R-Fe-B type rare earth sintered magnet and process for production of the same

1. An R—Fe—B based rare-earth sintered magnet comprising an R—Fe—B based rare-earth sintered magnet body that includes, as a main phase, crystal grains of an R 2 Fe 14 B type compound, including a light rare-earth element RL, which is at least one of Nd and Pr, as a major rare-earth element R, and a heavy rare-earth element RH, which is at least one element selected from the group consisting of Dy, Ho and Tb,

wherein at a depth of 20 μm under the surface of the R—Fe—B based rare-earth sintered magnet body, the crystal grains of the R 2 Fe 14 B type compound include an RH diffused layer at an outer periphery of the crystal grains, the RH diffused layer having an average thickness of 2 μm or less and having a composition of (RL 1-x RH x ) 2 Fe 14 B, where 0.2≦x≦0.75,

wherein at a depth of 500 μm under the surface of the R—Fe—B based rare-earth sintered magnet body, the crystal grains of the R 2 Fe 14 B type compound have an RH diffused layer with an average thickness of 0.5 μm or less in their outer periphery,

wherein the surface of the R—Fe—B based rare-earth sintered magnet body is a machined surface that is not covered with a metal layer of the heavy rare-earth element RH and that is a surface of the R—Fe—B based rare-earth sintered magnet body, from which its surface portion has already been removed after the heavy rare-earth element RH has been introduced into the R—Fe—B based rare-earth sintered magnet body from outside of the R—Fe—B based rare-earth sintered magnet body,

wherein the R—Fe—B based rare-earth sintered magnet body has a size of 1 mm to 4 mm as measured in a thickness direction,

wherein a difference ΔH cJ 1 in coercivity is 150 kA/m or less between:

the entire R—Fe—B based rare-earth sintered magnet body; and

a portion of the R—Fe—B based rare-earth sintered magnet body below a distance of 200 μm from the surface of the R—Fe—B based rare-earth sintered magnet body, and

wherein the magnet has an RH-RL-O compound in at least one grain boundary triple junction, which is located at a depth of 100 μm or less under the surface of the R—Fe—B based rare-earth sintered magnet body.

2. The R—Fe—B based rare-earth sintered magnet of claim 1 , wherein at a depth of 500 μm under the surface of the R—Fe—B based rare-earth sintered magnet body, the crystal grains of the R 2 Fe 14 B type compound include an RH diffused layer having a composition (RL 1-x RH x ) 2 Fe 14 B, where 0.2≦x≦0.75.

3. The R—Fe—B based rare-earth sintered magnet of claim 1 , wherein in a region of the R—Fe—B based rare-earth sintered magnet body between the depths of 20 μm and 500 μm under a surface of the R—Fe—B based rare-earth sintered magnet body, the crystal grains of the R 2 Fe 14 B type compound have an RH diffused layer in their outer periphery, and

wherein the greater the depth under the surface of the R—Fe—B based rare-earth sintered magnet body, the thinner the RH diffused layer gets in the outer periphery of the crystal grains of the R 2 Fe 14 B type compound.

4. The R—Fe—B based rare-earth sintered magnet of claim 1 , wherein the RH diffused layer has a uniform composition in which x has a dispersion of 10% or less at least within a single crystal grain.

5. The R—Fe—B based rare-earth sintered magnet of claim 1 , wherein the thickness of the RH diffused layer is 20% or less of the average grain size of the crystal grains of the R 2 Fe 14 B type compound.

6. The R—Fe—B based rare-earth sintered magnet of claim 1 , wherein in the crystal grains of the R 2 Fe 14 B type compound at the depth of 20 μm under the surface of the R—Fe—B based rare-earth sintered magnet body, the concentration of RH in the (RL 1-x RH x ) 2 Fe 14 B, where 0.2≦x≦0.75, layer is at least 6.0 mass % greater than that of RH at a core of the crystal grains.

7. The R—Fe—B based rare-earth sintered magnet of claim 1 , wherein in at least one of the crystal grains of the R 2 Fe 14 B type compound that are located at the depth of 100 μm or less under the surface of the R—Fe—B based rare-earth sintered magnet body, the concentration of RH in the RH diffused layer is smaller than the concentration of RH of the RH-RL-O compound in the at least one grain boundary triple junction, which surrounds the crystal grain of the R 2 Fe 14 B type compound, but greater than that of the rest of the at least one grain boundary triple junction other than the RH-RL-O compound.

