Glass substrate for magnetic disk and magnetic disk

1. A magnetic-disk glass substrate having a circular hole at a center, the magnetic-disk glass substrate comprising:

a pair of main surfaces; and

a side wall surface orthogonal to the main surfaces,

a substrate thickness of the magnetic-disk glass substrate being 0.635 mm or less,

a roundness of the circular hole being 1.5 μm or less, and

a difference between a maximum value and a minimum value of radii of three inscribed circles that are respectively derived from outlines in a circumferential direction at three positions spaced apart by 100 μm in a substrate thickness direction on the side wall surface of the circular hole being 3.5 μm or less.

2. The magnetic-disk glass substrate according to claim 1 , wherein

a surface roughness Rz of the side wall surface of the circular hole is 0.2 μm or less.

3. The magnetic-disk glass substrate according to claim 1 , wherein

an average coefficient of thermal expansion from 100° C. to 300° C. is 60×10 −7 (K −1 ) or less.

4. The magnetic-disk glass substrate according to claim 1 , wherein

with regard to a surface roughness of the side wall surface of the circular hole, in a case where a maximum height in the substrate thickness direction is Rz(t) and a maximum height in the circumferential direction is Rz(c), Rz(t)/Rz(c) is 1.2 or less.

5. The magnetic-disk glass substrate according to claim 1 , wherein

when a measurement point is provided every 30 degrees in the circumferential direction referenced on the center of the magnetic-disk glass substrate, and a radius of curvature of a shape of a portion between the side wall surface and a chamfered surface of the circular hole at the measurement point is derived, a difference in the radius of curvature between adjacent measurement points is 0.01 mm or less.

6. A magnetic disk in which at least a magnetic layer is formed on a main surface of the magnetic-disk glass substrate according to claim 1 .