The compounds FeMnAsxP1-x are very promising as far as commercial applications of the magnetocaloric effect are concerned. However, the theoretical literature on magnetocaloric materials still adopts simple molecular-field models in the description of important properties like the entropy variation that accompanies applied isothermal magnetic field cycling, for instance. We apply a Green function theoretical treatment for such analysis. The advantages of such approach are well-known since the details of the crystal structure are incorporated in the model, as well as a precise description of correlations between spins of the transition metal ions can be obtained. For the sake of simplcity we adopt a simple one-exchange parameter Heisenberg model, and the observed first-order phase transitions are reproduced by the introduction of a biquadratic term in the hamiltonian. Good agreement with experimental magnetocaloric data for FeMnAsxP1-x compounds is obtained, as well as an agreement with the magnetic field dependence for these properties predicted from the Landau theory of continuous phase transitions.
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