Magnetic properties and domain structures in Nd-Fe-B sintered magnets with Tb additive reacted and diffused from the sample surface

We investigated the magnetic properties and domain structures of Nd-Fe-B sintered magnets with a Tb additive reacted and diffused from the sample surface. We characterized these magnets using vibrating sample magnetometry, Kerr effect microscopy, and electron probe microanalysis. In the reacted sample (R-Tb) in this study, the Tb concentration was higher on its surface (∼30 μm depth) and in its interior grain boundaries than inside the subsurface crystal grains. The saturation polarization (Js) was about 6% lower in the reacted sample than in the non-reacted sample (N-Tb); which should come from the Js decrease in the Tb-substituted regions in the R-Tb sample. The substitution greatly improved the coercivity (Hc) of the samples from 1.02 MA/m (N-Tb) to 1.72 MA/m (R-Tb). To understand the reason of this increase, we measured the domain widths (dD) from the near-surface region (5-10 μm depth) of the polished R-Tb sample and from deeper below the surface in the R-Tb and N-Tb samples. The average dD from the near-surface region of the polished R-Tb sample was approximately 20% wider than the other polished planes, where the dD was about 1.0 μm for both samples. In addition to considering the reasons for this coercivity enhancement, we used a novel step method to compare the demagnetization behaviors of the R-Tb and N-Tb samples. The R-Tb sample likely had enhanced coercivity mainly because of the difficulty of nucleating magnetic reversal portions. This difficulty was mainly caused by the enlargement of the anisotropy field (Ha = 2K1/Js) because of the decrease in saturation polarization by the Tb diffusion and reaction. The ∼20% wider average dD on the R-Tb surface polished plane was also explained mainly by this Js decrease.