Glass substrate for information recording medium, method of manufacturing the substrate, magnetic recording medium using the glass substrate, and method of manufacturing the medium

A magnetic recording medium which has a thin film including at least a magnetic layer on a glass substrate having surface roughnesses Ra and Rmax, where Ra is representative of a center-line mean roughness and Rmax is defined as a maximum height representative of a difference between a highest point and a lowest point wherein:

• the surface roughness is specified by Rmax≦15 nm, Ra≦1 nm and Rmax/Ra≦30;
• said thin film is composed of substantially uniform crystal grains by controlling the surface roughness;
• and
• an average crystal grain size of the crystal grains falls within the range between 5 and 30 nm.

A method of manufacturing a glass substrate having a principle surface for an information recording medium, comprising the steps of:

• lapping said glass substrate;
• and
• polishing the principle surface by using a polishing liquid including a polishing grain, the polishing grain having a predetermined grain size;
• the grain size being selected so that said glass substrate has surface roughness which is specified by Rmax≦15 nm, Ra≦1 nm and Rq≦1.5 nm, where Ra is representative of a center-line mean roughness, Rmax is defined as a maximum height representative of a difference between a highest point and a lowest point and Rq is representative of a root mean square roughness.

A method of manufacturing a glass substrate for an information recording medium, comprising the steps of:

• lapping said glass substrate;
• polishing, as a first polishing step, said glass substrate by using a first polishing liquid including a first polishing grain, the first polishing grain having a first grain size between 1 and 3 μm;
• polishing, as a second polishing step, said glass substrate by using a second polishing liquid including a second polishing grain, the second polishing grain having a second grain size between 0.5 and 2 μm;
• polishing, as a third polishing step, said substrate by using a third polishing liquid including a third polishing grain, the third polishing grain having a third grain size of 0.5 μm or less.