Theory of defect-induced crystal field perturbations in rare earth magnets

TL;DR

This paper develops a theoretical framework to understand how point defects affect the crystal field and magnetic anisotropy in rare earth magnets, enabling efficient atomistic simulations of disordered materials.

Contribution

It introduces an analytical model for defect-induced crystal field perturbations in rare earth magnets, validated with first-principles calculations, and applicable to atomistic spin dynamics simulations.

Findings

Point defects significantly alter local anisotropy profiles.

The screened point charge model accurately describes crystal field perturbations.

Disorder can cause dramatic variations in magnetic anisotropy at the atomic level.

Abstract

We present a theory describing the single-ion anisotropy of rare earth (RE) magnets in the presence of point defects. Taking the RE-lean 1:12 magnet class as a prototype, we use first-principles calculations to show how the introduction of Ti substitutions into SmFe$_{12}$ perturbs the crystal field, generating new coefficients due to the lower symmetry of the RE environment. We then demonstrate that these perturbations can be described extremely efficiently using a screened point charge model. We provide analytical expressions for the anisotropy energy which can be straightforwardly implemented in atomistic spin dynamics simulations, meaning that such simulations can be carried out for an arbitrary arrangement of point defects. The significant crystal field perturbations calculated here demonstrate that a sample which is single-phase from a structural point of view can nonetheless have a dramatically varying anisotropy profile at the atomistic level if there is compositional disorder, which may influence localized magnetic objects like domain walls or skyrmions.