýrství, 2021. http://www.nusl.cz/ntk/nusl-443766.
Texte intégralRésumé :
Zařízení založená na spinových vlnách mají potenciál být využita ve výpočetní technice s nízkou spotřebou energie. Pro úspěšné využití je samozřejmě potřeba spojit více takových zařízení na jednom čipu, součástí čehož musí být zatáčení spinových vln zahnutými vlnovody. Problém zatáčení spinových vln v dipolárně výměnném režimu zatím nebyl uspokojivě vyřešen, vyzkoušené přístupy vedly ke ztrátě intenzity a fázové koherence. V této diplomové práci jsme zkoumali dva systémy, které by mohly být využity k zatáčení spinových vln. Prvním z nich jsou tenké metastabilní vrstvy slitiny železa a niklu. Paramagnetická metastabilní fcc vrstva, která byla epitaxně narostena na substrátu z mědi, může být transformována do stabilní ferromagnetické bcc fáze pomocí fokusovaného iontového svazku. Tato technika nám dává prostorovou kontrolu nad transformačním procesem a strategie skenování svazkem nám dokonce umožňuje určit směr mangetické anisotropie. Magnetické vlastnosti struktur vytvořených touto metodou a lom spinových vln mezi doménami s odlišným směrem magnetické anisotropie byly změřeny pomocí mikroskopie Brillouinova rozptylu světla. Druhým zkoumaným systémem jsou zvlněné vlnovody, jejichž zvlnění indukuje magnetickou anisotropii. Zvlnění magnetické vrstvy je vytvořeno depozicí nemagnetických vlnek na substrátu indukovanou fokusovaným elektronovým svazkem a následnou depozicí magnetického materiálu. Byly vyrobeny různé návrhy zatočených zvlněných vlnovodů a změřili jsme šíření spinových vln jejich zatáčkami pomocí mikroskopie Brillouinova rozptylu světla. Využili jsme také mikromagnetické simulace pro získání hlubšího porozumění zkoumané problematiky a pro hledání vhodných návrhů experimentů.
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Short, Geoffrey. « Study of magnetic anisotropy by Magnetic Circular X-ray Dichroism ». Thesis, University of York, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.310961.
Texte intégral
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Fard, Samad Moemen Bellah. « Modelling anisotropy in electrotechnical steels ». Thesis, Cardiff University, 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.263551.
Texte intégral
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Le, Laurent Ludovic. « Etude théorique de l'anisotropie magnétique dans des systèmes hybrides pour la spintronique moléculaire ». Thesis, université Paris-Saclay, 2020. http://www.theses.fr/2020UPASP071.
Texte intégralRésumé :
L'anisotropie magnétique est une propriété de grande importance aussi bien pour les applications (stockage magnétiques, ...) que pour son intérêt fondamental. Dans ce manuscrit, nous nous intéressons plus précisément au calcul de l'énergie d'anisotropie magnéto-cristalline (MCA) qui est issue du couplage spin-orbite. C'est une grandeur délicate à déterminer pour plusieurs raisons : elle est en général très faible (de l'ordre du meV par atome au maximum) et elle dépend crucialement de nombreux paramètres physiques et numériques. Elle est notamment fortement dépendante des symétries du système, du réseau cristallin, mais aussi de la nature des matériaux, ici des métaux de transition 3d. Nous nous intéressons plus particulièrement à des couches minces magnétiques pour lesquelles la MCA est calculée comme étant la différence d'énergie du système entre une aimantation parallèle à la surface du matériau et une aimantation orthogonale à celle-ci. Nous utilisons en particulier des codes basés sur théorie de la fonctionnelle densité (DFT, Quantum ESPRESSO et Quantum ATK) et un code de liaisons fortes (TB). La MCA étant obtenue comme une différence d'énergie entre deux orientations de spin en utilisant le théorème de force. L'objectif de cette thèse de doctorat est de comprendre et de décrire en détails le comportement de la MCA dans des films minces magnétiques et les différentes manières de modifier de manière contrôlée (ou non) cette grandeur. Grâce à nos outils de calcul, nous avons extrait des grandes tendances et mis en évidence les paramètres essentiels qui permettent de piloter l'anisotropie. L'une d'entre elles retient particulièrement notre attention, l'hybridation d'orbitales entre un métal de transition 3d et un atome de carbone non-magnétique, donnant lieu d'importantes variations d'anisotropie. De tels systèmes sont dits hybrides, et nous renseignent sur les capacités de transmission du magnétisme (polarisation, anisotropie) entre atomes voisinsMagnetic anisotropy is a property of great importance both for applications (magnetic storage, ...) and for its fundamental interest. In this manuscript, we are more precisely interested in the computation of the magneto-crystalline anisotropy energy (MCA) that results from spin-orbit coupling. It is a delicate quantity to determine for several reasons : it is generally very small (of the order of meV per atom at most) and depends crucially on many physical and numerical parameters. In particular, it is strongly dependent on the symmetries of the system, the crystal lattice, but also on the nature of the materials, in this case the transition metals 3d. We are more particularly interested in magnetic thin films for which the MCA is calculated as the difference in system energy between a magnetization parallel to the surface of the material and a magnetization orthogonal to it. In particular, we use codes based on density functional theory (DFT, Quantum ESPRESSO and Quantum ATK) and a tight-binding code (TB). The MCA being obtained as an energy difference between two spin orientations using the force theorem. The objective of this PhD thesis is to understand and describe in detail the behavior of the MCA in magnetic thin films and the different ways to tune and modify (controllably or not) its magnitude. Thanks to our computational tools, we have extracted the main trends and highlighted the essential parameters that allow us to control the anisotropy. One of them is of particular interest, the hybridization of orbitals between a 3d transition metal and non-magnetic carbon atom, giving rise to important variations in anisotropy. Such systems are called hybrid-systems, and give us information on the transmission of magnetic properties (polarization, anisotropy) between neighboring atoms
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Pessoa, Marcio Solino. « Anisotropia magnética em tricamadas epitaxiais Fe/Mn/Fe ». Universidade Federal de Goiás, 2010. http://repositorio.bc.ufg.br/tede/handle/tede/4211.
