Patent ID: 7875374

Claim:
A substrate for a donut-shape perpendicular magnetic recording medium having a perpendicular recording layer formed above the substrate, the substrate comprising: a main surface; an inner circumferential surface extending along an inner periphery of the main surface; and an outer circumferential surface extending along an outer periphery of the main surface, wherein, when a shape of the main surface is defined by a function Z(x,y) of x-y coordinates, a root mean square inclination angle (θsΔq), defined as an inverse tangent (tan −1 (sΔq)) of a root mean square inclination (sΔq), which is a root mean square over the entire main surface of a micro-region surface inclination (Δρ) of the main surface, expressed by the following equation, is 5° or less: Δ ⁢ ⁢ ρ = [ ( ∂ z ⁡ ( x , y ) ∂ x ) 2 + ( ∂ z ⁡ ( x , y ) ∂ y ) 2 ] 1 2 , wherein a surface shape of the substrate in a cross-sectional view is characterized as a sinusoidal curve, with a period λ of the curve being twice (D+δ) or greater, where D is an average grain diameter of magnetic grains in the perpendicular recording layer, and δ is an average interval between the magnetic grains.