Analysis of magnetization and demagnetization processes in Nd-Fe-B sintered magnets using the step method

Abstract

The magnetization and demagnetization processes in the 11 types of Dy-free Nd-Fe-B sintered magnets based on the NEOMAX-50 type magnets were investigated using the step method that is our original method of measurements and analyses of minor loops with step by step increased maximum applied fields. With the decrease in average grain sizes (DAVE), the single-domain grains (SDG) volume fraction increased and inversely that of the saturable multi-domain grains (SMDG) decreased, the changing behavior was monotonous. The dependence of coercivities on the DAVE, however, showed clear change around DAVE=3.0 μm, from the increase with decreasing DAVE to the opposite relationship less than DAVE ‹ 3.0 μm. We consider that the changing in the relationships should come from the ratios of the domain width (WD) to the DAVE, and conclude that the well cutting of magnetic interactions at the grain boundaries should realize the coercivity of μ0HC ≈ 2 T in fine grain size Dy-free samples.