Magnetic properties on bulk exchange-spring magnets in Nd-Fe-Co-V-B system formed by Spark Plasma Sintering

Bulk exchange-spring magnets of Nd₁₁Fe₇₂Co₈V_1.5B_7.5 and Nd₉Fe₇₆Co₈V₁B₆ alloys were prepared by Spark Plasma Sintering (SPS) using the rapidly quenched ribbon powders. The effects of SPS process conditions on magnetic properties and structures were investigated. SPS is a type of solid compression sintering similar to hot-pressing (HP) and can be performed at lower temperature with a shorter holding time. Bulk magnets reached to approximately full density at a sintering temperature of higher than 650 °C for 5 min. and had magnetically isotropic properties. They indicated the spring back phenomenon peculiar to exchange-spring magnets and consisted of multiphase of the Nd₂Fe₁₄B type phase and the α-Fe type phase. Grain growth did not occurred between the powders of the starting ribbons, but the fine crystals were observed inner the powders of the ribbons with an increase in the sintering temperature. Optimum preparation conditions and magnetic properties were as follows: composition, Nd₉Fe₇₆Co₈V₁B₆; surface velocity of the roller during melt spinning for starting ribbons, V_s = 20 m/s; SPS condition, at 675 °C for 5 min. under 490 MPa; magnetic and other properties, B_t = 0.99 T, H_cJ = 415 kA/m, (BH)_max = 107.8 kJ/m³, H_k/H_cJ×100 = 23%, spring back ratio = 40% and D = 7.59 Mg/m³.