R-T-B BASED RARE EARTH PERMANENT MAGNET

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PROBLEM TO BE SOLVED : To provide an R-T-B based sintered magnet which is suitable for a magnetic force-variable motor, and which has a high squareness ratio and minor curve flatness under the conditions of a low coercive force and a low magnetization magnetic field. SOLUTION : An R-T-B based permanent magnet has a composition formula represented by (R11-x(Y1-y-zCeyLaz)x)aTbBcMd, where R1 represents at least one rare earth element, excluding Y, Ce and La, T represents at least one transition metal including Fe, or a combination of Fe and Co essentially, and M represents Ga or a combination of Ga and one or more kinds of Sn, Bi, and Si; 0.4≤x≤0.7, 0.00≤y+z≤0.20, 0.16≤a/b≤0.28, 0.050≤c/b≤0.075, and 0.005≤d/b≤0.028. The R-T-B based permanent magnet has an R2T14B type tetragonal structure and includes main and grain boundary phases. The D10, D50 and D90 of the main phase crystal grains satisfy the following expressions : D50≤4.00 μm; and (D90-D10)/D50≤1.60. The coverage of the grain boundary phases is 70.0% or more. SELECTED DRAWING : Figure 1 COPYRIGHT : (C)2018, JPO&INPIT