Patent ID: 8394516

Claim:
A glass substrate for a magnetic recording medium, which is comprised of glass comprising, denoted as molar percentages, 56 to 75 percent of SiO 2 , 1 to 11 percent of Al 2 O 3 , more than 0 percent but equal to or less than 4 percent of Li 2 O, equal to or more than 1 percent but less than 15 percent of Na 2 O, equal to or more than 0 percent but equal to or less than 1 percent of K 2 O, and substantially no BaO, with a total content of alkali metal oxides selected from the group consisting of Li 2 O, Na 2 O, and K 2 O falling within a range of 6 to 15 percent, with a molar ratio (Li 2 O/Na 2 O) of a content of Li 2 O to a content of Na 2 O being less than 0.50, with a molar ratio {K 2 O/(Li 2 O+Na 2 O+K 2 O)} of a content of K 2 O to a total content of the above alkali metal oxides being equal to or less than 0.13, with a total content of alkaline earth metal oxides selected from the group consisting of MgO, CaO, and SrO falling within a range of 10 to 30 percent, with a total content of MgO and CaO falling within a range of 10 to 30 percent, with a molar ratio {(MgO+CaO)/(MgO+CaO+SrO)} of a total content of MgO and CaO to a total content of the above alkaline earth metal oxides being equal to or more than 0.86, with a total content of the above alkali metal oxides and alkaline earth metal oxides falling within a range of 20 to 40 percent, with a molar ratio {(MgO+CaO+Li 2 O)/(Li 2 O+Na 2 O+K 2 O+MgO+CaO+SrO)} of a total content of MgO, CaO, and Li 2 O to a total content of the above alkali metal oxides and alkaline earth metal oxides being equal to or more than 0.50, with a total content of oxides 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 , and Ta 2 O 5 being more than 0 percent but equal to or less than 10 percent, and with a molar ratio {(ZrO 2 +TiO 2 +Y 2 O 3 +La 2 O 3 +Gd 2 O 3 +Nb 2 O 5 +Ta 2 O 5 )/Al 2 O 3 } of a total content of the above oxides to a content of Al 2 O 3 content being equal to or more than 0.40, as well as the glass having a glass transition temperature of equal to or higher than 600° C., an average coefficient of linear expansion of equal to or higher than 70×10 −7 /° C. at 100 to 300° C., and a Young's modulus of equal to or higher than 80 GPa.