Glass for magnetic recording medium substrate and usage thereof

1. Glass for a magnetic recording medium substrate, which comprises:

essential components in the form of SiO 2 , Li 2 O, Na 2 O, and one or more alkaline earth metal oxides selected from the group consisting of MgO, CaO, SrO, and BaO;

wherein a molar ratio of a content of MgO to a combined content of MgO, CaO, SrO, and BaO (MgO/(MgO+CaO+SrO+BaO)) is equal to or greater than 0.80; and which has a Young's modulus of equal to or greater than 80 GPa, and a glass transition temperature of equal to or greater than 620° C., and

wherein an average coefficient of linear expansion at 100 to 300° C. is equal to or greater than 70×10 −7 /° C.

2. Glass for a magnetic recording medium substrate, which comprises:

essential components in the form of SiO 2 , Li 2 O and Na 2 O, and one or more alkaline earth metal oxides selected from the group consisting of MgO, CaO, SrO, and BaO;

wherein a molar ratio of a content of CaO to a combined content of MgO, CaO, SrO, and BaO (CaO/(MgO+CaO+SrO+BaO)) is equal to or less than 0.20; and which has a Young's modulus of equal to or greater than 80 GPa, and a glass transition temperature of equal to or greater than 620° C., and

wherein an average coefficient of linear expansion at 100 to 300° C. is equal to or greater than 70×10 −7 /° C.

3. Glass for a magnetic recording medium substrate, which comprises:

essential components in the form of SiO 2 , Li 2 O, Na 2 O, and one or more alkaline earth metal oxides selected from the group consisting of MgO, CaO, SrO, and BaO;

wherein a molar ratio of a content of MgO to a combined content of MgO, CaO, SrO, and BaO (MgO/(MgO+CaO+SrO+BaO)) is equal to or greater than 0.80; and which has an average coefficient of linear expansion at 100 to 300° C. of equal to or greater than 70×10 −7 /° C., and a glass transition temperature of equal to or greater than 620° C., and

wherein a specific modulus of elasticity is equal to or greater than 30 MNm/kg.

4. Glass for a magnetic recording medium substrate, which comprises:

essential components in the form of SiO 2 , Li 2 O, Na 2 O, and one or more alkaline earth metal oxides selected from the group consisting of MgO, CaO, SrO, and BaO;

wherein a molar ratio of a content of CaO to a combined content of MgO, CaO, SrO, and BaO (CaO/(MgO+CaO+SrO+BaO)) is equal to or less than 0.20; and which has an average coefficient of linear expansion at 100 to 300° C. of equal to or greater than 70×10 −7 /° C., and a glass transition temperature of equal to or greater than 620° C., and

wherein a specific modulus of elasticity is equal to or greater than 30 MNm/kg.

5. The glass for a magnetic recording medium substrate according to claim 3 , which has a Young's modulus of equal to or greater than 80 GPa.

6. The glass for a magnetic recording medium substrate according to claim 4 , which has a Young's modulus of equal to or greater than 80 GPa.

7. Glass for a magnetic recording medium substrate, which comprises:

essential components in the form of SiO 2 , Li 2 O Na 2 O, and one or more alkaline earth metal oxides selected from the group consisting of MgO, CaO, SrO, and BaO;

wherein a molar ratio of a content of MgO to a combined content of MgO, CaO, SrO, and BaO (MgO/(MgO+CaO+SrO+BaO)) is equal to or greater than 0.80; and which has a Young's modulus of equal to or greater than 80 GPa, and a glass transition temperature of equal to or greater than 620° C., and

wherein a specific modulus of elasticity is equal to or greater than 30 MNm/kg.

8. Glass for a magnetic recording medium substrate, which comprises:

essential components in the form of SiO 2 , Li 2 O and Na 2 O, and one or more alkaline earth metal oxides selected from the group consisting of MgO, CaO, SrO, and BaO;

wherein a molar ratio of a content of CaO to a combined content of MgO, CaO, SrO, and BaO (CaO/(MgO+CaO+SrO+BaO)) is equal to or less than 0.20; and which has a Young's modulus of equal to or greater than 80 GPa, and a glass transition temperature of equal to or greater than 620° C., and

wherein a specific modulus of elasticity is equal to or greater than 30 MNm/kg.

