Investigações de excitações magnéticas em gelos de spin bidimensionais

• Ano de defesa: 2012
• Autor(a) principal: Silva, Rodrigo da Costa
• Orientador(a): Pereira, Afrânio Rodrigues
• Banca de defesa: Ferreira Junior, Silvio da Costa , Ferreira, Sukarno Olavo , Mota, André Luiz
• Tipo de documento: Dissertação
• Tipo de acesso: Acesso aberto
• Idioma: por
• Instituição de defesa: Universidade Federal de Viçosa
• Programa de Pós-Graduação: Mestrado em Física Aplicada
• Departamento: Física Teórica e Computacional; Preparação e Caracterização de Materiais; Sensores e Dispositivos.
• País: BR
• Palavras-chave em Português: Monopolos magnéticos; Gelos de spin; Frustração geométrica
• Palavras-chave em Inglês: Magnetic monopoles; Spin ice; Geometrical frustration
• Área do conhecimento CNPq: CNPQ::CIENCIAS EXATAS E DA TERRA::FISICA::FISICA DA MATERIA CONDENSADA
• Link de acesso: http://locus.ufv.br/handle/123456789/4257

We study the magnetic excitations in two-dimensional spin ices on square lattices, by means of computer simulations. These materials are artificially produced by lithographic techniques, using ferromegnetic nano-islands arranged in a lattice especially organized in such a way, that, the lattice geometry together with the dipole-dipole interactions lead to an intrinsic frustration of the system. Our analysis, based only on the dipolar interaction between the islands, correctly reproduces the ground state experimentally observed recently. In addition, we verify the emergence of magnetic excitations that behave like magnetic monopoles, carrying isolated magnetic charge. These quasi-particles interact by means of the Coulomb potential plus a linear confining potential, which is attributed to the emergence of string-like excitations, linking the pair of opposite charges. However, we argue that the string has a configurational entropy, so that the string tension may decrease as the temperature is increased and then, free magnetic monopoles should be found in two-dimensional spin ices at a critical temperature. We also study the case where the artificial spin ice is built with missing nano-island (vacancy). For the case with only one vacancy, we found that the defect modify the potential obtained for the normal lattice. The vacancy behaves like a pair of opposite charges, carrying half charge of the usual magnetic monopole excitation. Our calculations show that when the smallest distance between the charges (or the string) is greater than 2a (a is the lattice spacing), the interaction of the string with the defect is negligible. However, for distances smaller than 2a, the string interacts with the defect more appreciable, in a very complicated way, which depends on the shape and size of the string. Our results indicate that, on each vertex where the string passes, there is an additional quadrupole moment and so the field created by the string decays very quickly.