Effect of quench rate on the intrinsic coercivity of iron-rare earth-boron permanent magnets

The effect of post-sintering cooling rate on the microstructure and magnetic properties of Fe-Nd-B based magnets has been studied. It has been shown that the intrinsic coercivity and energy product vary inversely as the cooling rate. In all cases, the magnet behaves as a nucleation controlled magnet characterized by a high initial permeability, although nucleation of reverse domains occurs at progressively lower fields as the cooling rate is increased. Samples quenched into water and oil exhibited extensive intergranular quench cracking. Scanning Auger elemental mapping as well as energy dispersive X-ray microanalysis showed the presence of oxygen, along with the rare earths, in the grain boundary phase. A qualitative argument for the influence of oxygen in reducing the magnetocrystalline anisotropy of the RE2Fe14B phase is presented. Finally, the effect of cooling rate on the intrinsic coercivity and energy product can be understood in terms of the effect of local strains, at the grain boundaries and interfaces, on the magnetocrystalline anisotropy of the RE2Fe14 phase. © 1988.