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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES
The ferromagnetic resonance (FMR) technique at the microwave frequencies of X-band (9.79 GHz) and Q-band (34 Ghz) was used to study, at room temperature, the magnetic anisotropy of Fe/Mn/Fe trilayers. The samples studied were grown by molecular beam epitaxy onto MgO substrates at the temperatures of 50 ° C, 150 ° C and 175 ° C, at the pressure of 4 x 10-11 Torr. The thicknesses of the distinct layers were 5 or 10 nm for the bottom Fe layer grown onto the MgO substrate, varied from 0.8 to 1.35 nm, for the Mn spacer layer, and was kept constant and equal to 5 nm, for the top Fe layer. The FMR spectra of the symmetrical trilayer at X and Q-band microwave frequencies show only the uniform resonance mode. Asymmetrical trilayers also show non resonant and/or non aligned modes at X-band frequency, but only the uniform mode at Q-band frequency. For some samples this result gives evidence of a non-collinear coupling between the Fe layers in the presence of low intensity magnetic fields. The out-of-plane angular dependence of the absorption fields show that the magnetization for all trilayers studied is in the plane of the sample. The magnetic anisotropy was deduced from the in-plane angular dependence of the uniform resonance mode at Q-band frequency, taking the free energy of the magnetic system as the result of contributions from Zeeman, demagnetizing, and in-plane cubic magnetocrystalline anisotropy energies. Fourfold in-plane anisotropy fields of 600 Oe and effective magnetizations above 16000 G were observed. The set of results obtained shows the high sensitivity and versatility of the FMR technique to study the magnetic properties of epitaxially grown monocrystalline nanometric structures.
A técnica de Ressonância Ferromagnética (RFM) nas frequências de micro-ondas de banda-X (9.79 GHz) e banda-Q (34 GHz) foi usada para estudar, à temperatura ambiente, a anisotropia magnética de tricamadas de Fe/Mn/Fe. As amostras estudadas foram crescidas por epitaxia de feixe molecular sobre substratos de MgO nas temperaturas de 50° C, 150° C e 175° C, à pressão de 4 x 10-11 Torr. A espessura da camada de Fe crescida sobre o MgO foi de 5 ou 10 nm, variou de 0.8 a 1.35 nm, para a camada separadora de Mn e foi mantida constante e igual a 5 nm, para a camada superior de Fe. Os espectros de RFM para a tricamada simétrica em bandas X e Q apresentam apenas o modo uniforme de ressonância. As tricamadas assimétricas apresentam também modos não ressonantes e/ou não alinhados em banda-X, mas apenas o modo uniforme em Banda-Q. Para algumas amostras, isto implica em um acoplamento não colinear entre as camadas de Fe na presença de campos magnéticos de baixa intensidade. A dependência angular do campo de ressonância fora do plano da amostra indica, para todas as amostras, que a magnetização se encontra no plano do filme. A anisotropia magnética das tricamadas foi deduzida a partir do estudo da dependência angular do campo de ressonância no plano da amostra, em banda-Q, tendo a energia livre do sistema contribuições de energias Zeeman, de desmagnetização e magnetocristalina cúbica. Campos de anisotropia cúbica da ordem de 600 Oe e magnetização efetiva superior a 16000 G foram observados. O conjunto dos resultados obtidos mostra a alta sensibilidade e versatilidade da técnica de RFM para estudar as propriedades magnéticas de estruturas monocristalinas nanométricas crescidas epitaxialmente.
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Bjork, Andreas. « Characterizing magnetic susceptibility and remanent magnetization of magnetite and hematite rich drill-core samples at Blötberget ». Thesis, Uppsala universitet, Geofysik, 2018. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-347975.
Texte intégralRésumé :
Laboratory magnetic measurements are used to develop a methodology to characterize the Kiruna-type Rare Earth Elements (REE) bearing apatite iron-oxide deposits at Blötberget in central Sweden. This high-grade ore deposit is known to have sharp boundaries between lens shaped main ore bodies of magnetite-rich ore, and a complex hematite-rich ore associated with pegmatites and skarn formation. The thesis covers laboratory magnetic measurements of 37 samples originating from eight drill cores and reference samples from previously mined area. It focuses on on-covering how the samples relate in terms of magnetic susceptibility, further its dependency on temperature, frequency, field and the orientation. The results are correlated with petrographic analysis previously performed on accompanying thin sections. The measurements show that magnetite with strong susceptibility contribution overshadow the hematite contribution in the samples. Transition changes in susceptibility are noticeable when crossing the Verwey temperature; -153°C, Curie temperature; 580°C and Néel temperature; 680°C. The Morin temperature appears at -60°C, or is missing. Linear relationships are identified between the magnitude difference in susceptibilities across transitions at high temperature and wt% magnetite and hematite have been identified. The Blötberget skarn and hematite-rich ore samples have a higher degree of susceptibility anisotropy than the other ore-types. Blötberget samples are dominated by multidomain characteristics in remanence, saturation and coercivity. High temperature measurements have shown that the magnetite is close to pure. The low temperature measurements suggest hematite is impure or bears a petrological footprint. The study also shows that rich iron ore samples sometimes can be at risk of being overlooked with standard methods of measuring susceptibilityLaboratorietekniska metoder kan användas som ett komplement till malmgeologi och geofysisk prospektering. I denna metodstudie karaktäriseras apatitjärnmalm från Blötberget, nära Grängesberg. En fyndighet bestående av linsformade malmkroppar rika på magnetit och ofta avskilda men komplexa hematitrika stråk. Studien är gjord 37 prover från totalt 8 borrkärnor, och lokaler som tillhörde produktion från gruvverksamhet under 1900-talet. Mätmetoderna fokuserar på att kartlägga malmens magnetiska egenskaper, och hur temperatur, frekvens, fältstyrka samt riktning påverkar dessa. Resultaten jämfördes med tidigare petrografisk studie av tillhörande tunnslip Resultaten visar att magnetit står för merparten av susceptibiliteten i proverna, men att även hematit kan urskiljas och kvantifieras. Temperaturberoende har påvisats vid övergångar för Verwey-temperatur; -153°C, Curie-temperatur; 580 °C, och Néeltemperatur; 680 °C. Den förväntade Morin-temperaturen vid -14°C, påträffades vid -60 °C eller saknas helt för flera av de hematitrika proverna. Magnetiskt anisotropa prover återfinns bland prover som identifierats som skarn eller hematitrika. Magnetisk granulometri visar karaktär av multidomäntyp med låg magnetisk coercivitet och hög satureringsförmåga. Högtemperaturmätningar av susceptibilitet visar på ren magnetit för prover från Blötberget. Samtidigt visar lågtemperaturemätningar att hematit sannolikt har inblandning av titanium eller bär på ett mer komplext förflutet. Studien visar också att det finns en risk i att enbart förlita sig på bulksusceptibilitet för prover rika på malm.
