EFEITO MAGNETOCALÓRICO EM SISTEMAS GEOMETRICAMENTE FRUSTRADOS

• Ano de defesa: 2024
• Autor(a) principal: Renata Mourão
• Orientador(a): Fabio Mallmann Zimmer
• Tipo de documento: Tese
• Tipo de acesso: Acesso aberto
• Idioma: por
• Instituição de defesa: Fundação Universidade Federal de Mato Grosso do Sul
• País: Brasil
• Palavras-chave em Português: Efeito Magnetocalórico, Sistemas desordenados, Clusters, Frustração Geométrica.
• Link de acesso: https://repositorio.ufms.br/handle/123456789/11082

In this work, we present a study of the magnetocaloric effect in spin glass systems that exhibit the presence of clusters and geometric frustration. We theoretically demonstrate that a strong enhancement of the magnetocaloric effect is achieved in geometrically frustrated cluster spin-glass systems just above the freezing temperature. We consider a network of clusters interacting randomly which have triangular structure composed of Ising spins interacting antiferromagnetically. The intercluster disorder problem is treated using a cluster spin glass mean-field theory, which allows exact solution of the disordered problem. This technique uses the replica method to derive an effective single-cluster model within the replica symmetry ansatz. The intracluster part can then be solved using exact enumeration. The coupling between the inter and intracluster problem incorporates the interplay between effects coming from geometric frustration and disorder. As a result, it is shown that the presence of geometrically frustrated clusters can lead to the onset of a cluster spin glass phase, even in the presence of very weak disorder. This cluster-glass phase emerges in a region where the replica symmetry is unstable. In this context, the freezing temperature is determined through the Almeida-Thouless eigenvalue analysis. The cluster spin glass order parameters, field-induced magnetization, and entropy behavior are presented for various sets of disorder levels, cluster sizes, and intensities of geometric frustration. The findings are discussed in terms of the magnetocaloric potential, focusing on the isothermal entropy variation and the isentropic curves for different relationships between disorder and frustration. The main conclusion is that it is precisely within a range of very weak disorder and small magnetic fields that the strongest isothermal entropy release is observed, indicando a interesting magnetocaloric potential. Keywords: Magnetocaloric Effect, Disordered Systems, Clusters, Geometric Frustration.