Microstructure and Magnetization Reversal of Iron/Rare Earth/Boron Permanent Magnets

摘要

An investigation of the microstructure and details of magnetization reversal of the new class of permanent magnets, Fe-Nd-B, was conducted. The microstructure consists of almost-defect-free grains of matrix Nd sub 2 Fe sub 14 B surrounded by a Nd-rich, oxygen stabilized, fcc phase. A third phase, Fe sub 4 NdB sub 4 , along with the rare earth sesquioxide was infrequently observed as isolated grains. It was shown that the magnet behaves as a nucleation-controlled magnet, i.e., the magnetization reversal is controlled by the domain wall nucleation step. Since the interiors of the matrix grains are defect-free, domain wall nucleation occurs due to defects at the grain boundaries and two-phase interfaces. One possible defect is oxygen. The effect of post-sintering heat treatments upon the intrinsic coercivity and energy product has been rationalized based upon the above observations. Increase in the cooling rate after sintering leads to a decrease in the intrinsic coercivity due to the effect of quenching stresses, caused by thermal expansion mismatch at the fcc phase-matrix interface. Thus, the combined effect of oxygen and the quenching stresses causes a large reduction in intrinsic coercivity and energy product. A magnetization reversal model has been proposed, which incorporates the effect of quenching stresses also. Predictions of the model agree with the effect of temperature and composition on the intrinsic coercivity and anisotropy field. Lorentz electron microscopic observations suggest that domain walls interact strongly with grain boundaries and the grain boundary fcc phase when they are parallel to the microstructural feature. Inclusions of size below 500A have been observed to pin domain walls, thus suggesting a possible mechanism for improving the magnetic properties. Further improvement in the intrinsic coercivity and energy product can come about only by making domain wall nucleation more difficult. (ERA citation 13:018583)