Ab-initio calculation of effective exchange interactions, spin waves, and Curie temperature in L2_1- and L1_2-type local moment ferromagnets

TL;DR

This paper uses first-principles calculations to analyze exchange interactions and spin waves in various ferromagnets, estimating Curie temperatures and comparing with experimental data, revealing the importance of accurate electronic structure modeling.

Contribution

It provides a detailed ab-initio study of exchange interactions and Curie temperatures in specific L2_1- and L1_2-type ferromagnets, highlighting the role of indirect exchange mediated by sp electrons.

Findings

Exchange interactions are long-range and RKKY-like.

Spin-wave dispersions agree with experimental data.

Estimated Curie temperatures can deviate when magnetic moments are overestimated.

Abstract

Employing first-principles electronic structure calculations in conjunction with the frozen-magnon method we study the effective exchange interactions and spin waves in local moment ferromagnets. As prototypes we have chosen three L2$_1$-type full Heusler alloys Cu$_2$MnAl, Ni$_2$MnSn and Pd$_2$MnSn, and the L1$_2$-type XPt$_3$ compounds with X= V, Cr and Mn. We have also included CoPt$_3$ which is a usual ferromagnet. In all compounds due to the large spatial separation ($\sim 4$ \AA) of the magnetic transition metal atoms, the 3\textit{d} states belonging to different atoms overlap weakly and as a consequence the exchange coupling is indirect, mediated by the \textit{sp} electrons. Calculated effective exchange parameters are long range and show RKKY-type oscillations. The spin-wave dispersion curves are in reasonable agreement with available experimental data. Using the calculated exchange parameters we have estimated the Curie temperatures within both the mean-field and the random-phase approximations. In local moment ferromagents deviations of the estimated Curie temperature with respect to the available experimental data occur when the ground-state electronic structure calculations overestimate the values of the spin magnetic moments as in VPt$_3$.