magnetic optical (MO)

In magneto-optical(MO) memories, two physical quantities act to store information: Magnetism and light or heat. According to the principle, first the uniformly aligned magnetic information layer is exposed to a magnetic field that does not cause any change at normal temperature.

The recording layer of the MO memory is an alloy of terbium, iron and cobalt, which is resistant to magnetic influences at normal ambient temperatures. Only by heating the information layer to approx. 200 °C can the coercive force of the alloy be lowered. This heating is performed by a laser beam and, in combination with induction, triggers a change in the magnetized layer. After cooling, the stored state is retained. This process can be repeated bit-selectively and almost as often as required, as long as the heating by the laser beam does not lead to structural changes in the material.

The stored information is read out using the Kerr effect, in which suitable substances reflect polarized light as a function of magnetization. The intensity change of the reflected laser beam is determined via polarization filters and converted into the stored information. The magneto-optical storage process is used on MO discs.