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Soares, Gabriel 1988. "Competição entre anisotropias perpendiculares em bicamadas de CoCrPt/Ni resolvida por ressonância ferromagnética". [s.n.], 2015. http://repositorio.unicamp.br/jspui/handle/REPOSIP/276925.
Pełny tekst źródłaDissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin
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Resumo: Desde a descoberta dos filmes finos magnéticos com anisotropia perpendicular, ou PMAs (Perpendicular Magentic Anisotropy, em inglês), se tornou possível aumentar drasticamente a densidade de dados em discos rígidos. Uma maneira de continuar a incrementar a densidade final de dados seria desalinhar o eixo de anisotropia uniaxial e o campo magnético de escrita. Para isso, necessita-se de materiais com anisotropia magnética intermediária, i.e, na qual o seu eixo fácil está entre a longitudinal ao plano e a sua perpendicular. A situação ótima seria com o eixo fácil a 450, teoricamente dobrando a densidade final. No entanto, fabricar grãos ou filmes com esta propriedade não é viável do ponto de vista industrial. Todavia, bicamadas podem ser utilizadas para imitar este comportamento, onde um filme com anisotropia longitudinal é depositado sobre um PMA. Além do mais, esta abordagem apresenta a possibilidade de maior velocidade de escrita. Bicamadas de CoCrPt/Ni podem ser utilizadas para este propósito, com a camada de CoCrPt como PMA e a de Ni com anisotropia longitudinal. O experimento de ressonância ferromagnética permite, a priori, resolver as contribuições de anisotropia de cada camada no sistema de bicamadas, juntamente com os mecanismos de amortecimento magnético. O objetivo principal desta dissertação de mestrado é introduzir os conceitos de experimentação e teoria desta técnica, e aplicá-los nas bicamadas de CoCrPt/Ni. Neste trabalho foram utilizados filmes já depositados por sputtering com 10 nm de Co66Cr22Pt12, seguidos de 5 a 40 nm de Ni, para investigar o efeito da camada com anisotropia longitudinal sobre o PMA. Um outro conjunto de amostras contém um espaçador de Ti entre as camadas magnéticas, a fim de se estudar a interação na interface entre elas. Os experimentos de ressonância foram realizados em varredura de frequência num analisador de rede vetorial com campos magnéticos aplicados longitudinais ou perpendiculares e numa cavidade de banda X (9,54GHZ) em função da orientação do campo aplicado. Foi verificado qualitativamente o decréscimo da energia de anisotropia longitudinal com menores espessuras de Ni nas bicamadas. No entanto, não foi possível observar a linha de absorção da camada de CoCrPt, e consequentemente, encontrar unequivocamente as constantes de anisotropia do sistema, muito menos confirmar o comportamento multiaxial encontrado. Não obstante, foi confirmado a natureza de curto alcance da interação entre as camadas. Nova experimentação em outras bandas de frequência é necessária, afim de identificar a absorção do CoCrPt. Uma perspectiva futura interessante é o estudo da largura de linha e o amortecimento magnético do sistema devido a interação envolvida
Abstract: Since the advent of perpendicular magnetic anisotropy media, or PMAs, it was possible to greatly enhance the data density of hard drives. A method to push forward the data density increase is to misalign the anisotropy and applied field axes. This can be achieved by tilted media, i.e., in which the anisotropy axis lays somewhere between in-plane (IP) and out-of-plane (OOP). The optimal condition is with the axis tilted by 45\textsuperscript{0}. However, produce this kind of material is not practible at industrial scale. Thus, composite media can be used to mimic this behavior, where an in-plane anisotropy media is deposited on top of a PMA. Nonetheless, this approach allows to reach faster switching rates. CoCrPt/Ni bilayers may be used for this purpose, with the CoCrPt serving as the PMA and the Ni as the IP anisotropy layer. The ferromagnetic resonance experiment (FMR) can be used to obtain information about the magnetic anisotropy, being able to characterize its constant for each layer in the bilayer system, together with the damping mechanisms. Therefore, the main goal of this master thesis is to introduce the FMR theory and experimentation and apply them in this system. The used films were already deposited by sputtering with 10 nm of Co.66Cr.24Pt.12 under a 5 to 40 nm Ni layer. Another set of samples with a Ti layer between the magnetic layers provides information of the interface coupling. The FMR experiments were partly performed in a broadband vector network analyzer (VNA) with applied magnetic fields IP or OOP, and in a X-band (9,54 GHz) cavity as function of the orientation of the applied magnetic field. It was qualitatively verified a decrease of the longitudinal magnetic anisotropy energy as a function of the Ni thickness in the bilayer system. However, the CoCrPt single layer absorption could not be observed, making it impossible to unequivocally identify each anistropy constant. Nonetheless, the short-range nature of the coupling was confirmed. Further experimentation in higher frequencies is needed in order to find the CoCrPt absorption. Also, promising effects in the damping mechanism due to the coupling are expected
Mestrado
Física
Mestre em Física
2013/1186360
CAPES
Villuendas, Pellicero Diego. "Magnetic deflagration in Mn₁₂-ac and Nd₅Ge₃ : new techniques and phenomena". Doctoral thesis, Universitat de Barcelona, 2016. http://hdl.handle.net/10803/396118.
Pełny tekst źródłaEl objetivo que persigue esta tesis es impulsar el estudio de las deflagraciones magnéti-cas gracias, por una parte al descubrimiento del fenómeno en un sistema nuevo y pro-metedor como es el compuesto intermetálico Nd5Ge3, y por otra a la presentación de un método nuevo de medición de las dependencias espacio-temporales de las mismas utilizando técnicas magneto-ópticas. Manteniendo el hilo conductor del fenómeno de las deflagraciones magnéticas, esta tesis doctoral se divide en dos partes. En la primera parte presento mis investigaciones en el estudio del sistema Mn12—ac. A partir de los tratamientos de los videos obtenidos se confirma la presencia de deflagraciones magnéticas. La segunda parte de la tesis está dedicada al compuesto intermetálico Nd5Ge3. Éste compuesto se trata de uno de los pocos sistemas en los que mediante un campo magnéti-co externo se induce espontáneamente un estado ferromagnético (FM) con gran irre-versibilidad proviniendo de un estado antiferromagnético (AFM). Además, los cambios magnéticos que experimenta el sistema, tanto dicha transición AFM—>FM como la in-versión de la magnetización en el estado FM, ocurren de forma muy abrupta, siendo también uno de los escasos sistemas que presenta esta propiedad. Dedico tres capítulos al estudio de sus propiedades magnéticas, térmicas y eléctri-cas, tanto estáticas como dinámicas. En esas medidas encuentro fenómenos interesantes, desde generación espontánea de voltaje durante las deflagraciones magnéticas, hasta la aparición de saltos espontáneos de la magnetización con el tiempo (manteniendo la tem-peratura y el campo magnético constantes), pasando por la obtención de términos de origen antiferromagnético en la dependencia térmica de la capacidad calorífica del estado ferromagnético saturado, o una magnetorresistencia gigante entre ambos estados, entre otros. En el sexto capítulo, las medidas experimentales confirman la existencia del fenómeno de la deflagración magnética en ambas fases, AFM y FM. La velocidad de propagación del frente obtenida en la teoría de deflagraciones se ajusta bien a los datos experimen-tales. Utilizando la bondad del ajuste, extrapolamos la velocidad teórica hacia campos magnéticos elevados y encontramos la posibilidad de que ésta iguale o supere la velocidad del sonido en el material. Lo más remarcable es que esta posible transición se observa en la extrapolación para campos menores de 50 kOe. Por lo que, en principio, reduciendo la temperatura podríamos ser capaces de obtener medidas de dicha transición. Sin em-bargo, el estudio de las deflagraciones espontáneas en función de la temperatura llevado a cabo en un criostato de dilución resultó un claro ejemplo de serendipia. En vez de alcanzar velocidades supersónicas, lo que encontré fueron unas discontinuidades de salto en los campos de deflagración espontánea no predichas. Por lo tanto, el capítulo pasa a enfocarse en su estudio, concluyendo que su origen está relacionado con propiedades intrínsecas del Nd5Ge3.
Fácio, Thais Josiani Silva. "Transporte eletrônico em estruturas híbridas : uma aproximação usando funções de green de não-equílibrio /". Ilha Solteira, 2016. http://hdl.handle.net/11449/136339.
