Anisotropy of exchange stiffness based on atomic-scale magnetic properties in rare-earth permanent magnet Nd$_2$Fe$_{14}$B

TL;DR

This study investigates the directional dependence of exchange stiffness in Nd2Fe14B magnets using atomistic and micromagnetic simulations, revealing its impact on coercivity and magnetic reversal processes.

Contribution

It connects atomistic and continuum models to analyze anisotropic exchange stiffness in Nd2Fe14B, highlighting its effects on magnetic properties and reversal behavior.

Findings

Exchange stiffness amplitude varies with crystal orientation.

Anisotropy of exchange stiffness influences coercivity.

Layered Nd structure affects magnetic exchange interactions.

Abstract

We examine the anisotropic properties of the exchange stiffness constant, $\mathcal{A}$, for rare-earth permanent magnet, Nd$_2$Fe$_{14}$B, by connecting analyses with two different scales of length, i.e., Monte Carlo (MC) method with an atomistic spin model and Landau-Lifshitz-Gilbert (LLG) equation with a continuous magnetic model. The atomistic MC simulations are performed on the spin model of Nd$_2$Fe$_{14}$B constructed from ab-initio calculations, and the LLG micromagnetics simulations are performed with the parameters obtained by the MC simulations. We clarify that the amplitude and the thermal property of $\mathcal{A}$ depend on the orientation in the crystal, which are attributed to the layered structure of Nd atoms and weak exchange couplings between Nd and Fe atoms. We also confirm that the anisotropy of $\mathcal{A}$ significantly affects the threshold field for the magnetization reversal (coercivity) given by the depinning process.