Fe-site-resolved anisotropy energies in Nd$_2$Fe$_{14}$B for atomistic spin dynamics

TL;DR

This paper investigates and improves the modeling of Fe anisotropy energies in Nd2Fe14B magnets for atomistic spin dynamics, proposing new models aligned with first-principles calculations.

Contribution

It introduces two corrected models for Fe anisotropy in ASD simulations, addressing previous discrepancies and incorporating anisotropic exchange effects.

Findings

First-principles torque calculations reveal Fe anisotropy contributions from anisotropic exchange.

Single-ion model cannot fully explain the observed Fe anisotropy.

Proposed simplified mean-field approach for Fe anisotropy modeling.

Abstract

Nd-Fe-B magnets are the most widely used high performance magnets in the world today, and remain the subject of both experimental and computational research aimed at understanding and optimizing them. Atomistic spin dynamics (ASD) is one technique which has been used in recent years to provide insight into magnetic properties relevant to coercivity, such as domain wall width. Although it is relatively clear how to model magnetocrystalline anisotropy arising from rare-earth atoms in these simulations, the contribution from the transition metal Fe is less obvious, due to the itinerant nature of the magnetism. Here, we examine previous treatments of Fe anisotropy in ASD simulations and identify a discrepancy with previously-published first-principles studies. We derive two models which correct this discrepancy, one based on single-ion theory and the other on anisotropic exchange, and test their performance by comparing to first-principles torque calculations on Y$_2$Fe$_{14}$B. The torque calculations show a contribution which cannot be explained by the single-ion model but arises naturally from (antisymmetric) anisotropic exchange. We propose practical strategies to model Fe anisotropy in future ASD simulations, including a simplified (mean-field) description of anisotropic exchange, which may have applications beyond R$_2$Fe$_{14}$B to the wider class of itinerant magnetic materials.