Pełny tekst źródłaBanca: Victor Ciro Solano Reynoso
Banca: Rodrigo Yoshikawa Oeiras
Resumo: Neste trabalho é proposto a investigação teórica de uma junção formada por metal supercondutor e um metal ferromagnético acoplados através de dois canais. O primeiro canal consiste de um acoplamento direto entre os metais, enquanto que o segundo canal é formado através um ponto quântico composto por um nível discreto. Para isso utilizamos como ferramenta as funções de Green de não-equilíbrio, por meio das quais obtemos o cálculo e as curvas para a corrente elétrica, números de ocupação e transmitância. É demonstrado que podemos alterar o tipo de spin no ponto quântico, e este sobrevive a presença de um campo magnético por meio de um efeito de interferência. Por meio deste resultado, é possível manipular um spin em um ponto quântico por meio de variáveis externas, o que pode ser de interesse em aplicações na computação quântica
Abstract: In this work is proposed the theorical investigation of a double-path junction formed by a ferromagnetic and a supercondutor lead. The first path connects superconductor and ferromagnet by an insulator barrier while in the second path these metals are connected by a single level quantum dot. We have used the nonequilibrium Green's functions to perform the calculations as well as to obtain the curves for electrical current, occupation numbers and transmittance. It is shown that the spin within the quantum dot can be manipulated by means of external parameters which can be of interesed in quantum computation applications
Mestre
Coy, Emerson. "Growth and characterization of new multiferroic materials". Doctoral thesis, Universitat de Barcelona, 2016. http://hdl.handle.net/10803/395177.
Pełny tekst źródłaLos materiales multiferroicos, en los que dos o más ordenes ferroicos tienen lugar en la misma fase, ha despertado gran interés en los últimos años debido, no solo al hecho de explorar nuevas propiedades físicas en los materiales, sino también a las implicaciones de las nuevas propiedades funcionales en las aplicaciones tecnológicas. De dichos materiales resultan especialmente interesantes aquellos que presentan un orden ferroeléctrico (FE) y ferromagnético (FM) debido a su aplicación directa en dispositivos magnetoelectrónicos. En este ámbito los materiales multiferroicos podrían tener una gran relevancia en una nueva generación de memorias magnéticas RAM (MRAM) de control eléctrico, no volátiles, en las que, si el acoplamiento magnetoeléctrico es suficientemente grande, se podría modificar el estado magnético no con un campo magnético sino con un campo eléctrico. Este hecho permitiría una reducción radical en el consumo de potencia y favorecería a su vez una mayor integración (la principal desventaja de las MRAMs para competir en el mercado), ya que el campo eléctrico, a diferencia del campo magnético, puede aplicarse de forma muy localizada. Por otro lado, dichos materiales multiferroicos podrían emplearse en una nueva generación de uniones túnel, en las que el carácter ferroeléctrico y ferromagnético permitiría codificar información en cuatro estados resistivos en lugar de en dos, como viene siendo hasta ahora en las convencionales uniones túnel magnéticas o ferroeléctricas, dando lugar a una nueva generación de memorias de cuatro estados. Los materiales con estructura perovskita, ABB '03, (A=Tierra Rara, Bismuto, Plomo e Ytrio) ofrecen una gran versatilidad a la hora de diseñar materiales funcionales debido a la gran variedad de cationes A, B y B' compatibles con tal estructura. Sin embargo en el caso de R(NiMn)03, estos óxidos han sido poco estudiados y muchos carecen de estudios detallados tanto en forma másica como en capa fina. Esta selección de cationes en la posición B y B' parece transformar la estructura perovskita la cual típicamente presenta un ordenamiento paramagnético (PM) en FM a temperaturas inferiores a la ambiente. El carácter multiferroico de estos materiales es típicamente aportado por el catión A en la formula perovskita, el cual puede ser un átomo de Bi, o Pb, para crear un multiferroico tipo 1. En los materiales de este tipo, por ejemplo el Bi2NiMnO6, la ferroelectricidad y el ferromagnetismo provienen de fuentes diferentes, el carácter FE es aportado por el catión A con -lone pairs electrons-, los cuales son electrones libres en la banda de valencia que no participan en las reacciones químicas del compuesto, mientras la combinación Ni2+ (d8) and Mn4+ (d3) aporta el FM. Pese al carácter multiferroico de estos materiales su acoplamiento magnetoelectrico, indispensable para sus aplicaciones industriales futuras, es débil, puesto que su FE y FM provienen de efectos independientes. Por otra parte la inducción de FE por distorsiones geométricas de la celda perovskitas, como es el caso de YMnO3 (YMO), es un caso interesante de considerar ya que la rotación de los octaedros Mn05 genera un cambio estructural importante, en el cual los oxígenos se desplazan a una posición más cercana al Y, esto sumado a una larga interacción de los dipolos conduce al material a un estado FE estable. Además la deformación de la celda genera un débil FM en este material, el cual proviene un pequeño giro en los espines del Mn ya sea debido a un dopaje con Li o por la deformación de la celda. Este comportamiento podría resultar interesante en la familia de perovskitas R(NiMn)03 las cuales presentan un fuerte FM. Esta tesis está dedicada al estudio de la perovskitas R(Ni0.5Mn0.5)O3 (Y, Sm, Nd y Pr) y Bi(Fe0.5Mn0.5)O6 crecidas en capa fina usando la técnica de depósito mediante ablación por láser pulsado. En primer lugar, esta tesis se centra en el crecimiento y caracterización de capas finas del compuesto Y(Ni0.5Mn0.5)O3 (YNMO) sobre substratos de titanato de estroncio, SrTiO3(001) (STO). Se estudia la influencia de los parámetros de depósito tales como temperatura, fluencia y frecuencia de ablación sobre la morfología y la calidad cristalina de las capas obtenidas. El estudio pone de manifiesto que las capas de YNMO crecidas sobre substratos de STO(001,011 y 111) son epitaxiales de YNMO y que la calidad cristalina y las relaciones epitaxiales entre la capa y el substrato son semejantes a las obtenidas en el compuesto YMO. En particular se observa un único dominio cristalino fuera del plano independientemente de la orientación del sustrato, mientras que dentro del plano se presentan varios dominios cristalinos. Por otra parte, los estudios de composición química revelan una difusión de Ti desde el sustrato hacía la capa de YMNO cuando se utilizan substratos STO(111).. Una vez optimizadas las condiciones de crecimiento del compuesto YNMO, se estudian sus propiedades magnéticas y dieléctricas. Todas las capas presentan una transición de fase paramagnetica a ferromagnética a una temperatura alrededor de 95K con un momento magnético de YNMO(001)= 4.35µB/f.u, YNMO(100) = 4,4 µB/f.u and YNMO(101) = 3,7µB/f.u, confirmando el carácter ferromagnético de las muestras. La caracterización dieléctrica revela el carácter FE de las capas de YNMO y lo que es más interesante, la existencia de anisotropía dieléctrica en las capas, ésta se pone de manifiesto en la ausencia de respuesta FE en capas YNMO sobre STO(001) que contrasta con la fuerte respuesta de las capas de YNMO sobre STO(111). Esta anisotropía puede tener su origen, a la luz de los recientes estudios teóricos, en el carácter impropio de la ferroelectricidad observada, a la luz de recientes estudios teóricos. La coexistencia de FM y FE muestra de manera conclusiva el carácter multiferroico del compuesto YNMO. En segundo lugar se han realizado estudios similares a los anteriores para el caso de capas finas de los compuestos del tipo R(Ni0.5Mn0.5)O3 (Sm, Nd y Pr) crecidas en STO(001). En este caso la influencia de la temperatura de depósito resulta ser un factor importante para la obtención, en todos los compuestos estudiados, de crecimiento epitaxial. Se observa que el cociente b/a entre las constantes red juega un factor importante en la epitaxia de las capas, siendo este cociente un factor determinante en el crecimiento mono-dominio o multi-dominio de las capas. Todas las muestras presentan transiciones PM a FM a temperaturas alrededor de 190K. Por último, se han crecido y estudiado capas finas del compuesto Bi(Fe0.5Mn0.5)O6 depositadas sobre STO(001). Las capas obtenidas son epitaxiales y crecen sometidas a estrés inducido por el substrato. Presentan comportamiento FM a temperatura ambiente pero con una débil señal de 7,42 emu/cm3 y 0,4 µB/f.u(Fe-Mn). La caracterización dieléctrica pone de manifiesto la influencia, a temperaturas superiores a la ambiente, de la presencia de campo magnético sobre las propiedades dieléctricas.