9. A magnetic recording medium substrate, which is comprised of the glass for a magnetic recording medium substrate according to claim 1 .

10. A magnetic recording medium substrate, which is comprised of the glass for a magnetic recording medium substrate according to claim 2 .

11. A magnetic recording medium substrate, which is comprised of the glass for a magnetic recording medium substrate according to claim 3 .

12. A magnetic recording medium substrate, which is comprised of the glass for a magnetic recording medium substrate according to claim 4 .

13. A magnetic recording medium substrate, which is comprised of chemically strengthened glass that has been prepared by chemically strengthening the glass for a magnetic recording medium substrate according to claim 1 .

14. A magnetic recording medium substrate, which is comprised of chemically strengthened glass that has been prepared by chemically strengthening the glass for a magnetic recording medium substrate according to claim 2 .

15. A magnetic recording medium substrate, which is comprised of chemically strengthened glass that has been prepared by chemically strengthening the glass for a magnetic recording medium substrate according to claim 3 .

16. A magnetic recording medium substrate, which is comprised of chemically strengthened glass that has been prepared by chemically strengthening the glass for a magnetic recording medium substrate according to claim 4 .

17. The magnetic recording medium substrate according to claim 9 , which is disk-shaped.

18. The magnetic recording medium substrate according to claim 10 , which is disk-shaped.

19. The magnetic recording medium substrate according to claim 11 , which is disk-shaped.

20. The magnetic recording medium substrate according to claim 12 , which is disk-shaped.

21. The magnetic recording medium substrate according to claim 13 , which is disk-shaped.

22. The magnetic recording medium substrate according to claim 14 , which is disk-shaped.

23. The magnetic recording medium substrate according to claim 15 , which is disk-shaped.

24. The magnetic recording medium substrate according to claim 16 , which is disk-shaped.

25. The magnetic recording medium substrate according to claim 13 , which is comprised of glass having a fracture toughness value of equal to or greater than 0.9 MPa·m 1/2 .

26. The magnetic recording medium substrate according to claim 14 , which is comprised of glass having a fracture toughness value of equal to or greater than 0.9 MPa·m 1/2 .

27. The magnetic recording medium substrate according to claim 15 , which is comprised of glass having a fracture toughness value of equal to or greater than 0.9 MPa·m 1/2 .

28. The magnetic recording medium substrate according to claim 16 , which is comprised of glass having a fracture toughness value of equal to or greater than 0.9 MPa·m 1/2 .

29. The magnetic recording medium substrate according to claim 13 , which is comprised of chemically strengthened glass in which a tensile stress distribution is convex in shape such that the convex shape does not contain indentations indenting to a compressive stress side in a stress profile in a virtual cross section perpendicular to two main surfaces as obtained by the Babinet method.

30. The magnetic recording medium substrate according to claim 14 , which is comprised of chemically strengthened glass in which a tensile stress distribution is convex in shape such that the convex shape does not contain indentations indenting to a compressive stress side in a stress profile in a virtual cross section perpendicular to two main surfaces as obtained by the Babinet method.

31. The magnetic recording medium substrate according to claim 15 , which is comprised of chemically strengthened glass in which a tensile stress distribution is convex in shape such that the convex shape does not contain indentations indenting to a compressive stress side in a stress profile in a virtual cross section perpendicular to two main surfaces as obtained by the Babinet method.

32. The magnetic recording medium substrate according to claim 16 , which is comprised of chemically strengthened glass in which a tensile stress distribution is convex in shape such that the convex shape does not contain indentations indenting to a compressive stress side in a stress profile in a virtual cross section perpendicular to two main surfaces as obtained by the Babinet method.

33. The magnetic recording medium substrate according to claim 13 , which is comprised of chemically strengthened glass in which an average value Tav of a tensile stress obtained by the Babinet method and a maximum value Tmax of the tensile stress satisfy the following expression (1):

T av/ T max≧0.5.

