EXCHANGE-ANISOTROPY IN FE-ND-B MAGNETS

Large exchange anisotropy (1.0 to 1.5 kOe) has been observed by magnetooptic Kerr effct (MOKE) at the surface of Fe-Nd-B sintered magnets. Three samples Fe79−xNd13+xB8 with x = 0, 1, and 2 atom %, were studied. These materials showed bulk intrinsic coercivities of 0.3, 4.2, and 7.7 kOe respectively. All samples showed MOKE coercivities below 0.6 kOe. Only the x = 1 sample exhibited significant MOKE exchange anisotropy. A three-layer model is assumed near the surface: outermost, nonmagnetic Nd2O3, soft Fe-rich subsurface layer, and magnetically hard Fe14Nd2B bulk. We speculate that the bulk is exchange-coupled to the soft Fe-rich layer in the x = 0, 1 samples but not in the x = 2 sample because of the greater Nd content of the grain boundaries for x = 2. This explains both the low bulk coercivity in x = 0 and x = 1 and the observation of exchange anisotropy for x = 1 but not for x = 2. © 1990 IEEE