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Zhang, Hongbin. « Relativistic Density Functional Treatment of Magnetic Anisotropy ». Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2009. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-25132.
Texte intégralRésumé :
Spin-orbit coupling (SOC) reduces the spatial symmetry of ferromagnetic solids. That is, the physical properties of ferromagnetic materials are anisotropic, depending on the magnetization direction. In this thesis, by means of numerical calculations with full-relativistic density functional theory, we studied two kinds of physical properties: surface magnetic anisotropy energy (MAE) and anisotropic thermoelectric power due to Lifshitz transitions. After a short introduction to the full-relativistic density functional theory in Chapter 2, the MAE of ferromagnetic thin films is studied in Chapter 3. For such systems, separation of different contributions, such as bulk magnetocrystalline anisotropy (MCA) energy, shape anisotropy energy, and surface/interface anisotropy energy, is crucial to gain better understanding of experiments. By fitting our calculating results for thick slabs to a phenomenological model, reliable surface MAE could be obtained. Following this idea, we have studied the MAE of Co slabs with different geometries, focusing on the effects of orbital polarization correction (OPC). We found that the surface anisotropy is mainly determined by the geometry. While OPC gives better results of orbital moments, it overestimates the MAE. In the second part of Chapter3, the effects of electric fields on the MAE of L10 ferromagnetic thin films are studied. Using a simple model to simulate the electric field, our calculations are in good agreement with previous experimental results. We predicted that for CoPt, even larger effects exist. Moreover, we found that it is the amount of screening charge that determines the magnetoelectric coupling effects. This gives us some clue about how to achieve electric field control of magnetization direction. In Chapter 4, Lifshitz transitions in L10 FePt caused by a canted magnetic field are studied. We found several Lifshitz transitions in ordered FePt with tiny features in DOS. Using a two-band model, it is demonstrated that at such transitions, the singular behaviour of kinetic properties is due to the interband scattering, and the singularity itself is proportional to the derivative of the singular DOS. For FePt, such singularity will be smeared into anomaly by chemical disorder. Using CPA, we studied the effects of energy level broadening for the critical bands in FePt. We found that for experimentally available FePt thin films, Lifshitz transitions would induce up to a 3% increase of thermopower as the magnetization is rotated from the easy axis to the hard axisSpin-Bahn-Kopplung reduziert die Symmetrie ferromagnetischer Festkörper. Das bedeutet, dass die physikalischen Eigenschaften ferromagnetischer Stoffe anisotrop bezüglich der Magnetisierungsrichtung sind. In dieser Dissertation werden mittels numerischer voll-relativistischer Dichtefunktional-Rechnungen zwei Arten physikalischer Eigenschaften untersucht: magnetische Oberflächen-Anisotropieenergie (MAE) und anisotrope Thermokraft durch Lifshitz-Übergänge. Nach einer kurzen Einführung in die relativistische Dichtefunktional-Theorie in Kapitel 2 wird in Kapitel 3 die MAE ferromagnetischer dünner Filme untersucht. In diesen Systemen ist es für ein Verständnis experimenteller Ergebnisse wichtig, verschiedene Beiträge zu separieren: Volumenanteil der magnetokristallinen Anisotropie (MCA), Formanistropie und Oberflächen bzw. Grenzflächenanisotropie. Durch Anpassen berechneter Daten für dicke Schichten an ein phänomenologisches Modell konnten verlässliche Oberflächen Anisotropien erhalten werden. In dieser Weise wurde die MAE von Co- Schichten mit unterschiedlichen Geometrien untersucht, wobei der Einfluss von Orbitalpolarisations-Korrekturen (OPC) im Vordergrund stand. Es wurde gefunden, dass die Oberflächenanisotropie hauptsächlich von der Geometrie bestimmt wird. Während OPC bessere Ergebnisse für die Orbitalmomente liefert, wird die MAE überschätzt. Im zweiten Teil von Kapitel 3 wird der Einfluss elektrischer Felder auf die MAE von dünnen ferromagnetischen Filmen mit L10-Struktur untersucht. Unter Verwendung eines einfachen Modells zur Simulation des elektrischen Feldes liefern die Rechnungen gute Übereinstimmung mit vorliegenden experimentellen Ergebnissen. Es wird vorhergesagt, dass für CoPt ein noch größerer Effekt existiert. Weiterhin wurde gefunden, dass die magnetoelektrische Kopplung von der Größe der Abschirmladung bestimmt wird. Dies ist eine wichtige Einsicht, um die Magnetisierungsrichtung durch ein elektrisches Feld kontrollieren zu können. In Kapitel 4 werden Lifshitz-Übergänge untersucht, die ein gekantetes Magnetfeld hervorruft. Es wurden mehrere Lifshitz-Übergänge in geordnetem FePt gefunden, welche kleine Anomalien in der Zustandsdichte hervorrufen. Mit Hilfe eines Zweiband-Modells wird gezeigt, dass an solchen Übergängen das singuläre Verhalten kinetischer Eigenschaften durch Interband- Streuung verursacht wird und dass die Singularität proportional zur Ableitung der singulären Zustandsdichte ist. In FePt wird durch chemische Unordnung diese Singularität zu einer Anomalie verschmiert. Der Einfluss einer Verbreiterung der Energieniveaus der kritischen Bänder in FePt wurde mittels CPA untersucht. Es wurde gefunden, dass in experimentell verfügbaren dünnen FePt-Filmen Lifshitz-Übergänge bis zu 3% Erhöhung der Thermokraft erzeugen, wenn die Magnetisierung von der leichten in die harte Richtung gedreht wird
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Rozatian, Amir Sayid Hassan. « Anisotropy and interface structure in magnetic multilayers ». Thesis, Durham University, 2004. http://etheses.dur.ac.uk/2825/.
