Propriedades termodinâmicas de sistemas magnéticos frustrados

Detalhes bibliográficos
Ano de defesa: 2013
Autor(a) principal: Griffith Mendonça Andrade Sousa
Orientador(a): Não Informado pela instituição
Banca de defesa: Não Informado pela instituição
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
Departamento: Não Informado pela instituição
País: Não Informado pela instituição
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.