Magnetic properties of isotropic and anisotropic Nd-Fe-B bonded magnets

Abstract A trial to make Nd-Fe-B rapidly quenched flakes anisotropic by grain growth during heat treatment in the magnetic field has been carried out. No magnetic anisotropy is observed, but the optimum heat treatment temperature is lowered by about 100°C by applying 14 kOe of magnetic field. There is, as yet, no explanation of this phenomenon. Another way of making anisotropic bonded magnets is to use pulverized powders of anisotropic dense die-upset magnets as a starting material. By examination of the process parameters for Nd-Fe-B die-upset magnets it was found from stress-strain curves that the Nd 14 Fe 80 B 6 ternary composition has the best workability and to achieve aligned material the compression ratio (defined as the inverse of reduction in height) should be larger than 4. Grain coarsening resulted in a decrease in coercivity ( i H c ) during die upsetting. Ga is the best additional element for enhancing i H c with a little sacrifice of remanence ( B r ) and no deterioration in hot workability. Further improvement of thermal stability by raising the Curie temperature ( T c ) of the Nd-Fe-B-Ga alloy can be achieved by Co substitution. The i H c of the powder made by pulverizing the die-upset magnets is nearly independent of particle size. Anisotropic bonded magnets were developed using this powder. The ( BH ) max of compression-moulded magnet with Ga addition and Co substitution, Nd 14 Fe 71.75 Co 7.5 B 6 Ga 0.075 , is 15 MGOe. The irreversible losses after exposure up to 140°C is lower than 5% ( P c =2), which is sufficient thermal stability for polymer-bonded magnets.