Boerner, Eric D. "Simulation of thermal decay and dynamic relaxation in ferromagnetic materials /". Diss., Connect to a 24 p. preview or request complete full text in PDF format. Access restricted to UC IP addresses, 2000. http://wwwlib.umi.com/cr/ucsd/fullcit?p9956455.
Pełny tekst źródłaPaula, Fagner Muruci de 1983. "Coerência quântica macroscópica em sistemas ferromagnéticos". [s.n.], 2011. http://repositorio.unicamp.br/jspui/handle/REPOSIP/277293.
Pełny tekst źródłaTese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin
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Resumo: Por muitas décadas, fenômenos quânticos foram observados com partículas microscópicas, tais como átomos, elétrons e fótons. No entanto, avanços na fabricação e controle de sistemas físicos com dimensões extremamente reduzidas vêm permitindo a manifestação de eventos quânticos em proporções gigantescas. Por exemplo, existem evidências de superposições quânticas com uma supercorrente composta por bilhões de elétrons num SQUID (Superconducting Quantum Interference Device). Motivados por tais evidências, nosso objetivo reside na busca de novos dispositivos capazes de exibir efeitos quânticos macroscópicos. Em particular, estamos interessados em sistemas ferromagnéticos que manifestem CQM (Coerência Quântica Macroscópica), isto é, ferromagnetos nos quais o campo de magnetização tunela periodicamente no tempo entre dois estados topologicamente distintos e degenerados. Nesta tese, sugerimos dois dispositivos: um ?o ferromagnético no qual uma parede de domínio tunela entre dois centros de aprisionamento arti?ciais; e um MQUID (Magnetic Quantum Interference Device), isto é, um análogo magnético do SQUID que permite efeitos de tunelamento com uma ¿supercorrente¿ formada por vórtices de spin. Esses dispositivos são úteis não só na exploração dos limites de validade da mecânica quântica, mas também abrem novas possibilidades de implementação de um bit quântico
Abstract: For many decades, quantum phenomena were observed with microscopic particles, such as atoms, electrons and photons. However, advancements in manufacture and control of physics systems with very small dimensions have allowed verifying quantum events in large proportions. For instance, there are evidences of quantum superposition with a supercurrent formed by billions of electrons on a SQUID (Superconducting Quantum Interference Device). Such evidences have driven our work in a way to investigate new devices that are capable to exhibit macroscopic quantum effects. In particular, we are interested in ferromagnetic systems that present MQC (Macroscopic Quantum Coherence), in other words, ferromagnets in which the magnetization ?eld tunnels periodically in time between two distinct and degenerate topological states. In this thesis, we have suggested two devices: a ferromagnetic wire in which a domain wall tunnels between two arti?cial pinning centers; and a MQUID (Magnetic Quantum Interference Device) that is a magnetic device analogous to SQUID that permit quantum tunneling effects with a supercurrent formed by spin vortices. These devices are useful to explore the limits of validity of quantum mechanics, as well they open new possibilities to put into operation a quantum bit
Doutorado
Física
Doutor em Ciências
Wang, Yutian. "Defect-induced ferromagnetism in SiC". Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2015. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-164623.
Pełny tekst źródłaStatuto, Nahuel. "Magnetic Excitations Induced by Surface Acoustic Waves and Spin-Polarized Currents". Doctoral thesis, Universitat de Barcelona, 2019. http://hdl.handle.net/10803/667710.
Pełny tekst źródłaLa tesis gira en torno al estudio de la dinámica de la magnetización en capas y multicapas delgadas ferromagnéticas. Sin embargo, los sistemas estudiados son diversos y pueden clasificarse por la técnica utilizada para la excitación de la dinámica de la magnetización. Este hecho queda plasmado en la estructura de la tesis que consta de una introducción general, Capítulo 1, y luego de dos partes independientes y separadas, a su vez, en varios capítulos. El orden en la exposición de los resultados pretende seguir una linea lógica para su compresión. Como contrapartida, los resultados son presentados sin seguir un orden cronológico. La primera parte de la tesis estudia la dinámica de la magnetización inducida por la aplicación de tensión dinámicamente sobre el material magnético, que al deformarlo induce en él un cambio en la dirección e intensidad de la anisotropía magnética. Por lo tanto, los estados magnéticos se ven afectados por esta variación y cambian para alinearse con la nueva dirección de anisotropía magnética induciendo dinámica en la magnetización. La segunda parte de la tesis estudia la dinámica de la magnetización inducida por la aplicación de corriente polarizada a través del material magnético que intercambia momento magnético con los espines magnéticos de los electrones de la corriente. Para que esta transferencia de momento magnético sea efectiva la densidad de corriente ha de ser elevada (~106-107 A/cm2) y para conseguirla se reduce hasta los 50-200 nm el diámetro del contacto eléctrico. Los materiales ferromagnéticos con grosor nanométrico usados en esta tesis son materiales magnéticos usados ampliamente en la investigación. Aparte del interés puramente científico, estos materiales son potencialmente aplicables en telecomunicaciones o tecnologías del almacenaje y transmisión de información a altas velocidades.
Pereira, Estéfani Marchiori. "Interação entre ferromagnetos e supercondutores em nanoestruturas fabricadas por ablação a laser e litografia por feixe de elétrons". Niterói, 2017. https://app.uff.br/riuff/handle/1/3949.
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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior
Conselho Nacional de Desenvolvimento Científico e Tecnológico
Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro
Dois fenômenos de natureza antagônica juntos em um sistema híbrido podem apresentar propriedades muito diferentes, e um exemplo é o sistema híbrido de supercondutividade e ferromagnetismo, onde diversos novos fenômenos podem ser observados, como vórtices espontâneos. Aqui, dois sistemas híbridos supercondutor-ferromagneto foram estudados: Um consistindo de uma camada de Nb(200 nm) entre duas camadas de nanopartículas ferromagnéticas de Ni( ∼5 nm), preparadas por ablação a laser, com os gases Ar e O2 para produção das nanopartículas; O outro consiste de uma primeira camada feita de nanodiscos ferromagnéticos de Py( ∼1 m) desenvolvidos com litografia por feixe de elétrons, com uma disposição de rede quadrada com determinadas distâncias entre nanodiscos adjacentes, cobertos por uma segunda camada de Al2O3, ambos depositados por pulverização catódica, e por fim uma terceira camada supercondutora de Nb(200 nm) preparada por ablação a laser. As nanopartículas de Ni no primeiro sistema estão em contato direto com a camada de Nb e como resultado, o efeito de proximidade está presente no sistema. Diferentemente, os nanodiscos de Py no segundo sistema estão eletricamente isolados da camada de Nb, que pode eliminar o efeito de proximidade, assim a interação entre nanodiscos magnéticos e o Nb supercondutor ocorre somente através dos campos magnéticos remanescentes dos nanodiscos de Ni. A microestrutura estudada mostra que as nanopartículas feitas em gás Ar e O2 possuem formatos muito diferentes: uma (preparada em Ar) é cubica e a outra (preparada em O2) é esférica. Os diferentes formatos das nanopartículas de Ni apresentam influência muito diferente sobre as propriedades supercondutoras da camada de Nb: a amostra com nanopartículas de Ni(Ar) não apresenta uma transição de vortex vidro e a amostra com nanopartículas de Ni(O2) mostra um estado de vortex vidro bem claro sem qualquer campo magnético externo aplicado, indicado pelas medidas V(I). No segundo sistema, as medidas de transporte indicam a formação de clusters de vórtices na camada supercondutora sobre os nanodiscos magnéticos devido aos momentos magnéticos deles, e os vórtices induzidos por um único nanodisco podem formar uma fase de vortex vidro. A dimensão do espaçamento entre discos desempenha também um papel muito importante. A amostra com uma distância muito grande entre nanodiscos não mostrou uma curva V(I) com formato ’S’ mas possui uma fase vortex vidro; quando diminui a distância entre discos, as curvas V(I) próximas à temperatura de transição vortex vidro deformaram para um formato ’S’, indicando que os vórtices induzidos pelos diferentes nanodiscos estão interagindo uns com os outros quando as distâncias entre discos são menores do que um valor crítico.
