Simultaneous increase in remanence and coercivity during grain refining of Nd-Fe-B deformed magnets

Abstract To explore the feasibility of deep grain-size refinement in overcoming the problem of traditional Br-Hci trade-off in bulk permanent magnets, the effect of deformation temperature on the texture strength, grain refining, and magnetic properties of slow-deformed Nd-Fe-B magnets were systematically studied. As a result, strong textures with gradual grain refining from conventionally large size of DL=650 nm to small nanoscale size of DL=53 nm (DL denotes the lateral size of the aligned plate-like grains) are observed as the temperature reduces from Td=650 oC to Td=450 oC. Moreover, magnetic observations show a simultaneous increase in remanence and coercivity as the grain refines from DL=650 nm to DL=127 nm. The increase in coercivity results from the grain size refinement as well as the smaller aspect ratio of the plate-like grains, and the increase in remanence results from the improved texture homogeneity. As compared with the DL=650 nm magnets, the simultaneous increase in remanence and coercivity leads to optimum enhancements of 57% in coercivity, 10% in remanence, and 25% in energy product, which demonstrate the feasibility of near-nanoscale grain refinement in overcoming the traditional Br-Hci trade-off for improved (BH)max values. The failure of higher coercivities in the DL≤80 nm magnets is closely related with the defect effects of the grain boundaries. Mechanisms explaining the grain refining and texture changing behavior were also discussed.