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 Δs 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 Δs are determined by the dimensionless parameter ζ (zeta) which depends on β, the steepness of the change of exchange forces with volume, and on α(p), the thermal expansion coefficient of the structural lattice. For ζ < 0 the change Δs 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 < ζ < 1 the two contributions partially cancel each other and the total entropy change is lower than the magnetic entropy change. For ζ > 1 the structural entropy dominates and a transition occurs upon heating from a low temperature paramagnet to a high temperature ferromagnet.