Influence of defect thickness on the angular dependence of coercivity in rare-earth permanent magnets

TL;DR

This study investigates how defect thickness influences the angular dependence of coercivity in Nd-Fe-B magnets, combining micromagnetic simulations with experimental validation to understand reversal mechanisms.

Contribution

It reveals the critical role of surface defect thickness in coercivity behavior and links simulation results with experimental grain boundary modifications.

Findings

Thinner defects favor nucleation-driven reversal.

Thicker defects promote domain wall de-pinning.

Experimental infiltration modifies grain boundaries, matching simulation trends.

Abstract

The coercive field and angular dependence of the coercive field of single-grain Nd$_{2}$Fe$_{14}$B permanent magnets are computed using finite element micromagnetics. It is shown that the thickness of surface defects plays a critical role in determining the reversal process. For small defect thicknesses reversal is heavily driven by nucleation, whereas with increasing defect thickness domain wall de-pinning becomes more important. This change results in an observable shift between two well-known behavioral models. A similar trend is observed in experimental measurements of bulk samples, where a Nd-Cu infiltration process has been used to enhance coercivity by modifying the grain boundaries. When account is taken of the imperfect grain alignment of real magnets, the single-grain computed results appears to closely match experimental behaviour.