Magnetic recording medium and magnetic recording and reproducing apparatus

1. A magnetic recording medium comprising:

a non-magnetic substrate on which at least a soft magnetic underlayer, an orientation control layer that controls an orientation of a layer that is located directly on the orientation control layer, a perpendicular magnetic layer having a magnetization easy axis that is mainly oriented perpendicular to the non-magnetic substrate, and a protective layer are disposed,

wherein the perpendicular magnetic layer includes a first magnetic layer, a second magnetic layer, a third magnetic layer, and a fourth magnetic layer from a side of the substrate in this order,

wherein the first magnetic layer, the second magnetic layer, the third magnetic layer, and the fourth magnetic layer are magnetic layers having a granular structure,

wherein magnetic particles, which constitute the first magnetic layer, the second magnetic layer, the third magnetic layer, and the fourth magnetic layer, are continuous columnar crystals,

wherein a first exchange coupling control layer is disposed between the first magnetic layer and the second magnetic layer,

wherein a second exchange coupling control layer is disposed between the second magnetic layer and the third magnetic layer,

wherein the third magnetic layer and the fourth magnetic layer are in contact with each other,

wherein the first magnetic layer, the second magnetic layer, the third magnetic layer, and the fourth magnetic layer are coupled ferromagnetically, and

wherein following relations are satisfied where Ku i is a magnetic anisotropic constant of an i-th magnetic layer, Ms i is a saturation magnetization of the i-th magnetic layer, and t i is a film thickness of the i-th magnetic layer,

Ku 1 >Ku 2 ,

Ku 2 >Ku 3 ,

Ms 1 ×t 1 >Ms 2 ×t 2 ,

Ms 2 ×t 2 >Ms 3 ×t 3 ,

Ku 3 <Ku 4 , and

Ms 3 ×t 3 <Ms 4 ×t 4 .

2. The magnetic recording medium according to claim 1 ,

wherein Ms 1 is in a range of from 1200 to 500 emu/cc,

wherein Ku 1 is in a range of from 12×10 6 to 5×10 6 erg/cc,

wherein Ms 2 is in a range of from 1100 to 400 emu/cc,

wherein Ku 2 is in a range of from 11 ×10 6 to 4 ×10 6 erg/cc,

wherein Ms 3 is in a range of from 1000 to 300 emu/cc,

wherein Ku 3 is greater than or equal to 4×10 6 and less than 9 ×10 6 erg/cc,

wherein Ms 4 is in a range of from 900 to 200 emu/cc,

wherein Ku 4 is greater than 4 ×10 6 and less than or equal to 9 ×10 6 erg/cc, and

wherein t 1 , t 2 , t 3 , and t 4 are in a range of from 1 to 10 nm.

3. The magnetic recording medium according to claim 1 , further comprising:

a fifth magnetic layer between the fourth magnetic layer and the protective layer,

wherein the fifth magnetic layer is a magnetic layer having a non-granular structure,

wherein magnetic particles, which constitute the first magnetic layer, the second magnetic layer, the third magnetic layer, the fourth magnetic layer, and the fifth magnetic layer, are continuous columnar crystals,

wherein the fourth magnetic layer and the fifth magnetic layer are in contact with each other,

wherein the first magnetic layer, the second magnetic layer, the third magnetic layer, the fourth magnetic layer, and the fifth magnetic layer are coupled ferromagnetically, and

wherein following relations are satisfied where Ku 5 is a magnetic anisotropic constant of the fifth magnetic layer, Ms 5 is a saturation magnetization of the fifth magnetic layer, and t 5 is a film thickness of the fifth magnetic layer,

Ku 4 >Ku 5 , and

Ms 4 ×t 4 >Ms 5 ×t 5 .

4. The magnetic recording medium according to claim 3 , wherein following relations are satisfied,

Ku 3 >Ku 5 , and

Ms 3 ×t 3 <Ms 5 ×t 5 .

5. The magnetic recording medium according to claim 3 ,

wherein Ms 5 is in a range of from 950 to 250 emu/cc,

wherein Ku 5 is in a range of from 8 ×10 6 to 1 ×10 6 erg/cc, and

wherein t 5 is in a range of from 1 to 10 nm.

6. A magnetic recording and reproducing apparatus comprising:

the magnetic recording medium according to claim 1 ; and

a magnetic head configured to record and reproduce information in and from the magnetic recording medium.