Two phenomena with antagonistic nature together in a hybrid system can have very different properties and one of the samples is the hybrid of superconductivity and ferromagnetism in which many new phenomena can be observed, such as spontaneous vortices. Here two superconducting-ferromagnetic hybrid systems have been studied: one consists of a Nb layer(200 nm) between two layers of ferromagnetic Ni nanoparticles( ∼5 nm), which is prepared by pulsed laser deposition(PLD) with Ar and O2 for the production of Ni nanoparticles; the other consists of a first layer made of ferromagnetic permalloy (Py) nanodisks( ∼1 m)developed by e-beam lithography, with the arrangement of square lattice with certain distances between two adjacent nanodisks, covered by a second layer of Al2O3, both deposited by magnetron sputtering, and finally a third layer of superconducting Nb(200 nm) prepared by PLD. The Ni nanoparticles in the first system are in direct contact with the Nb layer and as a result, the proximity effect in the system is presented. In contrast, the Py nanodisks in the second system are electrically insulated from the Nb layer which can eliminate the proximity effect, thus the interaction between the magnetic nanodisks and superconducting Nb is through the magnetic stray fields of Ni nanodisks only. The microstructure study shows that the nanoparticles made in Ar and O2 gases have very different shapes: one (prepare in Ar) is cubic and the other (prepared in O2) is spherical. The different shapes of the Ni nanoparticles have very different influence on the superconducting properties of the Nb layer: the sample with Ni (Ar) nanoparticles does not show a vortex glass transition and the sample with Ni (O2) nanoparticles shows a very clear votex glass state without any external magnetic field applied, indicated by the V(I) measurements. In the second studied system, the transport measurements indicate the formation of vortex clusters in the superconducting layer on the top of the magnetic nanodisks due to the magnetic moments of them and the vortices induced by a single nanodisk may form a vortex glass phase. The spacing dimension between the disks plays a very important role as well. The sample with very large distance between the nanodisks does not show an ’S’ shape V(I) curve but has a vortex glass phase; when decrease the distance between the disks, the V(I) curves near the vortex glass transition temperature deformed to a ’S’ shape, indicating that the vortices induced by different nanodisks are interacting with each other when the distance between the disks are smaller then an critical value.
Yin, Shuangye. "Ferroelectric and Ferromagnetic Alloy Clusters in Molecular Beams". Diss., Georgia Institute of Technology, 2006. http://hdl.handle.net/1853/11463.
Pełny tekst źródłaRamos, Rudnei de Oliveira. "Paredes de domínios em ferromagnetismo". Universidade de São Paulo, 1989. http://www.teses.usp.br/teses/disponiveis/43/43132/tde-01072015-153458/.
Pełny tekst źródłaWe determine the surface tension of a domain wall defect by using the semi classical approach in the context of field theory at finite temperature for the ? (?2)2 model with O(N) invariant interaction obtaining the critical temperature Tc of phase transition and the calculation of the critical exponents ? and ? applying then the obtained results to a description of a phenomenological model describing ferromagnetism.
Lussier, Alexandre Francois. "Ferromagnetism in cobalt-doped titanium dioxide". Diss., Montana State University, 2005. http://etd.lib.montana.edu/etd/2005/lussier/LussierA1205.pdf.
Pełny tekst źródłaBrydon, Philip M. R. "Coexistence of superconductivity and excitonic ferromagnetism". View electronic text, 2002. http://eprints.anu.edu.au/documents/disk0/00/00/07/63/index.html.
Pełny tekst źródłaAvailable via the Australian National University Library Electronic Pre and Post Print Repository. Title from title screen (viewed Mar. 27, 2003). "A thesis submitted for the degree of Honours in theoretical physics at the Department of Physics and Theoretical Physics, The Faculty of Science, The Australian National University" "Honours thesis submitted November 2002" Includes bibliographical references.
Knothe, Angelika Hildegard [Verfasser], i Andreas [Akademischer Betreuer] Buchleitner. "Quantum hall ferromagnetism in multicomponent systems". Freiburg : Universität, 2017. http://d-nb.info/1142738698/34.
Pełny tekst źródłaKnothe, Angelika Hildegard. "Quantum Hall Ferromagnetism in Multicomponent Systems". Thesis, Université Paris-Saclay (ComUE), 2017. http://www.theses.fr/2017SACLS595/document.
Pełny tekst źródłaThe present thesis deals with two-dimensional quantum Hall systems in which the electrons may be endowed with multiple discrete degrees of freedom. Quantum Hall ferromagnetism provides a framework to treat these electronic degrees of freedom as effective spins and isospins of the electrons. Different orderings of the electronic spins and isospins then characterise different possible phases of the system. Using this analogy, various aspects of the two-dimensional systems in the quantum Hall regime are explored theoretically by studying the corresponding spin and isospin structure. The work consists of three parts in which different two-dimensional materials are investigated in the quantum Hall regime. In any of the three projects presented within this thesis, Hartree Fock theory is employed to study the multicomponent spin and isospin system at the mean field level. All our considerations are stimulated directly by experimental results. We draw our main motivation from the key idea that purely theoretical investigations of abstract models may us allow to obtain deeper insights into the physical processes and mechanisms that determine the properties of the materials. This, in turn, we hope to allow conclusions about the experiments by providing possible explanations of the phenomena observed, as well as prospects for future investigations
Igarashi, Ricardo Noboru. "Estudo teórico de nanoestruturas magnéticas em superfícies metálicas". Universidade de São Paulo, 2012. http://www.teses.usp.br/teses/disponiveis/43/43134/tde-26032013-144125/.
Pełny tekst źródłaWe use the first principles RS-LMTO-ASA (Real Space - Linear Muffin- Tin Orbital - Atomic Sphere Approximation) method, in the framework of the Density Functional Theory and implemented to calculate noncollinear magnetic structures, to investigate the magnetic properties of nanostructures adsorbed on metallic surfaces. First, due to presence of the complex magnetic properties, we investigated magnetic nanostructures deposited on a ferromagnetic substrate. We have considered a variety of nanostructures such as adsorbed wires, pyramids, at and intermixed clusters with sizes varying from two to nine atoms deposited on Fe(001) and Fe(110). Our calculations reveal the long-range nature of exchange interactions between Mn-Mn and Mn-Fe atoms. Moreover, the presence of the strong dependence of these interactions on the local environment, the magnetic frustration, and the effect of spin-orbit coupling lead to the possibility of realizing complex noncollinear magnetic structures such as helical spin spiral and half-skyrmion. Finally, we also investigated FexCo1-x nanowires deposited on Pt(111) surface aiming to investigate materials with large local magnetic moment. Our results reveal that the Fe and Co spin magnetic moment are independent of the Fe concentration with the enhancement of the spin magnetic moment when compared with the FeCo bcc alloys, while the average spin magnetic moment is a linear function of the Fe concentration. This is in contrast to the Slater-Pauling model observed in the FeCo bcc alloys. The average orbital magnetic moment shows a linearly decreasing behavior with the Fe concentration which is in contrast to the behavior of FexCo1-x monolayer on Pt(111) surface.
Labarta, Amílcar. "Estudio de las propiedades magnéticas de ftalocianinas y sistemas ferromagnéticos diluidos". Doctoral thesis, Universitat de Barcelona, 1985. http://hdl.handle.net/10803/665977.
Pełny tekst źródłaMenéndez, Dalmau Enric. "Generation of ferromagnetism on non-magnetic materials". Doctoral thesis, Universitat Autònoma de Barcelona, 2008. http://hdl.handle.net/10803/3417.
Pełny tekst źródłaEls aliatges de Fe60Al40 (percentatge atòmic) i els acers inoxidables austenítics posseeixen una combinació de propietats estructurals i magnètiques que els converteix en materials amb cert potencial per a ser litografiats magnèticament. Des d'un punt de vista magnètic, mentre que els aliatges de Fe60Al40 ordenats atòmicament són paramagnètics a temperatura ambient, els aliatges de Fe60Al40 desordenats a nivell atòmic presenten un comportament ferromagnètic. Pel que fa als acers inoxidables austenítics, a partir de deformació mecànica, es pot induir la transformació en estat sòlid de la fase austenita (paramagnètica) a la fase martensita (ferromagnètica) en aquests aliatges ferris. A més, els processos de nitruració a temperatura moderada en acers inoxidables austenítics permeten transformar parcialment la fase austenita en la fase "austenita expandida", que és una solució sòlida sobresaturada de nitrogen que presenta un comportament ferromagnètic.
