Large Barkhausen jumps observed in Nd-Fe-B sintered magnets at very low temperatures

The demagnetization process for Nd₁₅Fe₇₇B ₈ sintered magnets was studied by precise loop measurements below 50 K. A zigzag variation of coercivity with temperature was observed below about 18 K. In this temperature range, the magnetization reverses with large discontinuous magnetization jumps. The origin of the jumps, which are caused by the sudden growth of residual reversed magnetic domains pinned at some defects, was investigated by measuring hysteresis loops at very low temperatures and analyzing them from the viewpoint of domain-wall nucleation and propagation. The temperature of 18 K coincides with the Curie temperature of the B-rich Nd_1.11Fe₄B₄ phase segregated in the Nd ₁₅Fe₇₇B₈ magnet. This behavior may be related to the soft magnetic properties of the B-rich phase and the demagnetizing field in the magnet