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/sub 79-x/Nd/sub 13+x/B/sub 8/ 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 Nd/sub 2/O/sub 3/, a soft Fe-rich subsurface layer, and magnetically hard Fe/sub 14/Nd/sub 2/B 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.<
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