US Patent 7891212 Magnetic disk glass substrate

A magnetic disk glass substrate including compressive stress layers at main surfaces and a tensile stress layer between the compressive stress layers formed by chemical strengthening. When the glass substrate has a thickness of less than 0.5 mm and the tensile stress layer has a thickness L and a tensile stress of Pt (kg/mm2), the following relation holds:0.4 (kg/mm)≦L·Pt≦2.0 (kg/mm).