Texte intégralRésumé :
It is believed that the interfacial structure can significantly affect the magnetic properties of magnetic multilayer thin films. X-ray scattering techniques provide a powerful method with which to study the bulk and interface morphology in these systems, and are therefore crucial in developing an understanding of the dominant factors influencing the magnitude of the perpendicular magnetic anisotropy (PMA).The inter-relation between magnetic and structural properties of a series of magnetic multilayer thin films is investigated. Magnetometry measurements on a series of Fe/Au multilayers showed that some samples exhibited in-plane magnetization. X-ray data and simulations showed that the interface roughness was high in these samples. However, the formation and propagation of uncorrelated roughness followed a systematic trend for surface growth. On the other hand, x-ray data and simulations for a single 100-bilayer sample showed that the interfaces are much better defined with significantly lower roughness. This was the only sample to show perpendicular anisotropy supposing the suggestion that the absence of PMA in all other samples is associated with high interface roughness. Magnetometry measurements of the PMA in Co/Pt multilayers show an increase in effective anisotropy at about 15 bilayers. X-ray data showed that the roughness of the interfaces was correlated in all samples and that the interfaces were sharp with no detectable interdiffusion. No systematic trend in roughness or crystallographic texture is detected with increasing bilayer repeat X-ray measurements on four series of Co/Pd multilayers show interface roughness independent of bilayer repeat number. For Co/Pt, the in-plane correlation length was independent of bilayer number while for Co/Pd and Fe/Au it increased. A saturation of the in-plane correlation length for the Au/Fe system where island growth of the Au occurs was observed. The out-of-plane correlation length increased with bilayer repeat for Co/Pt and Co/Pd. The interfaces in samples with higher PMA had a fractal parameter close to unity.
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Shirtcliffe, James Leslie. « Electrodeposited alloy nanostructures with tuneable magnetic anisotropy ». Thesis, University of Bristol, 2016. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.730831.
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Vopsaroiu, Marian. « Anisotropy and texture studies in magnetic media ». Thesis, University of Central Lancashire, 2002. http://clok.uclan.ac.uk/20302/.
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The rapid development of magnetic materials for recording media applications increased the demands for new and more precise experimental investigation techniques. In respect with these demands, this project is focused on experimental analyses of advanced particulate media and magnetic thin film samples. A new extended rotational remanence technique for anisotropy field measurements was developed. The technique is suitable for samples that contain aligned or partially aligned particles and provides both: in-plane anisotropy field distributions and the in-plane anisotropy field. This technique was also extended to out-of-plane anisotropy field measurements. Rotational hysteresis was introduced as an alternative method for anisotropy field measurements. This applies well in the case of samples without texture or samples having very small magnetic moment (i.e. thin films). The two techniques for anisotropy field measurement compare well and the experimental results were interpreted in terms of inter-particles interactions. Two measurement methods for determination of the demagnetizing field acting perpendicular to a sample plane were also developed. The first method is based on the in-plane and out-of-plane anisotropy field determination using an extended rotational remanence technique. The second method can provide the demagnetizing field starting from in-plane and out-of-plane transverse hysteresis loops. Comparison between the results from the two methods showed good agreement. Furthermore, the demagnetizing field values were used to calculate the magnetic coating thickness, so the two methods provide a non-destructive method for magnetic thickness measurements in film samples. The in-plane easy axis distribution (EAD) was experimentally determined using vector VSM techniques. Correlations between in-plane tape texture and magnetic thickness were obtained for a series of advanced MP tapes. A theoretical approach was used in order to relate the orientation ratio to EAD. The out-of-plane EAD was derived from numerical calculations. The out-of-plane distribution also showed a variation with the magnetic thickness. In addition, the microstructure and particle morphology of the advanced MP tapes, as well as the out-of-plane component of magnetization, have been investigated using Mossbauer Spectroscopy. Finally, 3D - EAD maps were produced. All VSM experiments were fully computer controlled and the routines were designed in the LabView environment as part of this project.
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Cubukcu, Murat. « Manipulation of magnetic anisotropy in ferromagnetic semiconductors ». Paris 6, 2010. http://www.theses.fr/2010PA066159.
