The reversal mechanism and coercivity of permanent magnetic materials

Abstract

The underlying mechanism of coercivity in permanent magnets has been a topic of intense interest for many years. It is motivated by the fact that the measured coercivity approaches only 20%–40% of the theoretical nucleation fields as derived from micromagnetic theory. We address this problem by proposing an analytical model, within the framework of the micromagnetic approximation, to examine the mechanism of magnetization reversal in hard magnetic materials. The exchange interaction between neighboring grains with different easy axes orientations can result in the formation of a domain wall-like magnetization structure (transition region) in the grain boundary. We propose that the transition region can propagate between neighboring grains provided that it is energetically favorable. The subsequent nucleation of a domain wall is shown to reduce the critical field considerably. Applying our model to a thin film, whose magnetic grains have a randomly oriented in-plane easy axis distribution, we have calculated the coercivity for the film to be 0.14HK, where HK is the anisotropy field. It is found that the coercivity decreases with increasing film thickness. For a material with a three-dimensional random easy axis distribution, we obtain the coercivity as 0.16HK. These results are substantially lower than that given by the Stoner-Wohlfarth model and are consistent with available experimental results.