R-Fe-B rare earth sintered magnet

1. An R—Fe—B based rare-earth sintered magnet comprising, as a main phase, crystal grains of an R 2 Fe 14 B type compound that includes Nd, which is a light rare-earth element, as a major rare-earth element R,

wherein the magnet includes a heavy rare-earth element RH (which is at least one of Dy and Tb) that has been introduced through the surface of the sintered magnet by diffusion, and

wherein the magnet has a region in which the concentration of the heavy rare-earth element RH in a grain boundary R-rich phase is lower than at the surface of the crystal grains of the R 2 Fe 14 B type compound but higher than at the core of the crystal grains of the R 2 Fe 14 B type compound.

2. The R—Fe—B based rare-earth sintered magnet of claim 1 , wherein if the concentration of Dy in the R—Fe—B based rare-earth sintered magnet is x (mass %) and if the temperature coefficient of an average coercivity H cJ from 20° C. through 140° C. is y (%/° C.), the magnet satisfies the inequality:

0.015 ×x− 0.57 ≦y≦ 0.023 ×x− 0.50.

3. The R—Fe—B based rare-earth sintered magnet of claim 1 , wherein if the concentrations of the heavy rare-earth elements Dy and Tb in the R—Fe—B based rare-earth sintered magnet are x1 (mass %) and x2 (mass %), respectively, and if the temperature coefficient of an average coercivity H cJ from 20° C. through 140° C. is y (%/° C.), the magnet satisfies the inequality:

0.015×( x 1+1.5 ×x 2)−0.57 ≦y≦ 0.023×( x 1+1.5 ×x 2)−0.50.

4. The R—Fe—B based rare-earth sintered magnet of claim 1 , wherein the region is located at a depth of 100 μm under the surface of the sintered magnet body.