Texte intégralRésumé :
Cette thèse présente une étude de l’anisotropie magnétique de semiconducteurs ferromagnétiques en couches minces et du couplage magnétique dans des bicouches formées à partir de ces matériaux et de métaux ferromagnétiques. Je me suis focalisé sur deux systèmes distincts : des films minces de l’alliage quaternaire GaMnAsP et des bicouches : MnAs/GaMnAs. Dans ces systèmes, j’ai étudié l’influence sur les anisotropies magnétiques d’une part, de la déformation biaxiale induite par le désaccord de maille avec le substrat et d’autre part, de la concentration en trous. Ces études ont été menées principalement par résonance ferromagnétique, mesures d’aimantation par SQUID, ainsi que grâce à des mesures de transport et de diffraction de rayons X à haute résolution. Deux séries de films de GaMnAsP caractérisées par des concentrations en Mn de 7 et 10% ont été étudiées. Pour chaque série, la concentration en P a été variée sur une large gamme de 0 à 20%. Les forts dopages en P sont intéressants car le régime de conduction peut changer, passant de métallique à bande d’impuretés. Ceci induit de profondes modifications de tous les paramètres magnétiques pertinents. Nous avons étudié les variations d’anisotropie magnétique avec la concentration en P. Une réorientation de l’aimantation avec la température a pu être mise en évidence pour une concentration 6% P. La relaxation de l’aimantation de ces systèmes a été étudiée via le facteur de Gilbert. Des mesures de RFM à haute fréquence ont permis d’étudier l’anisotropie magnétique de films de MnAs épitaxiés sur (111) et (100)GaAs. Un couplage d’échange ferromagnétique est mis en évidence pour les bicouches MnAs/GaMnAs
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Soldatov, Ivan. « Thermoelectric effects and anisotropy in magnetic films ». Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2016. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-197529.
Texte intégralRésumé :
It was the purpose of this thesis to contribute to a better understanding of spin caloritronic phenomena and thermoelectric effects as well as the anisotropy of magnetic thin films. Mostly this work was motivated by the recent discovery of the spin Seebeck effect (SSE) in Japan: a generation of a pure spin current across the interface between magnetic|nonmagnetic materials upon application of the temperature gradient along (transversal SSE) or across (longitudinal SSE) the interface.
As the experimental configuration for the TSSE involves the heat flows, special care to spurious temperature gradients has to be taken. Semiconducting GaMnAs and insulating yttrium iron garnet (YIG) magnetic thin films were investigated. As nonmagnetic material platinum was chosen, providing also the opportunity to detect pure spin currents via the spin Hall effect (SPHE) in it. Starting with the measurement of in-plane voltages, transverse to the in-plane temperature gradient and measured along the platinum stripes, by sweeping the external magnetic field at different in-plane directions (TSSE geometry), symmetric and asymmetric contributions were observed in both, the GaAs/GaMnAs/Pt and GaAs/GaMnAs systems. The former was clearly attributed to the planar Nernst effect, arising from an in-plane temperature gradient along the sample, while the latter was shown to be result of the anomalous Nernst effect (ANE) caused by spurious out-of-plane temperature gradients. Using the ANE constant that was measured upon deliberately applying an out-of-plane temperature gradient, it was estimated that a rather small temperature drop of ΔT z ≈ 12 nK across the 200 nm magnetic film thickness can be responsible for the appearance of spurious ANE signals, leading to the asymmetry of the thermovoltages registered in the PNE/TSSE configuration. Thus, the TSSE if it exists at all in the GaMnAs sample, is very likely to be negligible.
The transport measurements in the insulating YIG sample in TSSE/PNE configuration demonstrated no field dependencies, supporting the idea that the signals in the semiconducting GaMnAs film originated not from the TSSE, but rather from the conventional ANE. The voltages in ANE/LSSE configuration the characteristic LSSE signals were observed. This thesis is also devoted to the magnetic anisotropy investigation in GaMnAs thin film by applying wide-field Kerr microscopy simultaneously to galvanomagnetic measurements. Upon sweeping the magnetic field the development of magnetic domains was traced: at low temperatures (5 K) the sample exhibits a clear two-fold switching of magnetization, while at higher temperatures (above 20 K) only a one-step switching was detected, indicating a strong temperature dependence of the magnetic anisotropy. The same behavior was derived from the transport measurements in the PHE configuration. Rotating the field in the plane of the thin film and analyzing the resulting angle dependencies of the transversal voltage, we obtained the anisotropy constants K u and K c . The temperature dependence of the K u / K c -ratio showed a gradual substitution of the cubic anisotropy, prevailing at low temperatures, by a uniaxial one with rising temperature.
For the purpose of the magneto-optical investigation and in-depth analysis of the magnetic anisotropy, within the frame of this thesis a new hardware and software realization for quantitative Kerr microscopy was developed. The principles of such a technique were formulated already years ago by Rave et al, but the experimental realization was limited to materials with in-plane surface magnetization and static domain imaging due to the manual adjustment of the microscope sensitivity. The realization of a separation of in-plane|out-of-plane magnetic contrast, suggested in this thesis, disposes those complications and improves the method, providing the opportunity for a simultaneous measurement of both components of magnetization in an automatic regime and, thus, allowing the quantitative analysis of the magnetization reorientation processes in magnetic media.
This technique was successfully applied to the quantitative investigation of the magnetic domain structure in GaMnAs film, and was also demonstrated on a number of other materials, including a permalloy (Ni 81 Fe 19 ) patterned film element and the separation of the in-plane and out-of-plane components of magnetic contrast in sintered NdFeB polycrystals.
In summary, this work contributes to a deeper understanding of the contribution of the conventional thermoelectric effects to the exploding field of spin caloritronic. It points out that thermoelectric phenomena have to be treated with care and that magnetic anisotropy and its temperature dependence have to be considered in any analysis of spin caloritronic phenomena. The thesis extends the existing methods of investigating magnetic anisotropy of magnetic films by introducing a novel technical realization of quantitative Kerr microscopy.
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Satter, Md Abdus. « A theory for dilute magnetic alloys : the origin of magnetic anisotropy ». Thesis, University of Warwick, 1989. http://wrap.warwick.ac.uk/101517/.
Texte intégralRésumé :
In this thesis, a formalism for studying the anisotropic interaction between two substitutional magnetic impurities and the magnetic anisotropic effect in a dilute noble metal-transition metal magnetic alloy has been developed from relativistic scattering theory. The theoretical development and the computational techniques of this formalism are based on relativistic spin-polarised scattering theory and relativistic band structure frameworks. For studying the magnetic anisotropic effect a convenient ‘working’ frame of reference with its axes oriented along the fee crystal axes is set up. This formalism is applied to study the situation for two Fe impurities in both paramagnetic Au (heavy) and Cu hosts. For AuFe dilute alloy, the two impurity site interaction as a function of separation is not oscillatory and the anisotropic effect is found to be less than the two site interaction itself only by an order of magnitude. Apart from the anisotropic coupling of the two impurity spins to the separation vector, for the first time, another weak anisotropic coupling to the crystal axes is also contained in the two site interaction. These anisotropic effects are the results of the relativistic spin-orbit interaction which are incorporated into the formalism.
