Giant kinks in the entropy change temperature dependence of the magnetocaloric effect in layered phase-separated metals

Abstract

In this work, the validity of standard magnetocaloric (MCE) scenarios is revisited for the Hubbard model for a square (two-dimensional) lattice to describe a layered metal. Different types of magnetic ordering (ferrimagnetic, ferromagnetic, Néel and canted antiferromagnetic states) with magnetic transitions between them are considered to minimize the total free energy. The phase-separated states formed by such first-order transitions are also considered consistently. We employ the mean-field approximation to focus attention on the vicinity of a tricritical point, where the order of the magnetic phase transition changes from first to second and phase separation bounds merge. Two types of first-order magnetic transition can be found: PM-Fi, Fi-AFM; with further temperature growth, the phase separation boundaries between them merge and a second order transition, PM-AFM, is observed. The temperature and electron filling dependencies of the entropy change in the phase separation regions are investigated in detail in a consistent way. The dependence of the phase separation bounds on the magnetic field results in the existence of two different characteristic temperature scales. These temperature scales are indicated by giant kinks in the temperature dependence of the entropy, which are an exceptional attribute of phase separation in metals.