Isotropic bulk exchange-spring magnets with 34 kJ/m3 prepared by spark plasma sintering method

Densification of exchange-spring magnet ribbons of Nd_xFe_85-xCo₈V₁B₆ (x=7-11) alloys has been investigated. The starting ribbon powders containing amorphous parts were prepared by melt spinning and were compacted by using a spark plasma sintering (SPS) method. SPS is a type of solid compression sintering similar to hot-pressing and the sintering promotion factor is self-heat generation by electric discharges between particles when the on-off DC pulse is applied. The bulk magnets indicated the springback phenomenon, pointing to multiple phases of the Nd₂Fe₁₄B type and the α-Fe type, and had isotropic magnetic properties. With increasing Nd content, B_r decreased from 1.05 T to 0.82 T and H_cJ increased from 256 kA/m to 832 kA/m. These changes were equivalent to those of the starting ribbons following annealing. The grain sizes of these bulk magnets were about 40 nm when x=7-11. Under optimum preparation conditions, bulk Nd ₁₀Fe₇₅Co₈V₁B₆ magnet indicated (BH)_max=134 kJ/m³ (16.8 MGOe)