Propriedades termodinâmicas de sistemas magnéticos frustrados
Ano de defesa: | 2013 |
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Autor(a) principal: | |
Orientador(a): | |
Banca de defesa: | |
Tipo de documento: | Tese |
Tipo de acesso: | Acesso aberto |
Idioma: | por |
Instituição de defesa: |
Universidade Federal de Minas Gerais
UFMG |
Programa de Pós-Graduação: |
Não Informado pela instituição
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Departamento: |
Não Informado pela instituição
|
País: |
Não Informado pela instituição
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Palavras-chave em Português: | |
Link de acesso: | http://hdl.handle.net/1843/BUBD-9E3GE4 |
Resumo: | In this work, we present some thermodynamic properties of the anisotropic Heisenberg model with antiferromagnetic interactions in two and three dimensions. First, we study the zero temperature properties of the two-dimensional spatially anisotropic ferrimagnet with competing interactions, using the linear spin-wave theory, and considering pairs of mixed-spins in the set (1/2,1,3/2). For some values of the anisotropy parameter, we find a small region magnetically disordered in the phase diagram. Using a modified spin wave theory, the ordered phases of the two dimensional S=1 antiferromagnet with next and near next neighbor exchange interactions and easy axis single ion anisotropy on the square lattice are studied. We calculate the phase diagram at T = 0, and some thermodynamic quantities at finite temperatures. In the final part of the thesis, we use the bond operator technique to study the antiferromagnetic Heisenberg model with spin S=1 with easy plane anisotropy. We study the effects of frustration between nearest, next-nearest neighbor and next-next-nearest neighbors (NNN) of the quantum S=1 anisotropic antiferromagnetic Heisenberg model on a simple cubic lattice with single ion anisotropy. We calculate the phase diagram at zero temperature and the gap as a function of temperature in the disordered paramagnetic phase. Finally, we study the cubic lattice with interaction between nearest neighbors and a coupling between planes. We have considered an in-plane single ion anisotropy D and an intra-plane anisotropy Dx. Applying an external magnetic field h in the z direction, we calculate: i) the magnetization m in the direction of the field. ii) the magnetization mx in the plan perpendicular to the z axis. Our results indicate that the anisotropy Dx reduces mx and critical temperature Tc, while y and h induce long-range order in the system. |