The origin of coercivity enhancement in newly prepared high coercivity Dy-free Nd-Fe-B sintered magnets

Abstract

Coercivity enhancement in Nd-Fe-B sintered magnets without substitution of Dy for Nd is an important research target in the field of permanent magnets. Our newly prepared Dy-free Nd-Fe-B sintered magnets with fine crystal grain sizes of approximately 1.9 μm showed high coercivities near 1.6 MA/m. The characteristic magnetization behavior of the magnets that we have not observed previously in other Nd-Fe-B sintered magnets, under applied fields of 1.4–2.4 MA/m, appears to be responsible for the appearance of the high coercivity. In this study, we examine the origin of the characteristic magnetization mechanism and conclude that it was caused by the strong interaction between the domain walls (DWs) and the grain boundaries (GBs). We introduce our novel step method and investigate the characteristic mechanism using the method. This method can assist in clarifying whether the characteristic magnetization mechanism results from the hopping of DWs from one GB to another, and/or by magnetic reversal appearing to be like single-domain grains. The basis of our conclusion, that the latter mechanism (single-domain grain magnetic reversal) occurs in the magnetization, will be explained based mainly on the results obtained by the step method in this paper.