8. An R—Fe—B based rare-earth sintered magnet comprising an R—Fe—B based rare-earth sintered magnet body that includes, as a main phase, crystal grains of an R 2 Fe 14 B type compound, including a light rare-earth element RL, which is at least one of Nd and Pr, as a major rare-earth element R, and a heavy rare-earth element RH, which is at least one element selected from the group consisting of Dy, Ho and Tb,

wherein at a depth of 20 μm under the surface of the R—Fe—B based rare-earth sintered magnet body, the crystal grains of the R 2 Fe 14 B type compound include an RH diffused layer at an outer periphery of the crystal grains, the RH diffused layer having an average thickness of 2 μm or less and having a composition of (RL 1-x RH x ) 2 Fe 14 B, where 0.2≦x≦0.75,

wherein at a depth of 500 μm under the surface of the R—Fe—B based rare-earth sintered magnet body, the crystal grains of the R 2 Fe 14 B type compound have an RH diffused layer with an average thickness of 0.5 μm or less in their outer periphery,

wherein the surface of the R—Fe—B based rare-earth sintered magnet body is a machined surface that is not covered with a metal layer of the heavy rare-earth element RH and that is a surface of the R—Fe—B based rare-earth sintered magnet body, from which its surface portion has already been removed after the heavy rare-earth element RH has been introduced into the R—Fe—B based rare-earth sintered magnet body from outside of the R—Fe—B based rare-earth sintered magnet body,

wherein the R—Fe—B based rare-earth sintered magnet body has a size of more than 4 mm in the thickness direction,

wherein a surface region of the R—Fe—B based rare-earth sintered magnet body has a thickness of 1 mm as measured from a surface of the R—Fe—B based rare-earth sintered magnet body and includes:

a first layer portion with a thickness of 500 μm as measured from the surface; and

a second layer portion that is located deeper inside the R—Fe—B based rare-earth sintered magnet body than the first layer portion and that has a thickness of 500 μm,

wherein a difference ΔH cJ 2 in coercivity between the first and second layer portions is 300 kA/m or less, and

wherein the magnet has an RH-RL-O compound in at least one grain boundary triple junction, which is located at a death of 100 μm or less under the surface of R—Fe—B based rare-earth sintered magnet body.

9. The R—Fe—B based rare-earth sintered magnet of claim 8 , wherein at a depth of 500 μm under the surface of the R—Fe—B based rare-earth sintered magnet body, the crystal grains of the R 2 Fe 14 B type compound include an RH diffused layer having a composition (RL 1-x RH x ) 2 Fe 14 B, where 0.2≦x≦0.75.

10. The R—Fe—B based rare-earth sintered magnet of claim 8 , wherein in a region of the R—Fe—B based rare-earth sintered magnet body between the depths of 20 μm and 500 μm under a surface of the R—Fe—B based rare-earth sintered magnet body, the crystal grains of the R 2 Fe 14 B type compound have an RH diffused layer in their outer periphery, and

wherein the greater the depth under the surface of the R—Fe—B based rare-earth sintered magnet body, the thinner the RH diffused layer gets in the outer periphery of the crystal grains of the R 2 Fe 14 B type compound.

11. The R—Fe—B based rare-earth sintered magnet of claim 8 , wherein the RH diffused layer has a uniform composition in which x has a dispersion of 10% or less at least within a single crystal grain.

12. The R—Fe—B based rare-earth sintered magnet of claim 8 , wherein the thickness of the RH diffused layer is 20% or less of the average grain size of the crystal grains of the R 2 Fe 14 B type compound.

13. The R—Fe—B based rare-earth sintered magnet of claim 8 , wherein in the crystal grains of the R 2 Fe 14 B type compound at the depth of 20 μm under the surface of the R—Fe—B based rare-earth sintered magnet body, the concentration of RH in the (RL 1-x RH x ) 2 Fe 14 B, where 0.2≦x≦0.75, layer is at least 6.0 mass % greater than that of RH at a core of the crystal grains.

14. The R—Fe—B based rare-earth sintered magnet of claim 8 , wherein in at least one of the crystal grains of the R 2 Fe 14 B type compound that are located at the depth of 100 μm or less under the surface of the R—Fe—B based rare-earth sintered magnet body, the concentration of RH in the RH diffused layer is smaller than the concentration of RH of the RH-RL-O compound in the at least one grain boundary triple junction, which surrounds the crystal grain of the R 2 Fe 14 B type compound, but greater than that of the rest of the at least one grain boundary triple junction other than the RH-RL-O compound.