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

Abstract

This paper presents a study of the magnetocaloric effect at thefirst-order magneto-elastic phase transition. The entropy changeΔs at the transition temperature is given by the sum of the magnetic andthe structural contributions. By using a thermodynamic model, it isshown that the sign and amplitude of the structural contribution toΔs are determined by the dimensionless parameterζ (zeta) whichdepends on β, the steepness of the change of exchange forces with volume, and onαp, the thermal expansion coefficient of the structural lattice. Forζ < 0 thechange Δs can be larger than the magnetic contribution alone, because the structural entropy change and themagnetic entropy change have the same sign, giving rise to the giant magnetocaloric effect. For0 < ζ < 1 the two contributions partially cancel each other and the totalentropy change is lower than the magnetic entropy change. Forζ > 1 the structural entropy dominates and a transition occurs upon heating from a lowtemperature paramagnet to a high temperature ferromagnet.