Modeling of demagnetization processes in permanent magnets measured in closed-circuit geometry

Abstract

The hysteresis loops of nucleation-type magnets made of exchange-decoupled grains (i.e., sintered Nd-Fe-B magnets) reflect the discrete character of magnetization switching in such materials. Due to this discrete character, the experimental determination of coercivity depends on the measurement protocol. Finite element modeling allows us to investigate how the pattern of reversed grains develops during sample demagnetization performed under closed-circuit conditions, provided that the basic features of the hysteresigraph are known. Numerical modeling provides a quantitative understanding of the collective effects that are very pronounced in the closed-circuit configuration and shows how they affect both the slope of the demagnetizing curve and the sample coercivity. With a grain coercive field standard deviation adjusted to 0.1 T, it is numerically found that the difference in coercivity between closed- and open-circuit configurations is 40 kA/m, in good agreement with previous experimental data.