Investigation of magnetocaloric effect: Stoner approximation vs DMFT

Abstract

A comparative study of the magnetocaloric effect (MCE) in metals within the single-band Hubbard model on the face-centered cubic (fcc) lattice using both mean-field (Stoner) approximation (MFA) and dynamical mean-field theory (DMFT) is done. The MCE is investigated in the case of second order magnetic phase transition from ferromagnet to paramagnet. To ensure presence of itinerant ferromagnetism in the Hubbard model the special case of spectrum parameters generating giant van Hove singularity at the bottom of the band is considered, while the Fermi level $E_{\rm f}$ is in the vinicity of the band bottom. To compare MCE within MFA and DMFT temperature dependence of magnetization, total energy and finally entropy for a set of Coulomb interactions $U$ at zero and finite values of magnetic field $h$ for both methods were performed. Also one of the MCE potentials, isothermal entropy change, as a function of temperature $ΔS (T)$ for both MFA and DMFT is calculated. In the MFA, the expected maximum value of $ΔS (T)$ at the Curie temperature $T_C$ ($ΔS_{\rm max}$) quite significantly decreases while $U$ grows. Similar but much weaker decreasing of $ΔS_{\rm max}$ is found for DMFT results. The account of local quantum fluctuations results in larger values of $ΔS_{\rm max}$ within DMFT than within MFA. A peak width of $ΔS (T)$ at half height is approximately the same for both methods. Another effect of DMFT local quantum fluctuations is the destruction of anomalous Curie temperature $T_C$ dependence on $U$ present in MFA, which is invoked by an effect of giant van Hove singularity. However the relative cooling power (RCP) is very close in DMFT and MFA for the same model parameters and goes down upon $U$ increase.