The magnetocaloric effect in itinerant antiferromagnetic materials

Abstract

The Kondo lattice model is adopted to describe a variety of magnetic materials containing localized moments and conduction electrons. Within a mean-field approximation, we obtain the ground-state phase diagram including ferromagnetic (FM) and antiferromagnetic (AF) phases, as a function of the local FM exchange coupling JK and the conduction electron concentration n. The partial magnetizations are determined self-consistently as a function of temperature in presence of a magnetic field h. In the AF phase, we assume equivalent sublattices with independent magnetizations concomitant with charge ordering. The h–T phase diagram includes the homogeneous FM and AF phases, separated by a line of first-order metamagnetic transitions, and a PM phase at high temperatures. The magnetocaloric effect (MCE) is characterized by the isothermal entropy change ΔST, which can be evaluated by two alternative methods: (i) from the heat capacity or (ii) from the magnetization curves (via Maxwell relation). We verify the equivalence of the two methods, provided that the stable equilibrium states are taken into account. A metastable AF solution is allowed in a portion of the phase diagram. This feature may be associated to the presence of inhomogeneities in some compounds, which would lead to discrepancies between the two methods. The present approach describe the conversion from inverse to conventional MCE, as recently observed in some magnetocaloric materials with AF phases.