Magnetic recording medium having a cobalt-based alloy film for high density recording and magnetic storage device using same

1. A magnetic recording medium comprising an underlying film, a magnetic film and a protective film, which are formed in order on a substrate; wherein:

said magnetic film is a cobalt-base alloy film containing chromium and having a plurality of magnetic layers stacked without interposition of any non-magnetic layer;

said plurality of magnetic layers comprising first, second and third magnetic layers;

said first magnetic layer is disposed between said underlying film and said second magnetic layer;

said second magnetic layer being disposed between said first magnetic layer and said third magnetic layer;

said third magnetic layer being disposed between said second magnetic layer and said protective film;

the concentration of chromium contained in said first magnetic layer is lower than that of chromium contained in said second magnetic layer;

the first magnetic layer containing platinum; and

the concentration of chromium contained in said third magnetic layer is lower than the concentration of chromium contained in said second magnetic layer.

2. A magnetic recording medium according to claim 1 , wherein the sum of the concentration of cobalt and the concentration of platinum contained in said third magnetic layer is higher than the sum of the concentration of cobalt and the concentration of platinum contained in said second magnetic layer.

3. A magnetic recording medium according to claim 2 , wherein a thickness of said first magnetic layer is not smaller than about 0.8 nm and not larger than about 2.0 nm.

4. A magnetic recording medium according to claim 2 , wherein the concentration of chromium contained in said first magnetic layer is not lower than about 10 at. % and not higher than about 20 at. %.

5. A magnetic recording medium according to claim 2 , wherein said first magnetic layer having a platinum concentration of not higher than about 12 at. %.

6. A magnetic recording medium according to claim 2 , wherein said first magnetic layer is a cobalt-chromium-platinum layer.

7. A magnetic recording medium according to claim 2 , wherein the concentration of chromium contained in said second magnetic layer is not lower than about 20 at. %.

8. A magnetic recording medium according to claim 1 ,

wherein said plural magnetic layers further comprise a fourth magnetic layer formed over and in contact with said third magnetic layer; and

wherein the sum of the concentration of cobalt and the concentration of platinum contained in said third magnetic layer is lower than the sum of the concentration of cobalt and the concentration of platinum contained in said fourth magnetic layer.

9. A magnetic recording medium comprising at least a first magnetic layer, a second magnetic layer and a third magnetic layer, which are formed in order on a substrate through an underlying film; wherein:

said first magnetic layer is a cobalt-chromium-platinum alloy layer;

said second and third magnetic layers are each a cobalt-base alloy layer containing chromium, and platinum;

the concentration of chromium contained in said first magnetic layer is lower than the concentration of chromium contained in said second magnetic layer; and

the concentration of chromium contained in said third magnetic layer is lower than the concentration of chromium contained in said second magnetic layer.

10. A magnetic recording medium according to claim 9 , wherein the

sum of the concentration of cobalt and the concentration of platinum contained in said second magnetic layer is lower than the sum of the concentration of cobalt and the concentration of platinum contained in said third magnetic layer.

11. A magnetic recording medium according to claim 10 ,

wherein the thickness of said first magnetic layer is not smaller than about 0.8 nm and not larger than about 2.0 nm, the concentration of chromium contained in said first magnetic layer is not lower than about 10 at. % and not higher than about 20 at. %, and the concentration of platinum contained in said first magnetic layer is not higher than about 12 at. %; and

wherein the concentration of chromium contained in said second magnetic layer is not lower than about 20 at. %.

12. A magnetic recording medium according to claim 9 , further comprising a fourth layer formed over and in contact with said third magnetic layer;

wherein said fourth magnetic layer is a cobalt-base alloy layer containing chromium and platinum, and

wherein the sum of the concentration of cobalt and the concentration of platinum contained in said fourth magnetic layer is higher than the sum of the concentration of cobalt and the concentration of platinum contained in said third magnetic layer.

13. A magnetic recording medium comprising, on a substrate, an underlying film, a magnetic film and a protective film; wherein:

said magnetic film has a plurality of magnetic layers stacked without interposition of any non-magnetic layer;

said plurality of magnetic layers comprises first, second, third and fourth magnetic layers;

said first magnetic layer is disposed between said underlying film and said second magnetic layer;

said second magnetic layer is disposed between said first magnetic layer and said third magnetic layer;

said third magnetic layer is disposed between said second magnetic layer and said fourth magnetic layer;

said fourth magnetic layer is disposed between said third magnetic layer and said protective film;

said first magnetic layer is a cobalt-base alloy layer containing chromium;

said second, third and fourth magnetic layers are each a cobalt-base alloy layer containing chromium and platinum;

the concentration of chromium contained in said first magnetic layer is lower than the concentration of chromium contained in said second magnetic layer;

the concentration of chromium contained in said third magnetic layer is lower than the concentration of chromium contained in said second magnetic layer; and

the sum of the concentration of cobalt and the concentration of platinum contained in said fourth magnetic layer is higher than the sum of the concentration of cobalt and the concentration of platinum contained in said third magnetic layer.

14. A magnetic recording medium according to claim 13 , wherein the thickness of said first magnetic layer is not smaller than about 0.8 nm and not larger than about 2.0 nm, said first magnetic layer further contains platinum, the concentration of chromium contained in said first magnetic layer is not lower than about 10 at. % and not higher than about 20 at. %, and the concentration of platinum contained in said first magnetic layer is not higher than 1 about 2 at. %; and

wherein the concentration of chromium contained in said second magnetic layer is not lower than about 20 at. %.

15. A magnetic recording medium according to claim 13 , wherein the thickness of said first magnetic layer is not smaller than about 0.8 nm and not larger than about 2.0 nm.

16. A magnetic recording medium according to claim 13 , wherein a concentration of chromium contained in said first magnetic layer is not lower than about 10 at. % and not higher than about 20 at. %.

17. A magnetic recording medium according to claim 13 , wherein said first magnetic layer further contains platinum at a platinum concentration of not higher than about 12 at. %.

18. A magnetic recording medium according to claim 13 , wherein said first magnetic layer is a cobalt-chromium alloy layer or a cobalt-chromium-platinum layer.

19. A magnetic recording medium according to claim 13 , wherein the concentration of chromium contained in said second magnetic layer is not lower than about 20 at. %