Small-angle neutron scattering study of coercivity enhancement in grain-boundary-diffused NdFeB sintered magnets

• Tb-diffused NdFeB magnet has been studied by small-angle neutron scattering.
• The correlation length reduces about 15% in comparison with untreated magnet.
• The coercivity enhancement is related to the reduction in the effective defect size.

Isotropic NdFeB-based sintered magnets, before and after a (Tb-doped) grain-boundary diffusion process, have been investigated by means of magnetic-field-dependent small-angle neutron scattering. Compared to the untreated sample, we found a reduced correlation length in the grain boundary diffused sample which, in an applied-field range of 8.5 T, varies between 30 and 35 nm – about 15% smaller than in the untreated specimen. This observation is related to the increased magnetic anisotropy field of the nucleation sites for magnetization reversal, and may be explained by a reduction in the effective defect size responsible for the magnetic inhomogeneities.

(Top) Sketch of the neutron-scattering setup. The external magnetic field H0 is applied perpendicular to the wave vector k0 of the incident neutron beam; q denotes the momentum-transfer or scattering vector with |q| = q = 4πλ−1sinψ, where λ is the mean neutron wavelength (determined by the velocity selector) and 2ψ is the scattering angle. (Bottom) Sketch of the central conclusion of our paper (very much oversimplified). The correlation length lC is (at a given field) a measure for the size of inhomogeneously magnetized regions around defects (e.g., interfaces). Via the GBDP this length scale is reduced in isotropic Nd-Fe-B magnets.Figure optionsDownload as PowerPoint slide