Patent ID: 7622205

Claim:
A disk substrate for a perpendicular magnetic recording medium comprising a soft magnetic underlayer formed on a nonmagnetic disk-shaped base plate by means of an electroless plating method, wherein a thermal expansion coefficient of the soft magnetic underlayer is larger than a thermal expansion coefficient of the nonmagnetic disk-shaped base plate and a saturation magnetostriction constant As satisfies a relation As≧−1×10 −5 , wherein the soft magnetic underlayer exhibits a ratio Mrrδ/Mrcδ is in a range of 0.33 to 3.00, in which Mrrδ is a product of the thickness and residual magnetization obtained from a magnetization curve measured by applying a magnetic field along a circumferential direction of the disk substrate, and Mrrδ is a product of the thickness and residual magnetization obtained from a magnetization curve measured by applying a magnetic field along a radial direction of the disk substrate.