The reversal mechanism and coercivity of permanent magnetic materials

The underlying mechanism of coercivity in permanent magnets has been a topi c of intense interest for many years. It is motivated by the fact that the measured coercivity approaches only 20%-40% of the theoretical nucleation f ields as derived from micromagnetic theory. We address this problem by prop osing an analytical model, within the framework of the micromagnetic approx imation, to examine the mechanism of magnetization reversal in hard magneti c materials. The exchange interaction between neighboring grains with diffe rent easy axes orientations can result in the formation of a domain wall-li ke magnetization structure (transition region) in the grain boundary. We pr opose that the transition region can propagate between neighboring grains p rovided that it is energetically favorable. The subsequent nucleation of a domain wall is shown to reduce the critical field considerably. Applying ou r model to a thin film, whose magnetic grains have a randomly oriented in-p lane easy axis distribution, we have calculated the coercivity for the film to be 0.14H(K), where H-K is the anisotropy field. It is found that the co ercivity decreases with increasing film thickness. For a material with a th ree-dimensional random easy axis distribution, we obtain the coercivity as 0.16H(K). These results are substantially lower than that given by the Ston er-Wohlfarth model and are consistent with available experimental results. (C) 2000 American Institute of Physics. [S0021-8979(00)15808-6].