A magnetic disk is a type of storage device that uses a magnetization process to write, rewrite, and access data. It is a flat disc covered with a magnetic coating that stores data. Magnetic disks are a form of non-volatile memory and are widely used to store computer data, as well as audio and video signals. The magnetic surface of a disk is divided into many small sub-micrometer-sized magnetic regions, referred to as magnetic domains, each of which has a mostly uniform magnetization. Each magnetic region forms a magnetic dipole that generates a magnetic field. Magnetic disks are less expensive than RAM and can store large amounts of data, but the data access rate is slower than main memory because of secondary memory. Some examples of magnetic disks include hard disks, floppy disks, and magnetic tape. Hard disks are the most common type of magnetic disk and are used in most computers to store data.