Magneto-Optical disc is an optical disc format that uses a combination of optical and magnetic technologies. The sizes of discs are usually 3.5'' or 5.25'', and disk capacities are usually one of 128MB/230MB/540MB/640MB/1.3GB/2.6GB. A special Magneto-optical drive is required to read these discs.
In general magento-optical discs consist of a substrate medium upon which a ferromagnetic material is applied, originally crystalline in nature however metal alloys are used today. The write process consists of heating regions of the disc with a laser to a temperature above the material's curie point, thus causing the magnetic domains of the material to become paramagnetic. At this point application of an external magnetic field can set the magnetization vector of the material to have a specific orientation, only up and down orientations of the magnetization vector are allowed due to the specifics of the materials. Reading the disk is achieved by exposing the region to linearly polarised light which will have its plane of polarisation rotated in a specific direction depending on the orientation of the magnetization vector, this process is known as the Kerr effect. By determining the plane of polarisation of reflected light the data can be retrieved.
During writing, the magneto-optical disc needs to be erased, written, and then verified, which is a time-consuming process. Eventually, a technology called direct overwrite improved perfomance by eliminating the erase step.