Magnetic couplings and magnetocaloric effect in the GdTX (T=Sc, Ti, Co, Fe; X=Si, Ge) compounds

TL;DR

This study uses density functional theory and magnetic modeling to analyze the magnetocaloric effect in GdTX compounds, revealing how chemical substitutions influence magnetic interactions, transition temperatures, and the magnetocaloric response.

Contribution

It provides a detailed theoretical analysis of how chemical composition and structure affect magnetic couplings and the magnetocaloric effect in GdTX compounds, incorporating first-principles calculations and magnetic modeling.

Findings

Replacements Ti→Sc or Fe→Co induce antiferromagnetic interactions.

Large variations in transition temperature and magnetocaloric effect among compounds.

Universal behavior of the magnetocaloric effect after scaling by Curie temperature.

Abstract

We compute the magnetocaloric effect (MCE) in the GdTX (T=Sc, Ti, Co, Fe; X=Si, Ge) compounds as a function of the temperature and the external magnetic field. To this end we use a density functional theory approach to calculate the exchange-coupling interactions between Gd$^{3+}$ ions on each compound. We consider a simplified magnetic Hamiltonian and analyze the dependence of the exchange couplings on the transition metal T, the p-block element X, and the crystal structure (CeFeSi-type or CeScSi-type). The most significant effects are observed for the replacements Ti $\to$ Sc or Fe $\to$ Co which have an associated change in the parity of the electron number in the 3d level. These replacements lead to an antiferromagnetic contribution to the magnetic couplings that reduces the Curie temperature and can even lead to an antiferromagnetic ground state. We solve the magnetic models through mean field and Monte Carlo calculations and find large variations among compounds in the magnetic transition temperature and in the magnetocaloric effect, in agreement with the available experimental data. The magnetocaloric effect shows a universal behavior as a function of temperature and magnetic field in the ferromagnetic compounds after a scaling of the relevant energy scales by the Curie temperature $T_C$.