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Vo, Van Chi. « Epitaxial graphene on metal for new magnetic manometric systems ». Phd thesis, Université de Grenoble, 2013. http://tel.archives-ouvertes.fr/tel-00829379.
Texte intégralRésumé :
Graphene is a candidate for next generation spintronics devices exploiting its long spin transport length and high carrier mobility. Besides, when put in interaction with a ferromagnet, it may become an active building block, as suggested by recent surface science studies revealing few tenth of a Bohr magneton magnetic moments held by carbon atoms in graphene on iron, and a Rashba spin-orbit splitting reaching about 10 meV in graphene on a high atomic number element such as gold. The extent to which graphene may influence the properties, e.g. magnetic ones, of the materials contacted to it was barely addressed thus far. High quality hybrid systems composed of graphene in contact with magnetic thin layers or nanoclusters are playgrounds for exploring both aspects, the manipulation of the properties of graphene by interaction with other species, and vice versa. In graphene contacted to ultra-thin ferromagnetic layers for instance, strong graphene/ferromagnet interface effects could be employed in the view of manipulating the magnetization in the ferromagnet. The recently discovered close-to-perfect self-organization of nanoclusters on graphene, provides a way to probe magnetic interaction between clusters, possibly mediated by graphene. Three high quality hybrid systems relying on graphene prepared by chemical vapor deposition on the (111) surface of iridium have been developed under ultra-high vacuum (UHV): cobalt ultra-thin and flat films deposited on top of graphene, and intercalated at moderate temperature between graphene and its substrate, and self-organized cobalt- and iron-rich nanoclusters on the 2.5 nm-periodicity moiré between graphene and Ir(111). Prior to these systems, 10 nm-thick Ir(111) single-crystal thin films on sapphire were developed: they were latter employed as a substrate replacing bulk Ir(111) single-crystals usually employed. This new substrate opens the route to multi-technique characterizations, especially ex situ ones which were little employed thus far for studying graphene/metal systems prepared under UHV. Using a combination of in situ surface science techniques (scanning tunneling microscopy, x-ray magnetic circular dichroism, spin-polarized low-energy electron microscopy, auger electron spectroscopy, reflection high-energy electron diffraction) and ex situ probes (x-ray diffraction, transmission electron microscopy, Raman spectroscopy, MOKE magnetometry) the structural, vibrational, electronic, and magnetic properties of the three new graphene hybrid systems were characterized and confronted to first-principle calculations. Several striking features were unveiled. The interface between graphene and cobalt involves strong C-Co interactions which are responsible for a large interface magnetic anisotropy, capable of driving the magnetization out-of-the plane of the surface of an ultra-thin film in spite of the strong shape anisotropy in such films. The effect is maximized in the system obtained by intercalation between graphene and iridium, which comes naturally air-protected. Nanoclusters, on the contrary, seem to weakly interact with graphene. Small ones, comprising ca. 30 atoms each, remain super paramagnetic at 10 K, have no magnetic anisotropy, and it turns out difficult, even with 5 T fields to saturate their magnetization. Besides, the magnetic domains size seem to exceed the size of a single cluster, possibly pointing to magnetic interactions between clusters.
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Jiménez-Gallego, Juan Ramón. « Soluble cyanide-bridged cubes : structure, (photo) magnetism and redox behaviour ». Thesis, Paris 6, 2017. http://www.theses.fr/2017PA066548.
Texte intégralRésumé :
Cette thèse a porté sur l'étude des propriétés structurales et électroniques d'une série de complexes cubiques A-FeCo représentant des modèles moléculaires des CoFe-PBA. Ces cubes hétérobimétalliques neutres de formule A{Fe4Co4(CN)12} reproduisent une série d'unités cubiques uniques excisées du composé inorganique de type " bleu de prusse ". Le composé moléculaire octamétallique est composé de 4 ions FeIIBS, 3 ions CoIIIBS et 1 ion CoIIHS reliés par des ponts cyanures. Un monocation est hébergée à l'intérieur du cube. Les ions métalliques situés au coin du cube sont liés par des ligand tridentés de type scorpionate (Tp pour Fe et pzTp pour Co). La présence d'un ion Co (II) non équivalent induit un abaissement de la symétrie du Td à C3v. La DRX sur monocristaux a permis de déterminer avec précision l'emplacement du cation inséré dans le cube. En raison de la présence d'une charge négative formelle dans l'un des coins de la cage (celle accueillant l'ion Co (II)), deux types d'interactions entre le cation et le cube ont été postulés. L'une est une interaction électrostatique, et l'autre est l'interaction entre le cation (acide de Lewis) et les systèmes de cyanure (base de Lewis). Au sein de la famille des cubes contenant des ions alcalins, le Cs + interagit avec les 12 ponts cyanures alors que le K + n'interagit qu'avec trois d'entre eux. La spectroscopie RMN 133Cs a permis de confirmer une interaction chimique entre le césium (invité) et la cage (hôte). Ces cubes ont montré une remarquable capacité d'encapsuler différentes monocations de tailles et de nature différentesThis thesis has focused on the investigation of the structural and electronic properties of a series of A-FeCo cubic complexes representing molecular models of the CoFe-PBAs. These neutral heterobimetallic cubes of abbreviated formula A-{Fe4Co4(CN)12} reproduce a series of single cubic units excised from the Prussian Blue type framework. The octametallic anionic core is composed by 4xFeIILS, 3xCoIIILS and 1xCoIIHS connected via bridging cyanides. A monocation is encapsulated inside the cube. The metal ions situated at the corner of the cube are facially capped by tridentate “scorpionate ligand” (Tp for Fe and pzTp for Co). The occurrence of one non-equivalent Co(II) ion induces a lowering of the symmetry from Td to C3v. Single crystal XRD allowed determining precisely the location of the inserted cation in the cube. Because of the presence of a formal negative charge in one of the corner of the cage (that accommodating the Co(II) ion), two types of interactions between the cation and the cube have been postulated. One is an electrostatic interaction, and the other one is the interaction between the cation (Lewis acid) and the cyanide systems (Lewis base). Within the family of alkali ion containing cubes, the Cs+ interacts with the 12 cyanide bridges whereas the K+ interacts with only three of them. The 133Cs-NMR spectroscopy allowed confirming a through bond interaction between the guest caesium and the host cage
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Altarawneh, Moaz. « High frequency probes of superconductivity and magnetism in anisotropic materials in very high magnetic field ». Tallahassee, Florida : Florida State University, 2009. http://etd.lib.fsu.edu/theses/available/etd-10142009-142552/.
