Estudo de modelos de spins com interação Dzyaloshinskii-Moriya

Abstract

The critical properties of systems that exhibit magnetism have been the subject of intense research over the years. The study of the critical behavior of many magnetic, insulating and anisotropic materials can be approached through Ising and Heisenberg models, for exemple. Taking into account anisotropic effects in spin models usually results in the presence of tricritical behavior in the phase diagram. In particular, the Dzyaloshinskii-Moriya interaction consists of an important type of anisotropy, which has attracted attention over the last decades, playing an important role in the description of some classes of insulators, as well as in the study of phenomena involving chirality, among others. The presence of disorder in the models, introduced through random variables governed by probability distributions, also results in a change in critical behavior in relation to pure systems. In this thesis, the ferromagnetic anisotropic Heisenberg model with Dzyaloshinskii-Moriya interaction is studied in spin-3/2 and spin-2 versions within the pair approximation. The formalism can be applied to different types of lattice. However, the results presented here deal in detail with the case of the simple cubic lattice (q = 6). The phase diagrams and magnetic properties reveal the existence of tricritical points and first-order phase transitions. For the spin-1 model, cases involving the disorder generated from the inclusion of a random field are treated. In addition, some results with the application of the effective field theory, as well as the antiferromagnetic model, are discussed.