Entropy changes and the magnetocaloric effect in anisotropic magnetic materials

The magnetocaloric effect, the heating or cooling of magnetic materials upon magnetic field variation, is usually characterized by the entropy change in an isothermal process and the temperature change in an adiabatic process. The behavior of curves of the magnetocaloric quantities as a function of temperature strongly depends on the magnetic interaction inside the material. For example, for isotropic materials, these curves exhibit a sharp peak around the magnetic ordering temperature and fade away outside this temperature region. On the other hand, in anisotropic materials these curves present anomalies such as the table-like effect, a structure with two peaks, the inverse effect etc.

In this work, we theoretically discuss the magnetic properties and the magnetocaloric effect of anisotropic magnetic materials with two or more nonequivalent sites. For this purpose, we consider a model Hamiltonian of interacting magnetic moments with extra terms to account for the anisotropy and the nonequivalence of the sites in the crystalline lattice. In the first step, we perform a systematic analysis in terms of the model parameters to understand the physics behind the different behaviors observed in the curves of the caloric quantities. Afterwards, we apply the model to calculate the magnetocaloric quantities in the compounds RCu2Cd where R stands for rare earth element. Our obtained results of the entropy change are in reasonable agreement with the available experimental data.