Aquesta Tesi està basada en la generació de xarxes ordenades d'entitats ferromagnètiques a escala micro/nanomètrica, dins d'una matriu paramagnètica, en la superfície d'aliatges de Fe60Al40 i acers inoxidables austenítics. Aquest propòsit s'assoleix aprofitant les transicions magnètiques que tenen lloc en aquests materials després de sotmetre'ls a processos de deformació plàstica local (nanoindentació) i irradiació controlada amb ions (αs de feixos d'ions focalitzats i irradiació amb ions de gasos nobles a través de màscares en el cas del Fe60Al40 i processos de nitruració a través de màscares d'irradiació en els acers inoxidables austenítics). Cal esmentar que també s'ha dut a terme un estudi detallat de les modificacions a nivell estructural, mecànic i magnètic que ocorren en aquests materials una vegada s'han deformat mecànicament o irradiat amb ions.
In recent years, intense research is being pursued in the development of novel methods for the fabrication of arrays of ordered magnetic nanostructures. This is motivated, in part, by the technological applications of sub-micron magnetic structures, ranging from biomedicine to recording media, but it is also due to fundamental scientific reasons, since the behavior of magnetic materials at this length scale is often significantly different from that in the bulk.
Fe60Al40 (at. %) alloys and austenitic stainless steels show an interesting combination of magnetic and structural properties, which makes them turn into potential candidates to be magnetically patterned. Namely, from the magnetic point of view, whereas atomically ordered Fe60Al40 (at. %) is paramagnetic at room temperature, disordered Fe60Al40 becomes ferromagnetic. Concerning austenitic stainless steels, due to mechanical deformation, a phase transformation from the paramagnetic austenite phase to the ferromagnetic martensite phase can occur in these ferrous alloys. In addition, nitriding of austenitic stainless steels at moderate temperatures is able to partially transform the austenite phase into the supersaturated nitrogen solid solution, often called in the literature "expanded austenite" phase, which shows ferromagnetic behavior.
This Thesis is mainly focused on the generation of ordered arrays of micro/nanoscaled ferromagnetic entities (i.e., magnetic patterning), embedded in a paramagnetic matrix, at the surface of either Fe60Al40 (at. %) alloys or austenitic stainless steels. This is accomplished by taking advantage of the magnetic transitions which occur in these alloys upon local plastic deformation (nanoindentation) and controlled ion irradiation (focused ion beam and broad beam noble gas ion irradiation through shadow masks in FeAl alloys and ion beam nitriding through shadow masks in austenitic stainless steels). Furthermore, a detailed study of the structural, mechanical and magnetic changes which take place in these materials upon either mechanical deformation or ion irradiation is presented.
Saubi, Benjamin Haubiih. "Studies in ferromagnetism using electron polarization analysis". Thesis, University of Sussex, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.249391.
Pełny tekst źródłaTalut, Georg. "Ferromagnetismus in mit Fe implantierten GaN und TiO2". Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2010. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-27497.
Pełny tekst źródłaIn der vorliegenden Arbeit wurde versucht, mittels Ionenimplantation verdünnte magnetische Halbleiter auf der Basis von GaN und TiO2 herzustellen. In den meisten Fällen konnte anhand von Charakterisierungen der strukturellen und magnetischen Eigenschaf- ten nachgewiesen werden, dass der ferromagnetische Zustand auf das Vorliegen von entweder spinodaler Entmischung oder kristalliner Ausscheidungen zurückgeführt werden kann. Im Fall von Fe-implantiertem GaN konnten spinodale Entmischung, epitaktisch ausgerichtete alpha-Fe- oder epsilon-Fe3N-Nanokristallite für den Ferromagnetismus verantwortlich gemacht werden. Daneben wird die Bildung von gamma-Fe beobachtet. Bei TiO2 ist Ferromagnetismus ebenfalls auf die Ausscheidung von epitaktisch orientierten alpha-Fe-Clustern zurückzuführen. In Abhängigkeit von den Prozessparametern bei Temperungsexperimenten bildete sich eine Reihe unterschiedlicher Sekundärphasen. Eine kritische Auseinandersetzung mit den Literaturangaben zeigt die Wichtigkeit des Einsatzes sensitiver, sich ergänzender Messmethoden (wie CEMS, SQUID, XRD, EXAFS), um die Ursache des Ferromagnetismus auf dem Gebiet der verdünnten magnetischen Halbleitern zu finden
Cunha, Anderson Magno Chaves. "Correspondência entre ondas de spin de um ferromagneto em uma rede favo de mel e a banda de energia do grafeno". reponame:Repositório Institucional da UFC, 2014. http://www.repositorio.ufc.br/handle/riufc/8968.
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Spin waves are collective excitations that occur in magnetic materials. These excitations are caused by disturbances in the magnetic system. For example, a small change in temperature causes the precession of a magnetic dipole moment that interacts with neighboring leading to the spread of this disorder. This disturbance has wave character, and can propagate in the direction of any of the nearest neighbors. These waves of spin can be observed by some experimental methods, such as: the inelastic neutron scattering, inelastic scattering of light including Raman and Brillouin scattering, to name a few. The importance of spin waves emerges clearly when magnetoelectronic devices are operated at low frequencies. This situation, the generation of spin waves can sing in a significant loss of energy of these systems, because the excitation of such waves consumes a small part of the energy of the system, becoming important in the innovation process of electronic systems. These waves can be studied using mathematical models like the Heisenberg, Ising, among others. In this model, we can calculate the dispersion relation of the spin waves. The Heisenberg model can be written in terms of operators of creation and destruction through the Holstein-Primakoff transformations. The Hamiltonian that describes the spin waves is now written in terms of bosonic operators. This mathematical description is similar to Tight-Binding Hamiltonian for fermions. This Hamiltonian described, for example, graphene, a material that has recently been discovered and is being treated with much optimism for having a two-dimensional structure that leads to amazing properties. Many possibilities of applications for it have been studied. Our goal here is to make an analogy between the graphene and a magnetic system on a honeycomb lattice. In the magnetic system, we use the Heisenberg model to find the dispersion relations and understand the behavior of the spin waves of the same. While in graphene, we used the Tight-Binding model to find the energy spectrum. Underscoring we use a mathematically identical method for both and found that the curves for power modes have similar behaviors, respecting the particularities of each. Then, we calculate how these modes behave introduction of impurities in substitution sites on one or two lines of the crystal lattice.
Ondas de spin são excitações coletivas que surgem em materiais magnéticos. Essas excitações são causadas por perturbações no sistema magnético. Por exemplo, uma pequena variação na temperatura provoca a precessão de um momento de dipolo magnético que interage com seus vizinhos levando à propagação dessa perturbação. Essa perturbação tem caráter ondulatório, e pode se propagar na direção de qualquer um dos vizinhos próximos. Essas ondas de spin podem ser observadas através de alguns métodos experimentais, tais como: espalhamento inelástico de nêutrons, espalhamento inelástico de luz incluindo espalhamento Raman e Brillouin. A importância das ondas de spin surge claramente quando aparelhos magnetoeletrônicos são operados a baixas frequências. Nessa situação a geração de ondas de spin pode ser um processo significante na perda de energia desses sistemas, pois a excitação de tais ondas consome uma pequena parte da energia do sistema, as tornando importante no processo de inovação dos sistemas eletrônicos. Essas ondas podem ser estudadas através de modelos matemáticos como o de Heisenberg, Ising, dentre outros. Nesse modelo, podemos calcular a relação de dispersão das ondas de spin. O modelo de Heisenberg pode ser escrito em termos de operadores de criação e destruição através das transformações de Holstein-Primakoff. O Hamiltoniano que descreve as ondas de spin é agora escrito em termos de operadores bosônicos. Essa descrição matemática é semelhante ao Hamiltoniano Tight-Binding para férmions. Tal Hamiltoniano descreve, por exemplo, o grafeno, um material que foi descoberto recentemente e vem sendo tratado com muito otimismo, por ter uma estrutura bidimensional que leva a propriedades surpreendentes. Muitas possibilidades de aplicações para ele vêm sendo estudadas. Nosso objetivo aqui é fazer uma analogia entre o grafeno e um sistema magnético em uma rede favo de mel. No sistema magnético, utilizamos o Modelo de Heisenberg para encontrar as relações de dispersão e conhecer o comportamento das ondas de spin do mesmo. Enquanto no grafeno, utilizamos o modelo Tight-Binding para encontrar o espectro de energia. Ressaltando que utilizamos um método matematicamente idêntico para ambos e que as curvas encontradas para os modos de energia são idênticas. Então, calculamos como esses modos se comportam com a introdução de impurezas em substituição em sítios de uma ou duas linhas da rede cristalina.
