the present invention is related to the fabrication of a magnetic memory storage device consisting essentially of a head pad and a rigid information disk , which disk consists essentially of a rigid substrate having a coating of magnetic media on the surface thereof facing the head pad . more particularly , this invention is directed to a substrate for use in the disk . the substrate is selected from two different groups of glass - ceramics : the first group contains a sheet silicate as the predominant crystal phase ; and the second group contains a chain silicate as the predominant crystal phase .