  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.
