Grain-size dependent demagnetizing factors in permanent magnets

TL;DR

This paper investigates how grain size influences the demagnetizing factors and coercive fields in permanent magnets, revealing a size-dependent demagnetizing factor that affects magnetic performance.

Contribution

It introduces a size-dependent demagnetizing factor model for permanent magnets and compares experimental data with micromagnetic simulations to explain coercivity variations.

Findings

Demagnetizing factor varies from 0.2 to 1.22 with grain size

Coercivity decreases as grain size increases

Micromagnetic simulations support non-uniform magnetostatic field effects

Abstract

The coercive field of permanent magnets decreases with increasing grain size. The grain size dependence of coercivity is explained by a size dependent demagnetizing factor. In Dy free Nd$_2$Fe$_{14}$B magnets the size dependent demagnetizing factor ranges from 0.2 for a grain size of 55 nm to 1.22 for a grain size of 8300 nm. The comparison of experimental data with micromagnetic simulations suggests that the grain size dependence of the coercive field in hard magnets is due to the non-uniform magnetostatic field in polyhedral grains.