Magneto-Optical Disc and MiniDisc

Magneto-optical recording is thermally assisted magnetic recording: a laser warms a tiny spot of magnetic film so that a modest magnet can flip it, and when the spot cools the bit is locked in again. The Computer History Museum dates the first demonstration to 1967, when Di Chen at Honeywell stored data on thin manganese-bismuth (MnBi) films, and the first commercial 5.25-inch computer drives were announced in November 1988. Sony's MiniDisc, sold from 1992, put the same principle into a 64 mm consumer disc, and the last Sony MiniDisc stereo system shipped in March 2013.

How it works. The Museum and the CACM overview agree on the physics. At room temperature the film has high coercivity and does not respond to an applied field; heating it with a focused laser lowers its coercivity so a magnet can set the direction of magnetisation, and cooling freezes the bit. McDaniel adds that the magnetisation must be perpendicular to the film for read-out to work, even though in-plane (longitudinal) magnetisation is energetically favoured, which is why material selection is demanding, and that the applied field is about 600 times Earth's field. Reading uses the magneto-optic Kerr effect: the polarisation of reflected light turns one way or the other depending on the bit. Read power is lower than write power by a factor of 3 to 6.

Origins. The Museum says MO work began in the 1950s and 1960s at Bell Telephone, Honeywell, IBM, 3M and other labs in the US, Europe and Japan, and its reference list includes a 1958 paper by L. Mayer on Curie-point writing and a 1965 US patent (3,164,816). McDaniel notes that optical recording became feasible when low-cost diode lasers appeared. In the 1980s drives using amorphous magnetic films in removable cartridges reached the market.

Market launch. StorageNewsletter, abstracting its November 1988 issue, says that at Comdex (14-18 November 1988, Las Vegas) Sony Corporation of America and Advanced Graphic Applications announced themselves as the first to offer MO drives, with Canon, Ricoh and Maxtor to follow within months. They used the 5.25-inch format with 256 MB on one side and up to 650 MB on two sides; Verbatim alone offered a 3.5-inch drive with a 60 MB disc and 30 ms average seek time. The Museum credits Steve Jobs's 1988 presentation of the 256 MB drive in the NeXT workstation with introducing MO to a world-wide audience. NeXT's brochure describes a disc spinning at 3,000 rpm with an aluminium backing layer behind the MO film, and states that a write is an erase pass followed by a write pass and a verification pass (vendor description). That sequence is the plain reading of the brochure's own steps and is a likely contributor to the slow writes the Museum describes; the sources opened do not measure it.

Capacity race and the plateau. The Museum lists 5.25-inch discs from 256 MB to 9.2 GB and a long vendor list (HP, IBM, Kodak, Philips, Pioneer, 3M, Laser Byte, MaxOptix, MOST, Pinnacle Micro). Density tricks stayed inside the optical head. A Fujitsu and Sony press release of 21 March 2001 describes a 3.5-inch GIGAMO standard raised from 1.3 GB to 2.3 GB using magnetically induced super resolution (MSR) and land/groove recording with minimal changes to the optical head (vendor claim): track pitch fell from 1.10 micrometres in ISO/IEC 15041 (540 and 640 MB) to 0.90 micrometres (1.3 GB) and 0.67 micrometres (2.3 GB). McDaniel, writing in 2000, says hard-disk areal density by then grew far faster than MO's, which is the economic reason the format stayed a niche for rugged, removable and archival use.

MiniDisc. Sony proposed the Mini Disc on 16 May 1991 as a 2.5-inch disc with up to 74 minutes of digital audio, targeted for late 1992, using CD-type optical media for pre-recorded discs and MO media for recording. Yoshida's 1994 IEEE paper (a Sony author) says the aim was to replace the compact cassette, that cassette sales had been falling since 1989, and that the recordable disc is a 64 mm MO disc in a 72 x 68 x 5 mm cartridge, with a 780 nm laser and a 0.45 numerical aperture lens, a 1.6 micrometre track pitch and ATRAC compression of about 5:1 so that a disc a fifth the data capacity of a CD still plays 74 minutes. A buffer called Shock Resistant Memory, 4 Mbit holding about 12 seconds of compressed audio, covers read interruptions from knocks.

Magnetic-field modulation. MiniDisc recorded by holding the laser on at roughly 4.5 mW, which raised the film to about the Curie temperature (approximately 180 degrees C by Sony's account), while a contact magnetic head on the opposite side of the disc switched its field with the data. McDaniel calls the two schemes laser intensity modulation (LIM) and magnetic field modulation (MFM). Yoshida says MFM gives direct overwrite, can write marks of about 0.3 micrometres even with a 780 nm, NA 0.45 optic, tolerates disc tilt of plus or minus 1.5 degrees better than laser modulation, and that the contact head survives more than a million passes (all vendor claims from a Sony engineer).

MD Data and Hi-MD. Yoshida says MD Data, developed in July 1993, put about 140 MB on the same size of disc, with about 150 KB/s transfer. Sony's 8 January 2004 Hi-MD release gives three capacities on the same 780 nm, NA 0.45, 64.8 mm discs: 177 MB for the existing MD format, 305 MB for an 80-minute disc reformatted as Hi-MD, and 1.0 GB for a new Hi-MD disc that is, Sony says, the first to use domain wall displacement detection (DWDD), a kind of magnetic super resolution that uses the laser's temperature gradient to expand marks smaller than the beam spot for reading (vendor claim). Sony also expected cumulative MD media shipments of about 1.1 billion and 80 million compatible devices by the end of fiscal 2003 (vendor claim; forecast).

End of the line. The BBC (1 February 2013) reports that Sony ended portable MiniDisc Walkman shipments in 2011 and would ship its last MiniDisc stereo system in March 2013, while continuing to make the cartridges; it also says Sony claimed recordings would last more than 30 years (vendor claim), that user experiments showed magnets near a disc could erase it, and that the format had limited success outside Japan. A reseller reports that MO drive production ceased among all makers by 2010 and that Sony stopped producing MO media on 1 January 2015.

Link to HAMR. McDaniel, a Seagate researcher, wrote in 2000 that MO's thermomagnetic recording concept might be used to mitigate a looming write-ability problem in hard disks, whose media coercivity was being raised to keep ever-smaller bits stable. He describes Quinta, founded in 1996 and acquired by Seagate in 1997, which attached micro-optics and a microcoil to a flying slider (the Optically Assisted Winchester), and states that Seagate had exhibited prototypes but not marketed a product. Seagate's current HAMR page describes the same problem and fix in modern terms: very stable media are hard to write, so a laser diode on each head momentarily heats a tiny spot (vendor claim). The two sources support a conceptual kinship and a 1990s research programme; this card did not find a source tracing a direct engineering line from MO drives to shipping HAMR drives.