CD-Magneto Optical (CD-MO) is a compact disc format that uses magnetic fields for data storage. Defined by Philips and Sony in their 1990 Recordable CD Standard, informally known as the Orange Book), CD-MO discs can, at least theoretically, be rewritten an unlimited number of times.
CD-MO discs are constructed of an alloy of terbium ferrite and cobalt. The reading of an MO disc is based on the Kerr effect. In the Kerr effect, linear, polarized light is deflected when it is influenced by a magnetic field, and the plane of polarization is twisted. The MO method changes the magnetic characteristics of tiny areas on the disk's surface so that the reading laser beam is reflected differently on altered areas than on unaltered areas.
When writing to the disc, a laser beam is focused on an extremely small spot, and the alloy is heated to a specific temperature (called the Curie point) sufficient to cause the ferromagnetic properties of the aligned elementary particles to be lost. An electromagnet is positioned on the other side of the disc, changing the polarity of the particles, whose differences will be encoded as binary data for storage. Like other optical media, such as DVD and other CD formats, CD-MO is read by a laser beam, which makes it more reliable than a hard disk or a floppy disk. However, a strong magnetic field can corrupt the stored data.