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Previous issue date: 2016-02-26 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior (CAPES) / Esta Tese compreende um estudo te?rico sobre a influ?ncia da anisotropia magnetocristalina nas propriedades magn?ticas est?ticas e din?micas de filmes finos: monocamadas e tricamadas acopladas atrav?s dos campos de troca bilinear e biquadr?tico, para situa??es nas quais os sistemas s?o crescidos em dire??es n?o usuais [hkl] de baixa simetria. Usando uma teoria baseada em um modelo fenomenol?gico real?stico para descrever sistemas magn?ticos nanom?tricos, consideramos a energia magn?tica livre total incluindo a intera??o Zeeman, anisotropias magnetocristalinas c?bica e uniaxial, anisotropias desmagnetizante e de superf?cie, bem como os termos de troca. Os c?lculos num?ricos s?o conduzidos atrav?s da minimiza??o da energia magn?tica total a partir da determina??o das configura??es est?ticas de equil?brio. Consideramos par?metros experimentais da literatura para ilustrar nossos resultados em sistemas tipo Fe/Cr/Fe. Em particular, um total de seis diferentes cen?rios magn?ticos ? analisado para tr?s grupos de campos de troca e as dire??es de crescimento de baixa simetria [211] e [321]. Ap?s minimizarmos numericamente a energia total, utilizamos as configura??es de equil?brio para obter curvas de magnetiza??o e de magnetoresist?ncia com as respectivas fases magn?ticas e os campos cr?ticos de transi??es de fases. Estes resultados tamb?m s?o usados para a determina??o da fronteira de ocorr?ncia dos estados saturados. Dentro do contexto das ondas de spin, resolvemos a equa??o de movimento para estes sistemas a fim de encontrarmos as respectivas rela??es de dispers?o associadas. Os resultados mostram curvas de magnetiza??o e magnetoresist?ncia similares em ambos os cen?rios [211] e [321], com comportamento de transi??o magn?tica equivalente. No entanto, a combina??o entre simetrias peculiares e a influ?ncia da energia de troca resulta em propriedades cativantes, incluindo a gera??o de estados magn?ticos dependentes da dire??o de crescimento e aumento de incompatibilidade entre valores de campo de satura??o da magnetiza??o e magnetoresist?ncia para eixos anisotr?picos c?bicos intermedi?rios, particularmente no regime onde os campos de troca bilinear e biquadr?tico s?o compar?veis. As rela??es de dispers?o e os resultados est?ticos s?o coerentes, com as fases magn?ticas tamb?m presentes em ambos os modos ac?stico e ?ptico. Excita??es de Goldstone s?o observadas particularmente nos eixos anisotr?picos c?bicos intermedi?rios, um efeito relacionado ?s transi??es de segunda ordem e ? quebra espont?nea de simetria imposta pela combina??o entre o acoplamento biquadr?tico e as anisotropias c?bica e uniaxial. / This Thesis comprises a theoretical study about the influence of the magnetocrystalline anisotropy
on the static and dynamic magnetic properties of nanofilms: monolayers and trilayers coupled through
the bilinear and biquadratic exchange fields, for situations in which the systems are grown in
unusual [hkl] asymmetric directions. Using a theory based on a realistic phenomenological model
for description of nanometric systems, we consider the total free magnetic energy including the
Zeeman interaction, cubic and uniaxial anisotropies, demagnetizing and surface anysotropy energies,
as well as the exchange terms. Numerical calculations are conducted by minimizing the total
magnetic energy from the determination of equilibrium static configurations. We consider experimental
parameters found in the literature to illustrate our results for Fe/Cr/Fe trilayer systems. In
particular, a total of six different magnetic scenarios are analyzed for three regimens of exchange
fields and the [211] and [321] asymmetric growth directions. After numerically minimize the total
energy, we use the equilibrium configurations to calculate magnetization and magnetoresistance
curves with the respective magnetic phases and corresponding critical fields. These results are also
used to establish the boundary for occurrence of saturated states. Within the context of the spin
waves, we solve the equation of motion for these systems in order to find the respective associated
dispersion relations. The results show similar magnetization and magnetoresistance curves for
both [211] and [321] growth scenarios, including an equivalent magnetic transition behavior. However,
the combination of those peculiar symmetries and influence of the exchange energies results
in attractive properties, including the generation of magnetic states as a function of the asymmetric
degree imposed in the [hkl] growth orientations. There is also an increasing incompatibility
between the values of saturation fields of magnetization and magnetoresistance for the cases in
which a magnetic field acts along intermediate cubic anisotropic axes, particularly in the situations
where the bilinear and biquadratic exchange fields are comparable. The dispersion relations and
static results are consistent, the corresponding magnetic states are also present in both acoustic and
optical modes. Furthermore, Goldstone excitations are also observed for that particular cases of a
magnetic field acting in the intermediate axes, an effect related to transitions of second order and
to the spontaneous symmetry breaking imposed by the combination of the biquadratic energy with
the cubic and uniaxial anisotropies.
| Identifer | oai:union.ndltd.org:IBICT/oai:repositorio.ufrn.br:123456789/21269 |
| Date | 26 February 2016 |
| Creators | Celino, Klaydson Reinaldo |
| Contributors | 60992565472, http://lattes.cnpq.br/5085046065890176, Feitosa, Carlos Chesman de Ara?jo, 51233789449, http://lattes.cnpq.br/1150673024968512, Anselmo, Dory Helio Aires de Lima, 59764708404, http://lattes.cnpq.br/0554474279738500, Costa, Carlos Humberto Oliveira, 00440538351, http://lattes.cnpq.br/4056658417684725, Costa Filho, Raimundo Nogueira da, 23189975353, http://lattes.cnpq.br/0434393043515628, Bezerra, Claudionor Gomes |
| Publisher | Universidade Federal do Rio Grande do Norte, PROGRAMA DE P?S-GRADUA??O EM F?SICA, UFRN, Brasil |
| Source Sets | IBICT Brazilian ETDs |
| Language | Portuguese |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, info:eu-repo/semantics/doctoralThesis |
| Source | reponame:Repositório Institucional da UFRN, instname:Universidade Federal do Rio Grande do Norte, instacron:UFRN |
| Rights | info:eu-repo/semantics/openAccess |
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