The Magneto-Optical Disc

By the late 1980s storage had split in two and neither half was satisfactory. Magnetic media were fast and rewritable and either sealed away where you could not remove them, or removable and fragile enough that a loudspeaker on the desk could erase them. Optical media were tough and removable and could only be written once. The magneto-optical disc is the sibling of both, and it works by making the medium almost impossible to write on purpose. An amorphous alloy of terbium, iron and cobalt is sputtered a few tens of nanometres thick, with its easy axis PERPENDICULAR to the surface. At room temperature its coercivity is hundreds of kiloamps per metre — nothing in an office comes close. Heat a two-micrometre spot with the laser and that coercivity collapses towards zero as the Curie point approaches. Now a weak bias field, far too weak to have mattered a moment earlier, sets the direction of that one spot. Take the heat away and it is locked again. Reading uses a different effect entirely. Light reflected from a perpendicularly magnetised film comes back with its plane of polarisation rotated by about four tenths of a degree, and the SIGN of that rotation follows the magnetisation. That is the polar Kerr effect, and four tenths of a degree is a very small thing to measure, which is why every MO drive splits the return beam between two detectors and subtracts them. MEASURED, NOT FABRICATED. Sputtering rare-earth transition-metal alloys is not a bench process and this blueprint says so. What you can do is take a salvaged MO or MiniDisc medium, put it between crossed polarisers, and see the Kerr rotation for yourself — the written regions and the erased ones differ visibly.