Magnetic recording medium

1 . A magnetic recording medium comprising a substrate, an underlayer, an intermediate layer, and a magnetic layer in this order, the underlayer being made of Ru, the intermediate layer being made of an RuCo alloy, and the magnetic layer having a granular structure made up of a Co-containing ferromagnetic metal alloy and a non-magnetic oxide.

2 . The magnetic recording medium according to claim 1 , which is a longitudinal magnetic recording medium.

3 . The magnetic recording medium according to claim 1 , wherein the Co-containing ferromagnetic metal alloy is an alloy of cobalt with at least one element selected from Cr, Pt, Ni, Fe, B, Si, Ta, Nb, and Ru.

4 . The magnetic recording medium according to claim 1 , wherein the Co-containing ferromagnetic metal alloy is an alloy of Co—Pt—Cr, Co—Pt—Cr—Ta, Co—Pt—Cr—B, or Co—Ru—Cr.

5 . The magnetic recording medium according to claim 1 , wherein the non-magnetic oxide is an oxide of Si, Zr, Ta, B, Ti, Al, Cr, Ba, Zn, Na, La, In, or Pb.

6 . The magnetic recording medium according to claim 1 , wherein the non-magnetic oxide is an oxide of silicon.

7 . The magnetic recording medium according to claim 1 , wherein a molar ratio of the Co-containing ferromagnetic metal alloy to the non-magnetic oxide in the granular structure is from 95:5 to 80:20:

8 . The magnetic recording medium according to claim 1 , wherein the magnetic layer has a thickness of 5 to 60 nm.

9 . The magnetic recording medium according to claim 1 , wherein the magnetic layer has a thickness of 5 to 30 nm.

10 . The magnetic recording medium according to claim 1 , wherein the underlayer has a thickness of 5 to 50 nm.

11 . The magnetic recording medium according to claim 1 , wherein the intermediate layer has a thickness of 1 to 60 nm.

12 . The magnetic recording medium according to claim 1 , wherein the intermediate layer has a thickness of 3 to 30 nm.

13 . The magnetic recording medium according to claim 1 , which further comprises a gas barrier layer so that the substrate, the gas barrier layer and the underlayer are in this order, wherein the gas barrier layer contains a non-metallic single substance or a Ti compound with a non-metallic element.

14 . The magnetic recording medium according to claim 13 , wherein the gas barrier layer has a thickness of 5 to 200 nm.

15 . The magnetic recording medium according to claim 1 , wherein the substrate is a flexible polymer substrate.

16 . The magnetic recording medium according to claim 15 , wherein the flexible polymer substrate has a thickness of 10 to 200 μm.

17 . The magnetic recording medium according to claim 15 , wherein the flexible polymer substrate has a thickness of 10 to 100 μm.

18 . The magnetic recording medium according to claim 15 , wherein the flexible polymer substrate contains at least one of aromatic polyimide, aromatic polyamide, aromatic polyamide-imide, polyether ketone, polyether sulfone, polyether imide, polysulfone, polyphenylene sulfide, polyethylene naphthalate, polyethylene terephthalate, polycarbonate, cellulose triacetate, and fluorine resins.

19 . The magnetic recording medium according to claim 15 , which further comprises an undercoating layer containing at least one of polyimide resins, polyamide-imide resins, silicone resins and fluorine resins, so that the flexible polymer substrate, the undercoating layer and the underlayer are in this order.

20 . The magnetic recording medium according to claim 19 , wherein the undercoating layer has, at its surface, projections having a height of 5 to 60 nm.

21 . The magnetic recording medium according to claim 20 , wherein a density of the projections provided on the surface of the undercoating layer is 0.1 to 100/μm 2 .

22 . The magnetic recording medium according to claim 20 , wherein the projections are made by spherical silica particles.