The magnetocaloric effect at the first-order magneto-elastic phase transition

This paper presents a study of the magnetocaloric effect at the first-order magneto-elastic phase transition. The entropy change Delta s at the transition temperature is given by the sum of the magnetic and the structural contributions. By using a thermodynamic model, it is shown that the sign and amplitude of the structural contribution to Delta s are determined by the dimensionless parameter zeta (zeta) which depends on beta, the steepness of the change of exchange forces with volume, and on alpha(p), the thermal expansion coefficient of the structural lattice. For zeta < 0 the change Delta s can be larger than the magnetic contribution alone, because the structural entropy change and the magnetic entropy change have the same sign, giving rise to the giant magnetocaloric effect. For 0 < zeta < 1 the two contributions partially cancel each other and the total entropy change is lower than the magnetic entropy change. For zeta > 1 the structural entropy dominates and a transition occurs upon heating from a low temperature paramagnet to a high temperature ferromagnet.