Patent ID: 8222170

Claim:
A magnetic information recording medium comprising a magnetic recording layer formed on a glass substrate for an information recording medium, said glass substrate comprising, by mol %, 40 to 74.5% of SiO 2 , 7.5 to 15% of Al 2 O 3 , 1% or more MgO as RO, 0 to 25% of B 2 O 3 , the total content of Al 2 O 3 and B 2 O 3 being 7.5% or more, 1 to 12% of total RO in which R is at least one member selected from the group consisting of Mg, Ca, Zn, Sr and Ba, wherein MgO is an essential component and CaO, ZnO, SrO and BaO are optional components, 17 to 25% of total R′ 2 O in which R′ is Li, Na and K, 0 to 7% of TiO 2 and 0 to 7% of ZrO 2 , the glass substrate containing 10 to 18% of Li 2 O, 1 to 10% of Na 2 O and 2 to 8% of K 2 O as R′ 2 O, the total content of SiO 2 , Al 2 O 3 , B 2 O 3 , RO and R′ 2 O being 90% or more, the total content of RO and R′ 2 O being 18 to 29%.