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The coercivity,microstructure,and magnetic domain structure of Nd-Fe-B sintered magnets by grain boundary diffusion process (GBDP) with TbH3 nanoparticles were systematically investigated.Compared to the original magnet,the coercivity (Hci) of the GBDP magnets improved from 1702 to 2374 kA·m-1 with few remanence reduced from 1.338 to 1.281 T.Electron probe microanalysis (EPMA) analysis showed that Tb diffused along grain boundary,mainly concentrated in the boundary layer of the main phase,and formed a core-shell structure.Magneto-optical Kerr optical microscope (MOKE) analysis showed that there were two types of magnetic domain reversal in one grain: gradual reversal (GR) and abrupt reversal (AR).When the applied field decreased from saturated magnetic field,the reversal magnetic domain nucleated and then spread over the whole grain gradually,which was called GR.However,some grains kept the single domain state until Hh which was a value of reverse direction applied field in second quadrant in hysteresis loops.When the applied field increased above Hh,reversed magnetic domain would suddenly appear and occupy most of the area of the grain,which was called AR.That is because AR grains have higher reversed magnetic domain nucleation field (HRN2) than GR grains (HRN1).After GBDP,the area of AR region increased obviously and GR region decreased accordingly,indicating that the core-shell structure could change GR grain into AR grain.The coreshell structure could suppress flipping of the magnetization of the grains due to the large magnetic anisotropy of Tbrich shell.Therefore,large AR area led to high coercivity.