A memory device in which the storage medium moves during operation and in which digital information is stored in magnetic material thinly coated on a nonmagnetic substrate; the direction of magnetization of small localized areas of the magnetic material represents the stored information. Information is stored and accessed to and from the material by means of small electromagnets – read and write heads (see magnetic recording) – past which the storage medium is moved or rotated at high speed. The same electromagnet can be used for both reading and writing – a read-write head – using time-division multiplexing.

The nature of the device and the speed of operation depend on the substrate material, which is either flexible or rigid, and the physical geometry used. Magnetic disk and magnetic tape are the major forms of moving magnetic surface memory. They are nonvolatile memory and are used as backing store (see memory). Interchange of disks or reels of tape greatly increases the volume of information available to a given computer system.