Perpendicular magnetic recording disk having a permeability gradient

1. A perpendicular magnetic recording disk, comprising:

a soft magnetic underlayer (SUL);

an interlayer on the SUL; and

a perpendicular magnetic recording layer on the interlayer;

wherein the SUL has an increased permeability from an inner radius of the disk to an outer radius of the disk.

2. The perpendicular magnetic recording disk of claim 1 wherein the SUL comprises an antiparallel (AP) coupled SUL structure comprising:

a first SUL;

an AP coupling layer on the first SUL; and

a second SUL on the AP coupling layer.

3. The perpendicular magnetic recording disk of claim 2 wherein the thicknesses of the first SUL and the second SUL increase from the inner radius of the disk to the outer radius of the disk.

4. The perpendicular magnetic recording disk of claim 3 wherein the thicknesses of the first SUL and the second SUL increase by at least 40% from the inner radius of the disk to the outer radius of the disk.

5. The perpendicular magnetic recording disk of claim 2 wherein the thickness of the AP coupling layer increases from the inner radius of the disk to the outer radius of the disk.

6. The perpendicular magnetic recording disk of claim 5 wherein the thickness of the AP coupling layer increases by at least 10% from the inner radius of the disk to the outer radius of the disk.

7. A perpendicular magnetic recording disk, comprising:

a first soft magnetic underlayer (SUL);

an antiparallel (AP) coupling layer on the first SUL;

a second SUL on the AP coupling layer;

an interlayer on the second SUL; and

a perpendicular magnetic recording layer on the interlayer;

wherein the first SUL, the AP coupling layer, and the second SUL are formed such that an AP exchange field between the first SUL and the second SUL decreases from an inner radius of the disk to an outer radius of the disk.

8. The perpendicular magnetic recording disk of claim 7 wherein the permeability of the first SUL, the AP coupling layer, and the second SUL increases from the inner radius of the disk to the outer radius of the disk.

9. The perpendicular magnetic recording disk of claim 7 wherein the thicknesses of the first SUL and the second SUL increase from the inner radius of the disk to the outer radius of the disk.

10. The perpendicular magnetic recording disk of claim 9 wherein the thicknesses of the first SUL and the second SUL increase by at least 40% from the inner radius of the disk to the outer radius of the disk.

11. The perpendicular magnetic recording disk of claim 7 wherein the thickness of the AP coupling layer increases from the inner radius of the disk to the outer radius of the disk.

12. The perpendicular magnetic recording disk of claim 11 wherein the thickness of the AP coupling layer increases by at least 10% from the inner radius of the disk to the outer radius of the disk.

13. A magnetic disk drive system, comprising:

a recording head; and

a perpendicular magnetic recording disk readable and writable by the recording head, the perpendicular magnetic recording disk comprising:

a perpendicular magnetic recording layer;

an interlayer; and

a soft magnetic underlayer (SUL) structure;

wherein the SUL structure has an increased permeability from an inner radius of the disk to an outer radius of the disk.

14. The magnetic disk drive system of claim 13 wherein the SUL structure comprises an antiparallel (AP) coupled SUL structure comprising:

a first SUL;

an AP coupling layer; and

a second SUL.

15. The magnetic disk drive system of claim 14 wherein the thicknesses of the first SUL and the second SUL increase from the inner radius of the disk to the outer radius of the disk.

16. The magnetic disk drive system of claim 15 wherein the thicknesses of the first SUL and the second SUL increase by at least 40% from the inner radius of the disk to the outer radius of the disk.

17. The magnetic disk drive system of claim 14 wherein the thickness of the AP coupling layer increases from the inner radius of the disk to the outer radius of the disk.

18. The magnetic disk drive system of claim 17 wherein the thickness of the AP coupling layer increases by at least 10% from the inner radius of the disk to the outer radius of the disk.

19. A method of fabricating a perpendicular magnetic recording disk, the method comprising:

forming a soft magnetic underlayer (SUL);

forming an interlayer on the SUL; and

forming a perpendicular magnetic recording layer on the interlayer;

wherein the SUL has an increased permeability from an inner radius of the disk to an outer radius of the disk.

20. The method of claim 19 wherein forming an SUL comprises:

forming a first SUL;

forming an antiparallel (AP) coupling layer on the first SUL; and

forming a second SUL on the AP coupling layer.

21. The method of claim 20 wherein the thicknesses of the first SUL and the second SUL increase from the inner radius of the disk to the outer radius of the disk.

22. The method of claim 20 wherein the thickness of the AP coupling layer increases from the inner radius of the disk to the outer radius of the disk.

23. A method of fabricating a perpendicular magnetic recording disk, the method comprising:

forming a first soft magnetic underlayer (SUL);

forming an antiparallel (AP) coupling layer on the first SUL; and

forming a second SUL on the AP coupling layer;

wherein the first SUL, the AP coupling layer, and the second SUL are formed such that an AP exchange field between the first SUL and the second SUL decreases from an inner radius of the disk to an outer radius of the disk.

24. The method of claim 23 wherein the thicknesses of the first SUL and the second SUL increase by at least 40% from the inner radius of the disk to the outer radius of the disk.

25. The method of claim 23 wherein the thickness of the AP coupling layer increases by at least 10% from the inner radius of the disk to the outer radius of the disk.