Exchange interactions and temperature dependence of the magnetization in half--metallic Heusler alloys

TL;DR

This study investigates exchange interactions and temperature-dependent magnetization in half-metallic Heusler alloys using first-principles calculations, revealing the dominant magnetic interactions and estimating Curie temperatures with improved accuracy.

Contribution

It provides a detailed analysis of exchange interactions in various Heusler alloys and compares mean-field and RPA methods for Curie temperature estimation, highlighting the effectiveness of RPA.

Findings

RPA yields better Curie temperature estimates aligning with experiments.

Nearest Mn-Mn interactions dominate in half-Heusler alloys.

Inter-sublattice interactions are key in full-Heusler alloys.

Abstract

We study the exchange interactions in half-metallic Heusler alloys using first-principles calculations in conjunction with the frozen-magnon approximation. The Curie temperature is estimated within both mean-field (MF) and random-phase-approximation (RPA) approaches. For the half-Heusler alloys NiMnSb and CoMnSb the dominant interaction is between the nearest Mn atoms. In this case the MF and RPA estimations differ strongly. The RPA approach provides better agreement with experiment. The exchange interactions are more complex in the case of full-Heusler alloys Co$_2$MnSi and Co$_2$CrAl where the dominant effects are the inter-sublattice interactions between the Mn(Cr) and Co atoms and between Co atoms at different sublattices. For these compounds we find that both MF and RPA give very close values of the Curie temperature slightly underestimating experimental quantities. We study the influence of the lattice compression on the magnetic properties. The temperature dependence of the magnetization is calculated using the RPA method within both quantum mechanical and classical approaches.