Analysis of the magnetic field dependence of the isothermal entropy change of inverse magnetocaloric materials

Abstract

In this work, the magnetic field dependence of the inverse magnetocaloric (MC) effect is analyzed using a mean field approach for describing antiferromagnetic to ferromagnetic magnetoelastic transitions. The model is able to describe both second- and first-order transition through the introduction of a magnetovolume energy term. The power law dependence for the field dependence of the isothermal magnetic entropy change (ΔSiso∝ΔHn), has an exponent n with an overshoot above 2 for first-order transitions, while it is not present for the second-order case. This is in excellent agreement with previous phenomenological observations, supporting the validity of recently proposed criterion to distinguish between first- and second-order thermomagnetic transitions. A main difference with respect to direct MC effect is that negative values of the exponent n are obtained at temperatures close to the transition. This is ascribed to the reduction of the inverse MC response due to the influence of the unavoidable ferromagnetic to paramagnetic transition at higher temperatures. The obtained features are qualitatively compared to those of GdBaCo2O6 (antiferromagnetic to ferromagnetic magnetoelastic transition), showing a good agreement between both experiments and the model. The obtained information is extrapolated to understand the behavior of the exponent n for a Ni49Mn36In15 sample (magnetostructural transition).