When heated moderately and evenly, it cools to a crystalline state that has high reflectivity. It's the Magneto Optical disc, a technology spearheaded by Sony and introduced by Sony in 1989. At room temperature, MO discs are nearly immune to environmental magnetism. But when the recording laser heats a spot on the recording layer above the Curie point, a constant magnetic field, applied to the opposite side of the disc, can change the orientation. The laser switches on and off to create alternating spots of upwards and downwards magnetization.
Erasure requires a separate pass, with a reversed magnetic field.
Instead, the recording changes the polarity of the reflected light. MO read-out requires a special polarizing filter. But MO drives typically have one weakness. The first MO medium to overcome these limitations was Sony's MiniDisc. LIM-DOW Magneto Optical storage.