Surface modified R-Fe-B sintered magnet and its manufacturing method

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PROBLEM TO BE SOLVED : To provide a surface-modified R-Fe-B based sintered magnet exhibiting excellent corrosion resistance even under severe condition and having excellent magnetic characteristics, and to provide a production method therefor.SOLUTION : The surface-modified R-Fe-B based sintered magnet comprises : a composition consisting of 25-40 mass% of rare earth element : R; 0.6-1.6 mass% of B, 0-1.0 mass% of at least one kind of additive element : M selected from a group consisting of Al, Si, Ti, V, Cr, Mn, Ni, Cu, Zn, Ga, Zr, Nb, Mo, Ag, In, Sn, Hf, Ta, W, Pb, and Bi; the reminder where 0.01-2.5 mass% of the entire magnet is Fe substituted Co; and the inevitable impurities. The surface-modified part consists of a modified layer having at least four layers, and the modified layer has at least a main layer containing R, Fe, Co, B and oxygen, a Co-rich layer, an R-rich layer, and an outermost layer, in order from the inside of the magnet.