The recording layer(a magnetization film) which is used on the MO disk has a large coercive force at room temperature and the direction of magnetization cannot be altered by an external magnetic field caused by a magnet or a magnetic head.
The recording layer(a magnetization film) has the characteristic that coercive forcedecreases as temperature of a magnetization film is raised.
The temperature which coercive force of a magnetization film becomes zero is called the Curie temperature.
When the temperature of a magnetization film reaches the Curie temperature, the magnetization is easily altered by the external magnetic field.
Under this principle, recording and erasing data is performed using the laser beam heat source and external magnetic field(thermo-magnetic recording).

Writing: In the initial status(below Figure), the direction of magnetization of the recording layer is downward.
Powerful electromagnet(a magnetic head) is located above the disk and optical head is located under the disk. When writing data, upward external magnetic field is applied by an electromagnet. Laser beam is applied at the point of recording data '1', and the temperature at the point rises.
When it reaches the Curie temperature,the coercive force at the point is decreased and the magnetization is inverted due to the external magnetic field, thus recording data '1'.

Erasing: Upon erasing data, external downward magnetic field is applied while laser beam is continuously applied to the recording layer.
When the temperature reaches the Curie temperature, the direction of magnetization is returned to its initial status, thereby erasing data.