A rotatable storage medium usually in the form of a circular nonmagnetic rigid plate coated on both sides with magnetic material, followed by some form of lubricating layer(s). The disks themselves (the substrate) are made of aluminum alloy or of some glass—ceramic composite material. The magnetic coating on early disks was a ferric oxide in a binder. Current disks have a thin metallic film, such as cobalt/nickel or cobalt/chrome, which is created by vacuum deposition (i.e. sputtering). The lubricating layer is a coat of carbon a few angstroms thick, sometimes followed by a proprietary lubricant. Flexible magnetic disks (i.e. floppy disks) with oxide coating provide low-cost lightweight media, but are now falling into disuse.
Data is stored on and retrieved from magnetic disks by means of a disk drive. See also access time, fixed disk drive, Winchester technology, memory hierarchy.