R-T-B sintered magnet and method for producing the same

1. An R-T-B based sintered magnet comprising:

a light rare-earth element R L , which is at least one of Nd and Pr, a heavy rare-earth element R H , which is at least one of Dy and Tb, and Nd 2 Fe 14 B type crystals as a main phase; wherein

a first region, which includes the heavy rare-earth element R H in a first concentration of zero or more heavy rare-earth elements R H , and a second region, which includes the heavy rare-earth element R H in a second concentration that is higher than the first concentration, are stacked in layers such that the layers extend across an entire length or width of the R-T-B based sintered magnet; and

the first and second regions are sintered and combined together.

2. The R-T-B based sintered magnet of claim 1 , further comprising a shrinkage reducer M, which is at least one element selected from the group consisting of C, Al, Co, Ni, Cu and Sn.

3. The R-T-B based sintered magnet of claim 2 , wherein the shrinkage reducer M has a higher concentration in the first region than in the second region.

4. The R-T-B based sintered magnet of claim 2 , wherein the first region includes about 50 ppm to about 3,000 ppm of C as M1 that is one of the shrinkage reducers M.

5. The R-T-B based sintered magnet of claim 2 , wherein the first region includes at least one element selected from the group consisting of Al, Co, Ni, Cu and Sn as M2 that is another one of the shrinkage reducers M, the content of M2 being equal to or greater than about 0.02 mass %.

6. The R-T-B based sintered magnet of claim 1 , wherein each of the first and second regions has a thickness of at least about 0.1 mm and the magnet has a thickness of at least about 1.0 mm.

7. The R-T-B based sintered magnet of claim 1 , further comprising a region in which the heavy rare-earth element R H has diffused on a boundary between the first and second regions.

8. The R-T-B based sintered magnet of claim 1 , further comprising a region in which the concentration of the heavy rare-earth element R H has a gradient on a boundary between the first and second regions.

9. The R-T-B based sintered magnet of claim 8 , wherein a portion of the first and second regions, which covers the surface of the magnet at least partially, includes a portion in which the heavy rare-earth element R H has a constant concentration from the surface of the magnet toward the boundary.

10. A method for producing an R-T-B based sintered magnet including both a light rare-earth element R L , which is at least one of Nd and Pr, and a heavy rare-earth element R H , which is at least one of Dy and Tb, and Nd 2 Fe 14 B type crystals as a main phase, the method comprising the steps of:

providing a first material alloy powder, which includes either the heavy rare-earth element R H in a relatively low concentration or no heavy rare-earth elements R H at all, and a second material alloy powder, which includes the heavy rare-earth element R H in a relatively high concentration;

forming a composite compact including a first compact portion made of the first material alloy powder that extends across an entire length or width of the composite compact and a second compact portion made of the second material alloy powder that extends across the entire length or width of the composite compact; and

sintering the composite compact, thereby making a sintered magnet in which the first and second compact portions have been combined together.

11. The method of claim 10 , wherein the step of forming the composite compact includes:

a first forming process step for forming a temporary compact by loading a cavity, defined by a die, with one of the first and second material alloy powders and compressing the material alloy powder; and

a second forming process step for forming the composite compact by loading the cavity defined by the die with the other alloy powder and compressing the material alloy powder along with the temporary compact.

12. The method of claim 10 , wherein the step of forming the composite compact includes the steps of:

providing the first compact portion made of the first material alloy powder;

providing the second compact portion made of the second material alloy powder; and

compressing the first and second compact portions, thereby forming the composite compact in which the first and second compact portions have been combined together.

13. The method of claim 10 , wherein the step of forming the composite compact includes the steps of:

providing the first compact portion made of the first material alloy powder;

providing the second compact portion made of the second material alloy powder; and

stacking the first and second compact portions one upon the other, thereby forming the composite compact in which the first and second compact portions are in contact with each other.

14. The method of claim 10 , wherein the first and second material alloy powders include a shrinkage reducer M, which is at least one element selected from the group consisting of C, Al, Co, Ni, Cu and Sn, and the shrinkage reducer M has a higher concentration in the first material alloy powder than in the second material alloy powder.

15. The method of claim 10 , wherein the first material alloy powder has a finer particle size than the second material alloy powder.

16. The method of claim 10 , wherein in the step of forming the composite compact, the first compact portion made of the first material alloy powder has a higher green density than the second compact portion made of the second material alloy powder.