34. The magnetic recording medium substrate according to claim 14 , which is comprised of chemically strengthened glass in which an average value Tav of a tensile stress obtained by the Babinet method and a maximum value Tmax of the tensile stress satisfy the following expression (1):

T av/ T max≧0.5.

35. The magnetic recording medium substrate according to claim 15 , which is comprised of chemically strengthened glass in which an average value Tav of a tensile stress obtained by the Babinet method and a maximum value Tmax of the tensile stress satisfy the following expression (1):

T av/ T max≧0.5.

36. The magnetic recording medium substrate according to claim 16 , which is comprised of chemically strengthened glass in which an average value Tav of a tensile stress obtained by the Babinet method and a maximum value Tmax of the tensile stress satisfy the following expression (1):

T av/ T max≧0.5.

37. The magnetic recording medium substrate according to claim 13 , which is glass that has been chemically strengthened by immersion into a salt melt comprising sodium salt and potassium salt.

38. The magnetic recording medium substrate according to claim 14 , which is glass that has been chemically strengthened by immersion into a salt melt comprising sodium salt and potassium salt.

39. The magnetic recording medium substrate according to claim 15 , which is glass that has been chemically strengthened by immersion into a salt melt comprising sodium salt and potassium salt.

40. The magnetic recording medium substrate according to claim 16 , which is glass that has been chemically strengthened by immersion into a salt melt comprising sodium salt and potassium salt.

41. The magnetic recording medium substrate according to claim 37 , which is glass that has been chemically strengthened by immersing glass which comprises equal to or greater than 0.1 mole percent of Li 2 O into the salt melt.

42. The magnetic recording medium substrate according to claim 38 , which is glass that has been chemically strengthened by immersing glass which comprises equal to or greater than 0.1 mole percent of Li 2 O into the salt melt.

43. The magnetic recording medium substrate according to claim 39 , which is glass that has been chemically strengthened by immersing glass which comprises equal to or greater than 0.1 mole percent of Li 2 O into the salt melt.

44. The magnetic recording medium substrate according to claim 40 , which is glass that has been chemically strengthened by immersing glass which comprises equal to or greater than 0.1 mole percent of Li 2 O into the salt melt.

45. A magnetic recording medium substrate blank, which is comprised of the glass for a magnetic recording medium substrate according to claim 1 .

46. A magnetic recording medium substrate blank, which is comprised of the glass for a magnetic recording medium substrate according to claim 2 .

47. A magnetic recording medium substrate blank, which is comprised of the glass for a magnetic recording medium substrate according to claim 3 .

48. A magnetic recording medium substrate blank, which is comprised of the glass for a magnetic recording medium substrate according to claim 4 .

49. A magnetic recording medium, which comprises a magnetic recording layer on the magnetic recording medium substrate according to claim 9 .

50. A magnetic recording medium, which comprises a magnetic recording layer on the magnetic recording medium substrate according to claim 10 .

51. A magnetic recording medium, which comprises a magnetic recording layer on the magnetic recording medium substrate according to claim 11 .

52. A magnetic recording medium, which comprises a magnetic recording layer on the magnetic recording medium substrate according to claim 12 .

53. A magnetic recording apparatus of energy-assisted magnetic recording system, which comprises:

a heat-assisted magnetic recording head having a heat source to heat at least a main surface of a magnetic recording medium, a recording element member, and a reproduction element member; and

the magnetic recording medium according to claim 49 .

54. A magnetic recording apparatus of energy-assisted magnetic recording system, which comprises:

a heat-assisted magnetic recording head having a heat source to heat at least a main surface of a magnetic recording medium, a recording element member, and a reproduction element member; and

the magnetic recording medium according to claim 50 .

55. A magnetic recording apparatus of energy-assisted magnetic recording system, which comprises:

a heat-assisted magnetic recording head having a heat source to heat at least a main surface of a magnetic recording medium, a recording element member, and a reproduction element member; and

the magnetic recording medium according to claim 51 .

56. A magnetic recording apparatus of energy-assisted magnetic recording system, which comprises:

a heat-assisted magnetic recording head having a heat source to heat at least a main surface of a magnetic recording medium, a recording element member, and a reproduction element member; and

the magnetic recording medium according to claim 52 .