Glass for magnetic recording medium substrate, glass substrate for magnetic recording medium, and their use

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 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 glass transition temperature of equal to or higher than 650° C.

2. The glass for a magnetic recording medium substrate according to claim 1 , which comprises, denoted as molar percentages,

55 to 78 percent of SiO 2 ,

greater than 0 percent and equal to or less than 5 percent of Li 2 O,

2 to 15 percent of Na 2 O, and

10 to 25 percent of a combined content of MgO, CaO, SrO, and BaO.

3. The glass for a magnetic recording medium substrate according to claim 2 , which comprises 0 to 12 mole percent of Al 2 O 3 .

4. The glass for a magnetic recording medium substrate according to claim 1 , wherein a molar ratio, CaO/Na 2 O, of a content of CaO relative to a content of Na 2 O is equal to or less than 1.5.

5. The glass for a magnetic recording medium substrate according to claim 1 , which comprises one or more selected from the group consisting of ZrO 2 , TiO 2 , Y 2 O 3 , La 2 O 3 , Gd 2 O 3 , Nb 2 O 5 , Ta 2 O 5 , and HfO 2 .

6. The glass for a magnetic recording medium substrate according to claim 5 , wherein a combined content of SiO 2 , ZrO 2 , TiO 2 , Y 2 O 3 , La 2 O 3 , Gd 2 O 3 , Yb 2 O 3 , Nb 2 O 5 , Ta 2 O 5 , and HfO 2 is equal to or greater than 66 mole percent.

7. The glass for a magnetic recording medium substrate according to claim 1 , wherein a combined content of ZrO 2 , TiO 2 , Y 2 O 3 , La 2 O 3 , 2 , Nb 2 O5, Ta 2 O5, and HfO 2 ranges from 0 to 10 mole percent.

8. The glass for a magnetic recording medium substrate according to claim 1 , which comprises 0 to 6 mole percent of ZrO 2 .

9. The glass for a magnetic recording medium substrate according to claim 1 , wherein a combined content of SiO 2 and ZrO 2 is equal to or greater than 66 mole percent.

10. The glass for a magnetic recording medium substrate according to claim 1 , which has an average coefficient of linear expansion at 100 to 300° C. of equal to or greater than 50×10 −7 /° C., a Young's modulus of equal to or greater than 75 GPa, and a specific modulus of elasticity of equal to or greater than 25 MNm/kg.

11. A glass substrate for a magnetic recording medium, comprising:

a glass having 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;

wherein the glass has a transition temperature of equal to or higher than 650° C.; and

wherein the glass has a compressive stress layer proximate a surface of the substrate and formed by chemical strengthening.

12. The glass substrate for a magnetic recording medium according to claim 11 , which comprises 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 a Babinet method.

13. The glass substrate for a magnetic recording medium according to claim 11 , which comprises chemically strengthened glass in which an average value Tav of a tensile stress obtained by a Babinet method and a maximum value Tmax of the tensile stress satisfy the following equation (1):

T av/ T max≧0.5.

14. The glass substrate for a magnetic recording medium according to claim 11 , wherein the chemically strengthened glass comprises glass that has been chemically strengthened by immersion into a mixed salt melt of sodium salt and potassium salt.

15. The glass substrate for a magnetic recording medium according to claim 14 , which comprises 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 mixed salt melt.

16. The glass substrate for a magnetic recording medium according to claim 11 , which has a fracture toughness value K1c of equal to or greater than 0.8 MPam 1/2 .

17. The glass substrate for a magnetic recording medium according to claim 11 , wherein an arithmetic average surface roughness (Ra) over an area of 1 μm×1 μm on a main surface measured by an atomic force microscope at a resolution of 512×256 pixels is equal to or lower than 0.15 nm.

18. The glass substrate for a magnetic recording medium according to claim 11 , which has a plate thickness of equal to or lower than 0.5 mm.

19. A glass substrate for a magnetic recording medium, which comprises:

a chemically strengthened glass,

wherein said glass is characterized by (1) a glass transition temperature of equal to or higher than 650° C., and (2) a tensile stress distribution that 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 a Babinet method.

20. A glass substrate for a magnetic recording medium, which comprises:

a chemically strengthened glass,

wherein said glass is characterized by (1) a glass transition temperature of equal to or higher than 650° C., and (2) an average value Tav of a tensile stress obtained by a Babinet method and a maximum value Tmax of the tensile stress that satisfy the following equation (1):

T av/ T max≧0.5.

21. A glass substrate blank for a magnetic recording medium, 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 a glass transition temperature of equal to or higher than 650° C.

22. A method of manufacturing a glass substrate blank for a magnetic recording medium, which comprises: blending glass starting materials to provide glass comprising 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 a glass transition temperature of equal to or higher than 650° C.; melting the blended glass starting materials to provide a glass melt; and press molding the glass melt that has been provided to prepare a substrate blank comprised of the glass.

23. The method of manufacturing a glass substrate blank for a magnetic recording medium according to claim 22 , which comprises press molding the glass melt in the air.

24. A method of manufacturing a glass substrate for a magnetic recording medium, which comprises:

preparing a glass substrate blank for a magnetic recording medium by the method according to claim 22 ; and

processing the substrate blank.

25. A method of manufacturing a glass substrate for a magnetic recording medium, comprising:

processing a glass substrate blank for a magnetic recording medium 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 a glass transition temperature of equal to or higher than 650° C.

26. A magnetic recording medium, comprising:

a glass, said glass having 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;

wherein the glass has a transition temperature of equal to or higher than 650° C.; and

wherein the glass has a compressive stress layer proximate a surface of the substrate and formed by chemical strengthening, and

a magnetic recording layer on the glass substrate for a magnetic recording medium.

27. 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

a magnetic recording medium, comprising:

a glass, said glass having 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;

wherein the glass has a transition temperature of equal to or higher than 650° C.; and

wherein the glass has a compressive stress layer proximate a surface of the substrate and formed by chemical strengthening, and

a magnetic recording layer on the glass substrate for a magnetic recording medium.

28. The magnetic recording apparatus according to claim 27 , wherein a rotational speed is equal to or higher than 5000 rpm.

29. The magnetic recording apparatus according to claim 27 , which is equipped with a DFH (Dynamic Flying Height) head.