Hexagonal ferrite magnetic powder and method of manufacturing the same, and magnetic recording medium and method of manufacturing the same

1. A hexagonal ferrite magnetic powder which is manufactured by a glass crystallization method and which has an average plate diameter ranging from 15 to 25 nm, an average plate ratio ranging from 2.0 to 2.8 and a coercive force (Hc) ranging from 163 to 279 kA/m,

the powder containing, as an element substituting for Fe, only a pentavalent element selected from the group consisting of Nb and Ta, in a quantity of equal to or greater than 2.5 atomic percent when the sum of the pentavalent element and Fe is adopted as 100 atomic percent.

2. The hexagonal ferrite magnetic powder according to claim 1 , which contains the pentavalent element in a quantity ranging from 2.5 atomic percent to 6 atomic percent when the sum of the pentavalent element and Fe is adopted as 100 atomic percent.

3. The hexagonal ferrite magnetic powder according to claim 1 , which contains only Nb as the pentavalent element.

4. A magnetic recording medium comprising a magnetic layer comprising a hexagonal ferrite magnetic powder and a binder on a nonmagnetic support, wherein

the hexagonal ferrite magnetic powder is one manufactured by a glass crystallization method and has an average plate diameter ranging from 15 to 25 nm, an average plate ratio ranging from 2.0 to 2.8 and a coercive force (Hc) ranging from 163 to 279 kA/m,

the powder containing, as an element substituting for Fe, only a pentavalent element selected from the group consisting of Nb and Ta, in a quantity of equal to or greater than 2.5 atomic percent when the sum of the pentavalent element and Fe is adopted as 100 atomic percent.

5. The magnetic recording medium according to claim 4 , wherein the hexagonal ferrite magnetic powder contains the pentavalent element in a quantity ranging from 2.5 atomic percent to 6 atomic percent when the sum of the pentavalent element and Fe is adopted as 100 atomic percent.

6. The magnetic recording medium according to claim 4 , wherein the hexagonal ferrite magnetic powder contains only Nb as the pentavalent element.

7. The hexagonal ferrite magnetic powder according to claim 1 , wherein the coercive force (Hc) is within the range of from 167 to 279 kA/m.

8. The hexagonal ferrite magnetic powder according to claim 1 , wherein the coercive force (Hc) is within the range of from 175 to 279 kA/m.

9. The magnetic recording medium according to claim 4 , wherein the coercive force (Hc) is within the range of from 167 to 279 kA/m.

10. The magnetic recording medium according to claim 4 , wherein the coercive force (Hc) is within the range of from 175 to 279 kA/m.

11. A glass crystallization method of manufacturing a hexagonal ferrite magnetic powder comprising:

preparing a melt by melting a starting material mixture comprising a glass-forming component and a hexagonal ferrite-forming component;

rapidly cooling the melt to obtain a solid; and

heating the solid to precipitate a hexagonal ferrite magnetic powder, wherein

the hexagonal ferrite-forming component contains, as a component substituting for Fe, only a pentavalent element component selected from the group consisting of Nb 2 O 5 component and Ta 2 O 5 component,

the starting material mixture comprises a pentavalent element selected from the group consisting of Nb and Ta in a quantity of equal to or greater than 2.5 atomic percent when a sum of the pentavalent element and Fe is adopted as 100 atomic percent, and

the hexagonal ferrite magnetic powder has an average plate diameter ranging from 15 to 25 nm, an average plate ratio ranging from 2.0 to 2.8, and a coercive force (Hc) ranging from 163 to 279 kA/m.

12. The method of manufacturing a hexagonal ferrite magnetic powder according to claim 11 , wherein the pentavalent element component consists of Nb 2 O 5 component.

13. The method of manufacturing a hexagonal ferrite magnetic powder according to claim 11 , wherein the starting material mixture contains a pentavalent element selected from the group consisting of Nb and Ta in a quantity ranging from 2.5 atomic percent to 6 atomic percent when a sum of the pentavalent element and Fe is adopted as 100 atomic percent.

14. A method of manufacturing a magnetic recording medium comprising:

manufacturing a hexagonal ferrite magnetic powder; and

forming a magnetic layer using the hexagonal ferrite magnetic powder manufactured, wherein the hexagonal ferrite magnetic powder is manufactured by the following glass crystallization method:

preparing a melt by melting a starting material mixture comprising a glass-forming component and a hexagonal ferrite-forming component;

rapidly cooling the melt to obtain a solid; and

heating the solid to precipitate a hexagonal ferrite magnetic powder, wherein

the hexagonal ferrite-forming component contains, as a component substituting for Fe, only a pentavalent element component selected from the group consisting of Nb 2 O 5 component and Ta 2 O 5 component,

the starting material mixture contains a pentavalent element selected from the group consisting of Nb and Ta in a quantity of equal to or greater than 2.5 atomic percent when a sum of the pentavalent element and Fe is adopted as 100 atomic percent, and

the hexagonal ferrite magnetic powder has an average plate diameter ranging from 15 to 25 nm, an average plate ratio ranging from 2.0 to 2.8, and a coercive force (Hc) ranging from 163 to 279 kA/m.

15. The method of manufacturing a magnetic recording medium according to claim 14 , wherein the pentavalent element component consists of Nb 2 O 5 component.

16. The method of manufacturing a magnetic recording medium according to claim 14 , wherein the starting material mixture contains a pentavalent element selected from the group consisting of Nb and Ta in a quantity ranging from 2.5 atomic percent to 6 atomic percent when a sum of the pentavalent element and Fe is adopted as 100 atomic percent.