Exchange anisotropy in Fe-Nd-B magnets

Large exchange anisotropy (1.0 to 1.5 kOe) has been observed by magnetooptic Kerr effect (MOKE) at the surface of Fe-Nd-B sintered magnets. Three samples, Fe_79-xNd_13+xB₈ with x=0, 1, and 2 at.%, 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, a soft Fe-rich subsurface layer, and magnetically hard Fe₁₄Nd₂B bulk. The authors 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