Iakovlev, Ilia. "Internal Field NMR of Cobalt nanoparticles in catalysts : size effects and particle-support interaction". Electronic Thesis or Diss., Université Paris sciences et lettres, 2022. http://www.theses.fr/2022UPSLS079.
Pełny tekst źródłaThe chemical properties of supported Co nanoparticles depend on their size, morphology, crystal structure, and the strength of interaction with the support. These characteristics reflect themselves in the magnetic properties of ferromagnetic nanoparticles, namely in the distribution of the hyperfine magnetic field across the particle and thus in the resonant frequency of 59Co nuclei in a 59Co Internal Field NMR (IF NMR) experiment. This unconventional NMR technique is a powerful tool for studying the crystal and magnetic structure of ferromagnetic metallic cobalt as well the local environment of Co nuclei. However, the application of this technique is hindered by the complicated nature of the experimental spectra that are subject to contributions from various sources. Thus, the main aim of this work was to establish the effects on 59Co IF NMR spectra of two aspects particularly important in heterogeneous catalysis – particle size distribution and metal-support interaction.The effect of particle size distribution was demonstrated on a model sample of small Co nanoparticles supported on multi-walled carbon nanotubes where the transition of the particles from superparamagnetic to ferromagnetic state was observed using 59Co IF NMR spectroscopy. The temperature of such a transition for an individual particle depends on its volume, making it possible to use 59Co IF NMR to characterize particle size distribution in a sample.The influence of the support surface on the structure of the Co nanoparticles was investigated using samples supported on metastable alumina phases. According to the 59Co IF NMR, the surface of χ-Al2O3 promoted formation of larger Co particles and favored the hcp crystal structure. Using semi-empirical calculations, we suggest that the hydroxyl coverage of the alumina is the main support characteristic determining the Co nanoparticles structure
Law, Sarah Ann. "Magnetic excitations in ZrZnâ†2". Thesis, University of Cambridge, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.279921.
Pełny tekst źródłaLo, Chi Kuen. "Construction of a surface magneto-optical Kerr polarimeter and the study of ultra thin Fe films on Si(111)". Thesis, University of Sussex, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.239543.
Pełny tekst źródłaBroom, Darren Paul. "A structural study of a commercial metal hydride battery electrode material". Thesis, University of Salford, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.272622.
Pełny tekst źródłaJordan, Simon Mark. "The MOKE characterisation of magnetic multilayers". Thesis, University of York, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.284135.
Pełny tekst źródłaKennewell, Kimberly. "Surface and interface anisotropies measured using inductive magnetometry". University of Western Australia. School of Physics, 2008. http://theses.library.uwa.edu.au/adt-WU2008.0243.
Pełny tekst źródłaRichard, Caroline. "Interplay of ferromagnetism and superconductivity : (in hybrid structures)". Thesis, Grenoble, 2013. http://www.theses.fr/2013GRENY066/document.
Pełny tekst źródłaWhile ferromagnetism and conventional superconductivity appear as antagonist phases of nature, the proximity effect in hybrid S/F structures offers a unique opportunity to study their interplay. In particular, spin-triplet odd-frequency superconducting correlations may be induced in a diffusive ferromagnet.In a first part, we study the equilibrium current that may flow in hybrid S/F Josephson junctions. We exhibit signatures of odd-frequency triplet correlations. In particular, we predict the existence of a long range triplet current through a non-collinear bilayer ferromagnet with a peculiar superharmonic current phase relation. This can be viewed as the Josephson effect between a conventional superconductor and an effective triplet superconductor generated at the end of the bilayer ferromagnet. Then, we study the competition between triplet and singlet superconductivity in the temperature dependence of the critical current. Namely, the critical current flowing between two effective triplet reservoirs through a conventional superconducting layer may display a maximum at finite temperature
Murakami, Regina Keiko. ""Novos materiais magnéticos para imãs de alta performance"". Universidade de São Paulo, 2005. http://www.teses.usp.br/teses/disponiveis/43/43134/tde-14022006-182556/.
Pełny tekst źródłaThe aim of the present work was to develop new improved magnetic materials suitable for permanent magnets. Two kinds of materials were studied: (Nd,Sm)5(Fe,MT)17 based materials, were MT is a transition metal and, (Nd,Pr)FeB nanocrystalline materials (exchange spring magnets) with TiC additions. The 5:17 alloys were melted in an arc melting furnace followed by a long annealing (at least 30 days). We tried to improve the magnetic properties by means of chemical substitutions (Ti, Co, Mn, etc.) and/or by addition of interstitial atoms of deuterium or nitrogen. The samples were characterized by means of thermomagnetic analysis (TMA), low temperature magnetometry, X ray and neutron diffraction, and Mössbauer spectroscopy. The main results were: a) increase of Tc temperature (up to 70 ºC) and; b) determination of interstitial sites for deuterium. (Nd,Pr)FeB alloys with TiC additions were melted in an arc melting furnace, being processed in a melt spinner system. After the samples were heat treated at different temperatures. The promissing literature results for Nd2Fe14B+TiC were also obtained for Pr2Fe14B + TiC, but not for systems composed by Pr2Fe14B and Fe3B phases with TiC additions. However, good results were obtained in systems composed by Pr2Fe14B and α-Fe with TiC additions, with 30% increase on coercive field values Hc, and 15% increase on (BH)max.
Thompson, Guy Russell. "Mössbauer spectroscopic studies of the ferrimagnets Naâ‚‚M²âºFeF₇(M=Ni, Mn and Co) and nanophase barium ferrite". Thesis, University of Liverpool, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.240809.
Pełny tekst źródłaHe, Jiexuan. "Effects of spin current in ferromagnets". Diss., Columbia, Mo. : University of Missouri-Columbia, 2008. http://hdl.handle.net/10355/6060.
Pełny tekst źródłaThe entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file (viewed on August 4, 2009) Vita. Includes bibliographical references.
Soltan, Soltan. "Interaction of superconductivity and ferromagnetism in YBCO-LCMO heterostructures". Göttingen Cuvillier, 2005. http://deposit.ddb.de/cgi-bin/dokserv?idn=97407683X.
Pełny tekst źródłaChan, Jennifer Ann. "High temperature ferromagnetism in polymerised fullerene and graphitic systems". Thesis, Imperial College London, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.428595.
Pełny tekst źródłaAhilan, K. "Superconductivity on the border of itinerant ferromagnetism in UGe2". Thesis, University of Cambridge, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.595374.
Pełny tekst źródłaRodriÌguez, GonzaÌlez AgustiÌn. "The helical antiferromagnetism and ferromagnetism in GD-Y alloys". Thesis, University of Bristol, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.399394.
Pełny tekst źródłaMajidi, Muhammad Aziz. "Computational Studies of Ferromagnetism in Strongly Correlated Electronic Systems". University of Cincinnati / OhioLINK, 2006. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1148320220.
Pełny tekst źródłaThapa, Sunil. "Defects and Ferromagnetism in Transition Metal Doped Zinc Oxide". Bowling Green State University / OhioLINK, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=bgsu1467319340.
Pełny tekst źródłaKienert, Jochen. "Ferromagnetism and interlayer exchange coupling in then metallic films". Doctoral thesis, Humboldt-Universität zu Berlin, Mathematisch-Naturwissenschaftliche Fakultät I, 2008. http://dx.doi.org/10.18452/15834.
