Role of Domain Structure Change in the Coercivity Mechanism of Sintered Permanent Magnets

This paper reviews my research on the role of domain structures and domain wall motion, especially, in anisotropic sintered permanent magnets. The existence of domain wall motion in Nd–Fe–B sintered magnets during the intermediate stages of the demagnetization process has been revealed experimentally using the step method, susceptibility measurements of magnetically demagnetized samples, and direct Hall probe sensor measurements of flux densities in holes drilled in magnets. This evidence of multi-domain regions during demagnetization has shown that the magnetically reversed regions should be larger than several micrometers. To explain these experimental results, the critical size of the magnetic reversal center, which is a nanometer-sized nucleus, and the explosive growth of the nucleus to micrometer-sized reversed regions with volumes 105–106 times larger were discussed based on classical equations. Accordingly, the Stoner–Wohlfarth model, Kronmüller’s equation modified by considering the surface domain wall of the nucleus, and Becker’s model of growth of the reversal center were discussed. The phenomenological Ginzburg–Landau free energy curves were also determined for each case of magnetic reversal in the single-domain grain and in the saturable multi-domain grains in conjunction with the energy equations. ©2021 Japan Society of Powder and Powder Metallurgy.