The present invention provides a glass composition having a low density and having a specific elastic modulus high enough to reduce deflection. The glass composition includes, as components, in mol %: 60 to 75% SiO.sub.2; 5 to 15% Al.sub.2O.sub.3; 0.5 to 5% MgO; 0.5 to 10% CaO; 5 to 20% Li.sub.2O; 2 to 10% Na.sub.2O; and 0 to 5% K.sub.2O, and has a density of 2.48 g/cm.sup.3 or less. The glass composition has a specific elastic modulus of, for example, 33.5×10.sup.6 Nm/kg or more.

The present invention is a glass sheet composite in which the loss coefficient is 1×10.sup.−2 or more and the longitudinal wave acoustic velocity in the sheet thickness direction is 5.5×10.sup.3 m/s or more.

The area of a spacer that is to be provided between the adjacent substrates in a laminate including a plurality of substrates to keep the adjacent substrates apart from each other is smaller than those of the stacked substrates. When pressure is released to bring the laminate obtained by providing the spacers between the substrates from a pressed state in which 0.60 MPa of pressure is applied to the laminate in the stacking direction into a non-pressed state, an amount of change ΔW in thickness per spacer that is calculated from a change in the thickness of the laminate due to the release of pressure is 30 μm or less.

A substrate for a magnetic disk includes a substrate main body having a disk shape and an alloy film. The substrate has a thickness (T+D) of 0.520 mm or less, which is the sum of a thickness T of the substrate main body and a thickness D of the film formed on main surfaces of the substrate main body. The disk shape has an outer diameter of 90 mm or more. A ratio D/T of the thickness D to the thickness T is 0.025 or more. The thickness of the film formed on an outer circumferential edge surface of the substrate main body is greater than the thickness of the film formed on each of the main surfaces, and the thickness of the film formed on each of the main surfaces is 80% or more of the thickness of the film formed on the outer circumferential edge surface.

A glass disk for a magnetic recording medium of the present invention has a disk shape, and has a strain point of 695° C. to 780° C., a temperature at 10.sup.4.5 dPa.Math.s of 1300° C. or lower, and a Young's modulus of 78 GPa or higher.

Disks for use in hard disk drives (HDD) or other magnetic recording apparatus. The disks are configured based on a finding that internal stress within a disk can make the disk more resistant to shock forces. In one example, a disk is provided that has a substrate with a thickness of no more than 0.5 millimeters and an internal stress no less than 300 megapascals. The relatively high internal stress within the substrate of the disk serves to reduce the magnitude of deflections caused by mechanical shocks to an HDD in which the disk is installed, as compared to other disks of equal thickness but with relatively less internal stress. Multi-platter stacks of the disks are described. Methods are also described for fabricating such disks and for rejecting disks that do not meet certain internal stress-based criteria. Substrates are also described.

A disk-shaped substrate for a magnetic disk has a diameter D of 85 mm or more and a thickness T of 0.6 mm or less, and a material of the substrate has a Young's modulus E of 90 GPa or more.

Provided is a glass for a magnetic recording medium substrate, which is an amorphous glass, in which an SiO.sub.2 content is in a range of 54 mol % or more and 62 mol % or less, an MgO content is in a range of 15 mol % or more and 28 mol % or less, an Li.sub.2O content is in a range of 0.2 mol % or more, and an Na.sub.2O content is in a range of 5 mol % or less.

A magnetic-disk substrate has an average value of squares of inclinations that is 0.0025 or less, and a frequency at which squares of inclinations are 0.004 or more is 15% or less, in a case where samples of inclinations on a main surface are obtained at intervals of 10 nm. The main surface is configured to receive at least a magnetic recording layer thereon. The magnetic-disk substrate includes an outer circumferential end portion and an inner circumferential end portion, and the outer circumferential end portion and the inner circumferential end portion have chamfered portions.

Provided is a glass for a magnetic recording medium substrate or for a glass spacer for a magnetic recording/reproducing apparatus, in which the total content of Li.sub.2O, Na.sub.2O, K.sub.2O, B.sub.2O.sub.3, and ZnO (Li.sub.2O+Na.sub.2O+K.sub.2O+B.sub.2O.sub.3+ZnO) is in a range of 0 mol % or more and 3 mol % or less, the mole ratio of the total content of Al.sub.2O.sub.3 and MgO relative to the total content of SiO.sub.2 and CaO [(Al.sub.2O.sub.3+MgO)/(SiO.sub.2+CaO)] is in a range of 0.30 or more and 0.6 or less, the total content of SiO.sub.2 and Al.sub.2O.sub.3 (SiO.sub.2+Al.sub.2O.sub.3) is in a range of 64 mol % or more and 85 mol % or less, and the total content of SiO.sub.2, Al.sub.2O.sub.3, MgO, and CaO (SiO.sub.2+Al.sub.2O.sub.3+MgO+CaO) is in a range of 87 mol % or more and 98 mol % or less.