Magneto-optical Drive - Recent Progress

Recent Progress

At the Consumer Electronics Show in January 2004, Sony revealed a 1 gigabyte capacity MiniDisc known as "Hi-MD." This sixfold increase in capacity is performed using a magneto-optical trick. To record a standard MiniDisc the writer uses an infrared laser to heat a spot of ferromagnetic material on the disc to above its Curie point, then it is magnetised by a recording head as it cools. In contrast a high capacity MiniDisc uses tracks that are one sixth of the width of those used in standard MiniDiscs. By employing three magnetic layers, when a high capacity MiniDisc is read, the track expands to readable size. Specifically the three layers are, from read-face to print-face: a displacement layer, a switching layer, and a memory layer. When it isn't being read, the magnetic field in the memory layer is the same as those in the displacement and switching layers. When a laser shines on the track, the switching layer, which has a lower Curie point than the other layers, demagnetises. It decouples from the displacement layer, whose "magnetic fence" around the track weakens, temporarily causing the track to swell to a readable size. Hi-MD recorders can also double the capacity of regular minidiscs with special formatting that renders the disc unreadable (or writable) by non-Hi-MD minidisc recorders.

As with all removable storage media, the advent of cheap CD/DVD drives and flash memory has made them largely redundant. Magneto-optical disks in particular were expensive when new, and while highly reliable, the slow writing time also was a detriment.