Patent ID: 7767607

Claim:
A glass substrate for magnetic recording media, which is obtained by cutting the glass into a doughnut shape; mirror-polishing at least both sides of the glass; and immersing the glass in a molten salt containing monovalent cations which have the same or larger radii as compared with radii of sodium ions for ion exchanging treatment of sodium ions with the monovalent cations, wherein the glass essentially comprises: 60 to 70 wt % SiO 2 ; 5 to 20 wt % Al 2 O 3 ; 0 to 1 wt % Li 2 O; 3 to 18 wt % Na 2 O; 0 to 9 wt % K 2 O; 0 to 10 wt % MgO; 1 to 15 wt % CaO; 0 to 4.5 wt % SrO; 0 to 1 wt % BaO; 0 to 1 wt % TiO 2 ; and 0 to 1 wt % ZrO 2 , wherein the sum of Li 2 O, Na 2 O and K 2 O is from 5 to 25 wt %, and the sum of MgO, CaO, SrO and BaO is from 5 to 20 wt %.