Pełny tekst źródłaThis thesis is concerned with the ferromagnetic Kondo lattice (s-d, s-f) model for film geometry. The spin-fermion interaction of this model refers to materials in which localized spins interact with mobile charge carriers like in (dilute) magnetic semiconductors, manganites, or rare-earth compounds. The carrier-mediated, indirect interaction between the localized spins comprises the long-range, oscillatory RKKY exchange interaction in the weak-coupling case and the short-range double-exchange interaction for strong spin-fermion coupling. Both limits are recovered in this work by mapping the problem onto an effective Heisenberg model. The influence of reduced translational symmetry on the effective exchange interaction and on the magnetic properties of the ferromagnetic Kondo lattice model is investigated. Curie temperatures are obtained for different parameter constellations. The consequences of charge transfer and of lattice relaxation on the magnetic stability at the surface are considered. Since the effective exchange integrals are closely related to the electronic structure in terms of the density of states and of the kinetic energy, the discussion is based on the modifications of these quantities in the dimensionally-reduced case. The important role of spin waves for thin film and surface magnetism is demonstrated. Interlayer exchange coupling represents a particularly interesting and important manifestation of the indirect interaction among localized magnetic moments. The coupling between monatomic layers in thin films is studied in the framework of an RKKY approach. It is decisively determined by the type of in-plane and perpendicular dispersion of the charge carriers and is strongly suppressed above a critical value of the Fermi energy. Finally, the temperature-dependent magnetic stability of thin interlayer-coupled films is addressed and the conditions for a temperature-driven magnetic reorientation transition are discussed.
Júnior, Gerson Ferreira. "Ferromagnetismo no regime Hall quântico inteiro via teoria do funcional de densidade". Universidade de São Paulo, 2011. http://www.teses.usp.br/teses/disponiveis/76/76131/tde-31082011-100156/.
Pełny tekst źródłaThe quantum Hall effect arises in two dimensional electron gases (2DEG) under high magnetic fields B. The magnetic field quantizes the planar motion of the electrons into cyclotron orbits given by the Landau levels. In this regime the transversal (Hall) resistivity ρxy shows plateaus as a function of B at integer sub-multiples of e2/h, i.e., ρxy = ν-1 e2/h, where n is the filling factor of the Landau levels. The longitudinal resistivity ρxx shows peaks at the transition between the plateaus of ρxy. In principle, ρxx is an indirect measure of the density of states at the Fermi level g(εF), so that the peaks indicate when the Fermi level εF crosses a Landau level. Therefore, a density-B-field diagram n2D-B of the ρxx ~ g(εF) peaks shows a topological map of the electronic structure of the system. In two-subband systems, ρxx( n2D, B) shows ringlike structures due to crossings of spin-split Landau levels from distinct subbands [experiments from the group of Prof. Jiang (UCLA)] that could lead to ferromagnetic instabilities. We study these instabilities using the density functional theory (DFT) to calculate the electronic structure, and Ando\'s model (Kubo formalism) for ρxx and ρxy. At higher temperatures (340 mK) we also obtain the ringlike structures in ρxx. At lower temperatures (70 mK) we see broken rings due to quantum Hall ferromagnetic phase transitions. Tilting B by theta with respect to the 2DEG normal we find that the ring structure shrinks. Our results show that the angle of full collapse depends on a competition between the exchange term from the Coulomb interaction (Pauli principle) and the anticrossing of Landau levels due to the finite angle theta. Additionally, at the instabilities we observe hysteresis. Sweeping the B field up or down near these regions we obtain two different solutions with distinct total energies, corresponding to the ground state and an excited state of the many-body system. This result, together with previous results of our group [Freire & Egues (2007)], are the first realizations of the theoretical prediction of the possibility of excited states as local minima of the ground state energy functional [Perdew & Levy (1985)]. The model proposed here shows an excellent agreement with the experiments. Additionally, the systematic and experimentally verified observation of excited states corroborates the predictions of Perdew & Levy. Similar ideas as presented here when applied to the electronic structure and conductance of quantum wires with an in-plane magnetic field show ferromagnetic instabilities at crossings of the wire transverse modes [Master Thesis of Filipe Sammarco, IFSC/USP], also with excellent experimental agreement. This strengthen the range of validity of the model proposed in this Thesis.
Iwamoto, Wellington Akira 1979. "Estudos das propriedades magnéticas dos filmes finos de GaAs dopado com Mn e de Zn1-xCoxO". [s.n.], 2007. http://repositorio.unicamp.br/jspui/handle/REPOSIP/278526.
Pełny tekst źródłaDissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin
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Resumo: Semicondutores ferromagnéticos (FM) são compostos de grande interesse tecnológico devido à possibilidade de combinar suas cargas e o grau de liberdade de spin para produzir dispositivos eletrônicos. Em particular, filmes finos semicondutores dopados com metais de transição têm se tornado foco de intensa investigação científica desde a descoberta do ferromagnetismo com razoável temperatura de Curie [1] ¿[4]. Exemplos de semicondutores magnéticos diluídos (DMS) são os filmes finos de GaAs dopado com Mn e ZnO dopado Co. Nessa dissertação, nós apresentamos experimentos de Ressonância Paramagnética Eletrônica (EPR) e de susceptibilidade magnética para os filmes finos amorfos e cristalinos de GaAs dopados com Mn e filmes cristalinos de ZnO dopados com Co, com a finalidade de explorar as propriedades magnéticas globais e locais nesses DMS. Para todos os filmes amorfos de GaAs dopados com Mn, os nossos resultados indicaram a ausência de qualquer ordenamento ferromagnético entre as temperaturas 300 > T > 2 K ao contrário dos filmes cristalinos que foi observado ferromagnetismo em T < 110 K. Além do mais, observamos nas medidas de EPR uma única linha associada aos íons localizados de Mn 2+ para os filmes finos amorfos de GaAs dopados com Mn e g ~ 2,01, o qual se manteve inalterado com a temperatura. Para nossos filmes cristalinos de GaAs foram observados modos ferromagnéticos para T < TC. Alguns filmes amorfos de GaAs dopado com Mn, foram hidrogenados e, para estes, encontramos que dopagem de hidrogênio, torna o filme mais cristalino, e que sua influência nas propriedades magnéticas é somente causada pela sua variação no grau de cristalinidade. Para os filmes cristalinos de ZnO dopado com Co, os experimentos de EPR mostraram que somente para os filmes com concentração de 10 % de Co um modo ferromagnético pôde ser observado. E através de medidas de magnetização foi observada uma magnetização de saturação máxima MS ~ 1,1 µB/Co para o filme com concentração de 10 % de Co, sendo que a magnetização decresce para concentrações maiores de Co. Isso indica que o loop ferromagnético encontrado nos filmes não pode estar associado a simples precipitação de Co. Uma comparação entre as propriedades magnéticas do filmes de 10 % de Co e da possível fase espúria, ZnCo2O4, mostraram propriedades magnéticas dos filmes de ZnO dopado com Co não parecem estar associados com esta fase
Abstract: Ferromagnetic semiconductors (FM) are compounds of technological interest due to the possibility of combining their charge and spin degrees of freedom when producing electronic devices. In particular, semiconductor thin films doped with transition metal have become focus of intense scientific investigation since ferromagnetism with reasonably high Curie temperatures (racing from few Kelvin to room temperature) was found in theses films [1-4]. Examples of such dilute magnetic semiconductor (DMS) are Mn-doped GaAs and Co-doped ZnO thin films. ZnO is a direct bandgap II-VI semiconductor with a wurtzite-type structure. In this word, we have performed studies of EPR and magnetic susceptibility in Co-doped ZnO and Mn-doped GaAs thin films in order to further explore the global and local magnetic properties of these intriguing DMS. For the Mn-doped GaAs samples, our results show the absence of ferromagnetic ordering for the amorphous films in the 300 > T > 2 K temperature range, in contrast to the ferromagnetism found in crystalline films for TC< 110 K. A single EPR line with a temperature independent g-value (g ~ 2) is observed for the amorphous films and the behavior of this ESR linewidth depends on the level of crystallinity of the film. For the Mn-doped GaAs crystalline films, only a ferromagnetic mode is observed for T < TCwhen the film is ferromagnetic. Regarding the effect of H-doping in the properties of Mn-doped GaAs amorphous films, the Mn 2+ ESR line was found to be nearly unaffected by the presence of hydrogen apart of slightly linewidth changes induced by the changes in the film crystallinity. Hydrogen doping has no direct effects in the magnetic properties of Mn-doped GaAs films. For the Co-based films, the ESR experiments show that only the Zn0,90Co0,10 O film presented a strong anisotropic FMR. The magnetization data show that ~ 10% of Co-doped ZnO films produce the maximum Ms ~ 1,1 µB/Co in the series. The absence of FMR for films with higher Co Concentration indicates that the observed FM loops cannot be associated with simply precipitation of pure Co ions, but more work needs to be done to complete rule out the contribution of other magnetic secondary phases
Mestrado
Física da Matéria Condensada
Mestre em Física
Santos, Antonio Domingues dos. "Anisotropias induzidas em ligas ferromagnéticas amorfas". Universidade de São Paulo, 1991. http://www.teses.usp.br/teses/disponiveis/43/43133/tde-17072012-113147/.
