R-T-B based sintered magnet

1. An R-T-B based sintered magnet comprising “R”, “T”, and “B”, wherein

“R” represents a rare earth element including at least Tb,

“T” represents a metal element other than rare earth elements including at least Fe, Cu, Mn, Al, and Co,

“B” represents boron or boron and carbon,

a content of “R” is 28.0 to 32.0 mass % with respect to 100 mass % of a total mass of the R-T-B based sintered magnet,

a content of Cu is 0.04 to 0.50 mass % with respect to 100 mass % of a total mass of the R-T-B based sintered magnet,

a content of Mn is 0.02 to 0.10 mass % with respect to 100 mass % of a total mass of the R-T-B based sintered magnet,

a content of Al is 0.15 to 0.30 mass % with respect to 100 mass % of a total mass of the R-T-B based sintered magnet,

a content of Co is 0.50 to 3.0 mass % with respect to 100 mass % of a total mass of the R-T-B based sintered magnet,

a content of “B” is 0.85 to 1.0 mass % with respect to 100 mass % of a total mass of the R-T-B based sintered magnet, and

Tb2/Tb1 is 0.49 or more and 0.80 or less, where Tb1 (mass %) denotes an average content of Tb at a surface portion of the R-T-B based sintered magnet, and Tb2 (mass %) denotes an average content of Tb at a core portion of the R-T-B based sintered magnet, and

the R-T-B based sintered magnet includes a plurality of main phase grains and grain boundaries, Tb is diffused at least into the core portion of the R-T-B based sintered magnet, and Tb is present in a shell of a main phase grain being a core-shell grain, the shell being defined as the part at which a molar ratio of the heavy rare earth element to the light rare earth element is two times or more the molar ratio at a core of the main phase grain being a core-shell grain.

2. The R-T-B based sintered magnet according to claim 1 , wherein “R” consists of heavy rare earth elements and at least one light rare earth element, and the heavy rare earth elements consist of substantially only Dy and Tb.

3. The R-T-B based sintered magnet according to claim 1 , wherein “R” consists of a heavy rare earth element and at least one light rare earth element, and the heavy rare earth element consists of substantially only Tb.

4. The R-T-B based sintered magnet according to claim 1 , wherein “T” further comprises Ga and

a content of Ga is 0.08 to 0.30 mass % with respect to 100 mass % of a total mass of the R-T-B based sintered magnet.

5. The R-T-B based sintered magnet according to claim 1 , wherein

“T” further comprises Zr and

a content of Zr is 0.10 to 0.25 mass % with respect to 100 mass % of a total mass of the R-T-B based sintered magnet.

6. The R-T-B based sintered magnet according to claim 1 , wherein

“T” further comprises Ga and Zr,

a content of Ga is 0.08 to 0.30 mass % with respect to 100 mass % of a total mass of the R-T-B based sintered magnet, and

a content of Zr is 0.10 to 0.25 mass % with respect to 100 mass % of a total mass of the R-T-B based sintered magnet.

7. The R-T-B based sintered magnet according to claim 1 , wherein Ga/Al is 1.30 or less by mass ratio.

8. The R-T-B based sintered magnet according to claim 1 , wherein the content of “R” is 29.0 to 31.5 mass %.

9. The R-T-B based sintered magnet according to claim 1 , wherein the content of Co is 1.0 to 2.5 mass %.

10. The R-T-B based sintered magnet according to claim 1 , wherein the content of Cu is 0.10 to 0.50 mass %.

11. The R-T-B based sintered magnet according to claim 1 , wherein the content of Al is 0.18 to 0.30 mass %.

12. The R-T-B based sintered magnet according to claim 1 , wherein the content of Mn is 0.02 to 0.06 mass %.

13. The R-T-B based sintered magnet according to claim 1 , wherein the content of “B” is 0.90 to 1.0 mass %.