From: Disk

Magneto-optical disks are plastic or glass disks coated with a
compound (often TbFeCo) that has special properties. The disk is read
by shining a low-intensity laser (originally infrared, but experiments
are being conducted all the way up to blue, I believe; the shorter the
wavelength the higher the possible density, all things being equal
(which they never are)) onto the media and examining the polarization
of the reflected light. To write, a higher-intensity laser is used to
heat the material up to its Curie point, where it becomes susceptible
to a magnetic field. When the media cools again, its state is
"frozen". The polarity of the reflected light during a read depends on
the polarity of the magnetic field under which the media was last
cooled. Once it has cooled it is no longer suceptible to magnetic
fields. Thus, it can be compared in a sense to paleomagnetism.

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