Inverse magnetocaloric effect and phase separation induced by giant van Hove singularity in itinerant ferromagnetic metal
A thermodynamic theory based on Landau grand potential expansion for ferromagnetic-paramagnetic phase transitions is developed for an electronic phase-separated state. It is rigorously shown that ferromagnetic phase involved in the phase-separated state exhibits negative magnetic susceptibility in the vicinity of~tricritical point. Thus, an entropy of the magnetically ordered phase may increase when the magnetic field is applied, which implies positive sign of the total magnetic entropy change $ΔS$ within magnetocaloric effect~(MCE). The electronic phase separation and MCE are considered within the Hubbard model for~face-centered cubic lattice with giant van Hove singularity of electron density of states at the band bottom. Within the Hartree-Fock approximation it is shown that such model of itinerant magnet exhibits the~first-order ferromagnet-paramagnet phase transition~(FOPT) with electronic phase separation and inverse magnetocaloric effect deep inside the phase-separated region. Temperature dependence of $ΔS$ for the mean-field solution of the non-degenerate Hubbard model is analyzed in detail for different band filling values. The possibility to control $ΔS$ sign by changing both temperature and band filling of magnetocaloric materials is demonstrated. This is important to interpret a lot of experimental data, possible technological applications, and further theoretical developments.