Texte intégralRésumé :
Thesis (Ph. D.)--Florida State University, 2009.Advisor: James Brooks, Florida State University, College of Arts and Sciences, Dept. of Physics. Title and description from dissertation home page (viewed on May 3, 2010). Document formatted into pages; contains xiii, 118 pages. Includes bibliographical references.
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Ribow, Mirko [Verfasser]. « Voltage control of magnetic anisotropy in ultrathin epitaxial magnetic layers / Mirko Ribow ». Halle, 2018. http://d-nb.info/1162892781/34.
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Zhou, Wei. « Oblique Angle Deposition Effects on Magnetron-Sputtered Metal Films ». Miami University / OhioLINK, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=miami1501067883261477.
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Goswami, Tamal. « Theoretical investigation on aromaticity, magnetic exchange coupling and magnetic anisotropy in metal based systems ». Thesis, University of North Bengal, 2015. http://ir.nbu.ac.in/handle/123456789/1851.
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Khadra, Ghassan. « Magnetic and structural properties of size-selected FeCo nanoparticle assemblies ». Thesis, Lyon 1, 2015. http://www.theses.fr/2015LYO10145/document.
Texte intégralRésumé :
Dans ce travail, nous nous sommes intéressés aux propriétés magnétiques intrinsèques (moments et anisotropie magnétiques) de nanoparticules bimétalliques fer-cobalt. Pour cela, des agrégats FeCo dans la gamme de taille 2-6 nm ont été préparés en utilisant la technique MS-LECBD (Mass Selected Low Energy Cluster Beam Deposition) et enrobés en matrice in − situ afin de les séparer, d'éviter leur coalescence pendant les recuits et de les protéger à leur sortie à l'air. Dans un premier temps, les propriétés structurales (dispersion de taille, morphologie, composition, structure cristallographique) ont été étudiées en vue de corréler directement les modifications des caractéristiques magnétiques des nanoparticules, à leur structure et à l'ordre chimique obtenu après traitement thermique haute température. D'autre part, pour mettre en évidence les effets d'alliages à cette échelle, des références d'agrégats purs de fer et de cobalt ont été fabriquées et étudiées en utilisant les mêmes techniques. Par microscopie électronique en transmission à haute résolution, diffraction anomale et absorption de rayons X (high resolution transmission electron microscopy (HRTEM), anomalous x-ray diffraction (AXD) and extended x-ray absorption fine structure (EXAFS), nous avons mis en évidence un changement structural depuis une phase A2 chimiquement désordonnée vers une phase B2 type CsCl chimiquement ordonnée. Cette transition a été validée par nos résultats obtenus par magnétomètrie SQUID et dichroïsme magnétique circulaire (x-ray magnetic circular dichroism (XMCD))Over the past few decades, use of nanostructures has become widely popular in the different field of science. Nanoparticles, in particular, are situated between the molecular level and bulk matter size. This size range gave rise to a wide variety physical phenomena that are still not quite understood. Magnetic nanoparticles are at their hype due to their applications in medical field, as a catalyst in a wide number of chemical reactions, in addition to their use for information storage devices and spintronics. In this work, we are interested in studying the intrinsic magnetic properties (magnetic moments and anisotropy) of FeCo nanoparticles. Thus, in order to completely understand their properties, mass-selected FeCo nanoparticles were prepared using the MS-LECBD (Mass Selected Low Energy Cluster Beam Deposition) technique in the sizes range of 2-6 nm and in − situ embedded in a matrix in order to separate them, to avoid coalescence during the annealing and to protect during transfer in air. From a first time, the structural properties (size, morphology, composition, crystallographic structure) of these nanopar- ticles were investigated in order to directly correlate the modification of the magnetic properties to the structure and chemical ordering of the nanoparticles after high temperature treatment. In addition to the bimetallic FeCo nanoparticles, reference Fe and Co systems were also fabricated and studied using the same techniques. The structural properties were investigated using high resolution transmission electron microscopy (HRTEM), anomalous x-ray diffraction (AXD) and extended x-ray absorption fine structure (EXAFS) where a phase transition from a disordered A2 phase to a chemically ordered CsCl B2 phase was observed and further validated from the magnetic findings using SQUID magnetometry and x-ray magnetic circular dichroism (XMCD)
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Chilcote, Michael A. « Controlling Anisotropy in Organic-Based Magnets for Coherent Magnonics ». The Ohio State University, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=osu1546565729485741.
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Kugler, Zoe [Verfasser]. « Perpendicular anisotropy in magnetic tunnel junctions / Zoe Kugler ». Bielefeld : Universitätsbibliothek Bielefeld, Hochschulschriften, 2012. http://d-nb.info/1023862891/34.
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Baker, Alexander A. « Tailoring of magnetic anisotropy and interfacial spin dynamics ». Thesis, University of Oxford, 2016. https://ora.ox.ac.uk/objects/uuid:475aee28-ddc5-49f3-a517-686d90d01d7b.
