Glass substrate for information recording medium and magnetic recording medium having the substrate

A magnetic recording medium having a glass substrate, wherein:

• the glass substrate contains undissolved concentrations of ZrO2,
• the glass substrate further contains Li2O,
• the glass substrate has a highly polished surface,
• the undissolved concentrations of ZrO2 are left on the polished surface as projections when the glass substrate is polished,
• a content of ZrO2 is selected within the range of 2.8 mol % or less in order to reduce a height of each of the projections so that the polished substrate is suitable for use for a magnetic recording medium, and
• Li2O has a viscosity of 20000 poise or more at a liquid phase temperature of the glass.

A magnetic recording medium having a glass substrate, wherein:

• the glass substrate contains undissolved concentrations of ZrO2,
• the glass substrate further contains TiO2 and B2O3,
• the glass substrate has a highly polished surface,
• the undissolved concentrations of ZrO2 are left on the polished surface as projections when the glass substrate is polished,
• a content of ZrO2 is selected within the range of 2.8 mol % or less in order to ensure a height of each of the projections to be within a predetermined range in order to allow the polished substrate to be suitable for use for a magnetic recording medium,
• a content of TiO2 falls within the range between 0.2 and 3.0 mol %,
• a content of B2O3 is 0.9 mol % or less,
• the glass is composed of a chemically reinforced glass,
• the chemically reinforced glass has a compressive stress layer having a thickness exceeding 100 μm when the glass is immersed in a chemical reinforcing process solution of a mixed salt containing 60 weight % of KNO3 and 40 weight % of NaNO3 which is kept at 400° C. for 4 hours.

A magnetic recording medium having a glass substrate, wherein:

• the glass substrate contains undissolved concentrations of ZrO2,
• the glass substrate further contains TiO2 and B2O3,
• the glass substrate has a highly polished surface,
• the undissolved concentrations of ZrO2 are left on the polished surface as projections when the glass substrate is polished,
• a content of ZrO2 is selected within the range of 2.8 mol % or less in order to ensure that a height of each of the projections is within a predetermined range in order to allow the polished substrate to be suitable for use for a magnetic recording medium,
• a content of TiO2 falls within the range between 0.2 and 3.0 mol %,
• a content of B2O3 is 0.9 mol % or less,
• the glass substrate has a specific modulus of 32.0×106 Nm/Kg or more.

A magnetic recording medium having a glass substrate, wherein:

• the glass substrate contains undissolved concentrations of ZrO2,
• the glass substrate has a polished surface which is precisely polished,
• the undissolved concentrations of ZrO2 are left on the polished surface as projections when the glass substrate is precisely polished, and
• a content of ZrO2 is selected within such a range that a height of each of the projections is within a predetermined range in order to allow the polished substrate to be suitable for use for a magnetic recording medium.

A magnetic recording medium having a glass substrate, wherein:

• the glass substrate contains undissolved concentrations of raw material components of the glass substrate,
• the glass substrate has a highly polished surface,
• the undissolved concentrations of raw material components are left on the polished surface as projections when the glass substrate is precisely polished, and
• a content of the raw material component is selected within such a range that a height of each of the projections is within a range that makes the substrate suitable for use for a magnetic recording medium.