Magnetic recording medium for high density recording
View Patent ↗A bit error rate is improved and, at the same time, an aging change due to thermal fluctuation is decreased. In one embodiment, a magnetic recording medium has first, second, third, and fourth magnetic layers stacked over the underlayer film on a substrate. The product (Brt2) of the residual magnetic flux density and film thickness of the second magnetic layer is smaller than the product (Brt3) of the residual magnetic flux density and film thickness of the third magnetic layer. The second magnetic layer has a thickness larger than that of the third magnetic layer and is anti-ferromagnetically coupled with the first magnetic layer by way of the first non-magnetic intermediate layer. The fourth magnetic layer is formed by way of a second non-magnetic intermediate layer above the third magnetic layer. The product (Brt4) of the residual magnetic flux density and film thickness of the fourth magnetic layer is from about 47% to 52% of the product (Brt_total) of the residual magnetic flux density and film thickness of the entire magnetic layers in a state of residual magnetization with the external magnetic field removed.
1. A magnetic recording medium comprising:
a substrate;
an underlayer film formed above the substrate;
a magnetic film comprising a first magnetic layer, a first non-magnetic intermediate layer, a second magnetic layer, a third magnetic layer formed directly on the second magnetic layer, a second non-magnetic intermediate layer and a fourth magnetic layer stacked above the underlayer film; and
a protective layer formed above the magnetic film;
wherein:
a product (Brt2) of residual magnetic flux density and film thickness of the second magnetic layer is smaller than a product (Brt3) of residual magnetic flux density and film thickness of the third magnetic layer;
the second magnetic layer has a thickness larger than a thickness of the third magnetic layer;
the second magnetic layer is coupled anti-ferromagnetically with the first magnetic layer by way of the first non-magnetic intermediate layer;
the fourth magnetic layer is formed by way of the second non-magnetic intermediate layer above the third magnetic layer; and
a product (Brt4) of residual magnetic flux density and film thickness of the fourth magnetic layer is from about 47% to 52% of a product (Brt_total) of residual magnetic flux density and film thickness of entire magnetic layers in a state of residual magnetization with an external magnetic field removed.
2. A magnetic recording medium according to claim 1 ,
wherein each of the first, second, third, and fourth magnetic layers is a magnetic layer comprising cobalt (Co) as a main component, and each of the first and second non-magnetic intermediate layers is a non-magnetic intermediate layer comprising ruthenium (Ru) as a main component.
3. A magnetic recording medium according to claim 2 ,
wherein the second magnetic layer comprises a Co—Cr—Pt—B—Ta alloy.
4. A magnetic recording medium according to claim 1 ,
wherein magnetization directions of the first magnetic layer and the second magnetic layer are anti-parallel to each other, and magnetization directions of the third magnetic layer and the fourth magnetic layer are parallel to each other.
5. A magnetic recording medium according to claim 1 , wherein the first non-magnetic layer comprises Ru and has a thickness of about 0.5 nm.
6. A magnetic recording medium according to claim 1 , wherein the second non-magnetic layer comprises Ru and has a thickness of about 0.6 nm.
7. A magnetic recording medium according to claim 1 , wherein the fourth magnetic layer comprises Ru and has an additional concentration of B at about 12% or less.
8. A magnetic recording medium according to claim 1 , wherein the fourth magnetic layer has a composition of about Co-12 at. % Cr-13 at. % Pt-10 at. % B.
9. A magnetic recording medium according to claim 1 , wherein the second magnetic layer has a composition of about Co-22 at. % Cr-14 at. % Pt-6 at. % B-2 at. % Ta.