The double exchange model is used to describe different magnetocaloric materials containing localized magnetic moments and itinerant electrons. The model includes the Hund rule exchange J between the localized spins and the conduction electrons. To describe the thermodynamic properties of the model, we follow the same procedure adopted in a previous paper within a mean field approximation. By using the equation of motion method, we apply a higher-order decoupling for the conduction electron Green's functions. The magnetism of the localized moments is described in terms of an effective Heisenberg model. We obtain a simple description of the magnetization curves and the isothermal entropy change delta S. The results exhibit an additional low-temperature bump in the delta S curves at low concentrations n separated from the usual maximum at the critical temperature. The method can also be addressed to the Kondo lattice compounds, in the case of a negative couping J.
展开▼