Pełny tekst źródłaIn this thesis we will present a series of studies related to induced magnetic anisotropies (Kind) in amorphous alloys. In order to get a more general view of this theme, we used several different kinds of annealings. The theoretical analysis of the data of Kind and magnetic after-effect (MAE) was performed using a model based on two-level systems (TLS). From the analysis of the experimental data we get a large actiyation energy spectrum. These energies are related to the relaxation times, through the Arrheniu \'s expression: = 0 exp(E/kT), where the pre-factor 0 is of the order of the inverse of the Debye frequency . We constructed a furnace for thermal annealing of the amorphous alloys, which operates within an electromagnet producing 6 kOe. We also wrote, the computer programs for the analysis of the experimental data. These facilities, together with the hysteresis loop tracer permitted the followings studies of the induced anisotropies in amorphous alloys: 1) A study of the kinetics of the induced anisotropy by annealing in the temperature range from 190 to 250 °C, in as cast amorphous ribbons of composition Co70.4Fe4.6Si15B10. Using the TLS model we obtained the activation energy spectrum. It presents two peaks in the energy range from 1.50 to 1.85eV and a pre-factor 0= 1.6x10-13 s. 2) Using a 5 KOe magnetic field we studied the effects of a Field annealing treatment in samples pre-annealed at 400 °C for 10 minutes. The isothermal annealings were made in Co70-XMnXSi14B9 with x = 2 and 6, in the temperature range from 240 to 325 °C. In this case we observed for these two compositions a larger pre-factor ( 10-8s) than before. The activation-energy spectra, for the both composition, are found in the energy range from 1.10 to 1.55eV. 3) Another experiment was done using samples of Co67Fe4Mo1Si12B16. We applied a tensile stress (800MPa) to the ribbon and measured the magnetic anisotropy energy. We observed a continuous variation of this energy and, after removal of the stress, the sample recuperated its initial condition, showing a process characteristically anelastic. We studied the effects of mechanic stress on the magnetic properties of samples of composition Co67Fe4Mo1Si12B16. We worked in two directions: 4) We studied the induced anisotropy in pre-annealed samples, submitted to annealing in the range from 200 to 400 °C, under a tensile stress of 500MPa and without applied stress. From these resul ts we can separate a plastic and an anelastic component in the induced anisotropy. 5) In other, we studied the behavior of the MAE, for samples with and with and without applied tensile stress, in the range from 300 to 500 K. The results obtained show neither plastic nor anelastic effects on the activation energies of the processes involved in the MAE. On the other h~nd we can see a strong alteration in the intensity of the MAE, due the stress.
Yang, Chunlei. "Studies of the spintronic systems of ferromagnetic GaMnAs and non-magnetic InGaAs/InAlAs two dimensional electron gas /". View abstract or full-text, 2005. http://library.ust.hk/cgi/db/thesis.pl?PHYS%202005%20YANG.
Pełny tekst źródłaKang, Hun. "A Study of the Nucleation and Formation of Multi-functional Nanostructures using GaN-Based Materials for Device Applications". Diss., Georgia Institute of Technology, 2006. http://hdl.handle.net/1853/14094.
Pełny tekst źródłaMishra, Subodha. "Theory of photo-induced ferro-magnetism in dilute magnetic semiconductors". Diss., Columbia, Mo. : University of Missouri-Columbia, 2006. http://hdl.handle.net/10355/4413.
Pełny tekst źródłaThe entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file (viewed on August 6, 2007) Includes bibliographical references.
Gester, Matthias. "Structural and magnetic properties of epitaxial Fe and Co films on GaAs substrates". Thesis, University of Cambridge, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.338052.
Pełny tekst źródłaGillies, Murray Fulton. "An investigation of the magnetic properties of spin-valves using transmission electron microscopy". Thesis, University of Glasgow, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.294195.
Pełny tekst źródłaVavekis, Konstantinos. "Structural, magnetic and superconducting properties of fulleride salts". Thesis, University of Sussex, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.362276.
Pełny tekst źródłaRoberts, Hywel Gwynedd. "Magnetism and transport in nanostructured domain wall systems". Thesis, University of Bath, 2008. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.486849.
Pełny tekst źródłaZhang, Yi. "Fermi Liquid Study of Exotic Modes in Magnetically Ordered Systems". Thesis, Boston College, 2014. http://hdl.handle.net/2345/bc-ir:103563.
Pełny tekst źródłaThe Landau Fermi liquid theory is a very successful theory in condensed matter physics. It provides a phenomenological framework for describing thermodynamics, transport and collective modes of itinerant fermionic systems. In 1957, Silin described the spin waves in polarized Fermi liquids based on Landau Fermi liquid theory, which are related to series of components of the spherical harmonic expansion of the fermi surface. It has been proved by Pomeranchuck that for the Fermi surface to be stable, the Landau parameters should satisfy the relation: $F_l^{s,a}>-(2l+1)$. Whenever the relation is violated, there will exist an instability of the Fermi surface known as a Pomeranchuck instability, such as the Stoner ferromagnetism when $F_0^a→ -1^+$, or phase separation when $F_0^s→ -1^+$. In 1959, Abrikosov and Dzyaloshinskii developed a ferromagnetic Fermi liquid theory(FFLT) of itinerant ferromagnetism based on Landau Fermi liquid theory, whose microscopic foundations were established later by Dzyaloshiskii and Kondratenko. Further studies had been made of this state using a generalized Pomeranchuck instability based on the FFLT of Blagoev, Engelbrecht and Bedell and Bedell and Blagoev. In this thesis, I study a magnetically ordered system with spin orbit magnetism, where the order parameter has a net spin current and no net magnetization in both two dimension and three dimension. Starting from a Fermi liquid theory, similar to that for a weak ferromagnet, I have shown that this excitation emerges from an exotic magnetic Fermi liquid state that is protected by a generalized Pomeranchuck condition. I derive the propagating mode using the Landau kinetic equation, and find that the dispersion of the mode has a $sqrt q$ behavior in leading order in 2D. I also find an instability toward superconductivity induced by this exotic mode, and a further analysis based on the forward scattering sum rule strongly suggests that this superconductivity has triplet pairing symmetry. I perform similar studies in the 3D case, with a slightly different magnetic system and find that the mode leads to a Lifshitz-like instability most likely toward an inhomogeneous magnetic state in one of the phases. I also study the collective modes in itinerant ferromagnetic system, which is related to the $F_0^a$ pomeranchuck instability. Using FFLT, I obtained the well-known magnon (Nambu-Goldstone) mode and a gapped mode that was first found by Bedell and Blagoev. I have identified this mode as the Higgs boson (amplitude mode) of a ferromagnetic metal. This is identified as the Higgs since it can be shown that it corresponds to a fluctuation of the amplitude of the order parameter. I use this model to describe the itinerant-electron ferromagnetic material MnSi. By fitting the model with the existing experimental results, I calculate the dynamical structure function and see well-defined peaks contributed from the magnon and the Higgs. From my estimates of the relative intensity of the Higgs amplitude mode I expect that it can be seen in neutron scattering experiments on MnSi
Thesis (PhD) — Boston College, 2014
Submitted to: Boston College. Graduate School of Arts and Sciences
Discipline: Physics
Sibley, Lara Ann. "Measurements on the itinerant ferromagnet ZrZn₂ under hydrostatic pressure". Thesis, University of Cambridge, 2011. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.609915.
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