Texte intégralRésumé :
Spin transfer in magnetic multilayers offers the possibility of a new generation of ultra-fast, low-power spintronic devices. New ways to control the resonance frequency and damping in ultrathin films are actively sought, fuelling study of the precessional dynamics and interaction mechanisms in such samples. One effect that has come under particular scrutiny in recent years is the spin-transfer torque, wherein a flow of spins entering a ferromagnet exerts a torque on the magnetisation, inducing precession. A flow of spin angular momentum is usually generated through a spin-polarised electrical current, but a promising alternative is the pure spin current emitted by a ferromagnet undergoing ferromagnetic resonance (FMR). This allows spins to be transferred without a net charge flow. The physics of the generation, transmission and absorption of pure spin currents is a developing field, and holds great promise for both industrial applications and as a means to study fundamental physical phenomena in exotic materials. This thesis presents an investigation into the magnetodynamics of ferromagnetic thin films and heterostructures grown by molecular beam epitaxy and studied using vector-network analyser ferromagnetic resonance (VNA-FMR), x-ray magnetic circular dichroism, vibrating sample magnetometry and x-ray detected ferromagnetic resonance (XFMR). Particular attention is paid to the anisotropy of damping processes that occur in thin films, and the different coupling mechanisms that can exist across non-magnetic spacer layers in spin valves and magnetic tunnel junctions. It is first shown that the static and dynamic magnetic properties of thin Fe films can be effectively tailored by dilute doping with Dy impurities, which introduces a sizeable anisotropy of Gilbert damping. The mechanism underlying this effect is discussed, as is the concurrent modification of the spin and orbital contributions to the magnetic moment. The focus then turns to magnetodynamics of ferromagnetic films coupled across a nonmagnetic spacer layer, examining how different materials permit different interactions. First, an insulating MgO layer is used to separate the FM layers; it is found that this attenuates a spin current in under 1~nm, but permits a static interaction for at least 2 nm. XFMR measurements are used to ascertain the different contributions of the two interactions, and shed light on their interplay. Next, the same techniques are applied to spin valves with a spacer layer of the topological insulator (TI) Bi2Se3. TIs are the subject of much attention in the physics community, as they hold the potential for dissipationless transport, extremely high spin-orbit torques, and a host of novel physical effects. Here, their ability to absorb and transmit a pure spin current is studied, testing their suitability for incorporation into existing device schemata. VNA-FMR measurements confirm that the TI functions as an efficient angular momentum sink. XFMR measurements, however, demonstrate the presence of a weak interaction between the two ferromagnets, able to persist up to at least 8~nm, and possibly mediated by the topological surface state. Finally, the angle-dependence of spin pumping through a Cr barrier is examined, finding that a strong anisotropy of spin pumping from the source layer can be induced by an angular dependence of the total Gilbert damping parameter in the spin sink layer. VNA-FMR measurements show that anisotropy is suppressed above the spin diffusion length in Cr, which is found to be 8 nm, and is independent of static exchange coupling in the spin valve. XFMR results confirm induced precession in the spin sink layer, with isotropic static exchange and an anisotropic dynamic exchange. Taken together, these studies provide an insight not only into the magnetisation dynamics of thin films (and ways to modify them) but a demonstration of the power of ferromagnetic resonance techniques, and their applicability across materials and concepts. The results offer valuable information on the transmission and absorption of spin currents by different materials, and several mechanisms by which enhanced spin torques and angular control of damping may be realized for next-generation spintronic devices.
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Steinbach, Gabi, Michael Schreiber, Dennis Nissen, Manfred Albrecht, Ekaterina Novak, Pedro A. Sánchez, Sofia S. Kantorovich, Sibylle Gemming et Artur Erbe. « Field-responsive colloidal assemblies defined by magnetic anisotropy ». American Physical Society, 2019. https://monarch.qucosa.de/id/qucosa%3A70641.
Texte intégralRésumé :
Particle dispersions provide a promising tool for the engineering of functional materials that exploit self-assembly of complex structures. Dispersion made from magnetic colloidal particles is a great choice; they are biocompatible and remotely controllable among many other advantages. However, their dominating dipolar interaction typically limits structural complexity to linear arrangements. This paper shows how a magnetostatic equilibrium state with noncollinear arrangement of the magnetic moments, as reported for ferromagnetic Janus particles, enables the controlled self-organization of diverse structures in two dimensions via constant and low-frequency external magnetic fields. Branched clusters of staggered chains, compact clusters, linear chains, and dispersed single particles can be formed and interconverted reversibly in a controlled way. The structural diversity is a consequence of both the inhomogeneity and the spatial extension of the magnetization distribution inside the particles. We draw this conclusion from calculations based on a model of spheres with multiple shifted dipoles. The results demonstrate that fundamentally new possibilities for responsive magnetic materials can arise from interactions between particles with a spatially extended, anisotropic magnetization distribution.
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Mozul, K., A. Ishchenko, A. P. Kryshtal, L. P. Olkhovik et Z. I. Sizova. « Magnetic Anisotropy of Ultra-small Nanocrystals of CoFe2O4 ». Thesis, Sumy State University, 2012. http://essuir.sumdu.edu.ua/handle/123456789/35365.
Texte intégralRésumé :
Ferrimagnetic nanoparticles of CoFe2O4 with dimensions of 4-16 nm were synthesized by pyrolysis of a
mixture of acetylacetonates of iron and cobalt. In the temperature range 300-500 K investigated field dependence
of magnetization up o 18 kOe. Found a significant contribution of "surface" anisotropy to the effective
anisotropy of the nanoparticles.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/35365
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Mireles, Hector Córdova. « Step induced magnetic anisptropy of Fe/W(100) / ». Digital version:, 2000. http://wwwlib.umi.com/cr/utexas/fullcit?p9992874.
Texte intégral