What Is Magnetooptic Recording?

Magnetooptic recording is a data storage technology that uses a focused laser beam and an applied magnetic field to write, read, and erase information on a thin magnetic film, combining the high spot definition of optical focusing with the erasability of magnetic media. Stored bits are represented as microscopic domains of opposite magnetization on a disk coated with a magnetically hard, amorphous rare-earth transition-metal alloy. Reading employs the polar magneto-optic Kerr effect, in which the polarization of reflected light rotates in opposite directions depending on the direction of magnetization at the illuminated point, allowing a polarization-sensitive detector to sense the bit state without contact. The technology was demonstrated in 1967 by Di Chen at Honeywell using manganese bismuth (MnBi) thin films, and commercial systems using terbium-iron-cobalt (TbFeCo) alloy disks were introduced by manufacturers including Canon, Sony, and Fujitsu in the 1980s and 1990s.

Magnetooptic recording occupies a historical niche between optical discs and magnetic hard drives: it offered the rewritability of magnetic tape, the removability of optical discs, and areal densities competitive with hard disk drives at the time of its commercial peak.

Thermomagnetic Writing and Erasure

Writing a bit requires locally reducing the coercivity of the recording layer so that a small applied magnetic field can reverse the domain magnetization. At ambient temperature, the high coercivity of TbFeCo alloys prevents any external field from switching the magnetization. A focused laser pulse heats the targeted spot to near the material's Curie temperature, typically 150 to 200 degrees Celsius, where coercivity drops sharply. A bias electromagnet positioned near the disk surface then defines the magnetization direction of the heated spot as it cools below the Curie point and the coercivity rises again. Erasing a track simply requires applying the bias field in the opposite polarity during a laser scan pass. The physical principles of magneto-optical recording describe in detail how the thermomagnetic writing process depends on thermal diffusion length, the laser pulse duration, and the temperature profile of the Curie transition.

Readout Using the Kerr Effect

Data is read with a lower-power continuous laser, well below the threshold for thermally disturbing the recorded domains. The reflected beam passes through a polarizing beamsplitter, which separates the components of the reflected light whose polarization has been rotated in one direction from those rotated in the other. A differential detector subtracts the two intensities to produce a signal proportional to the Kerr rotation angle, which is typically 0.2 to 1.0 degrees for TbFeCo alloys. This small rotation angle limits the signal-to-noise ratio in comparison with the reflectivity modulation used in conventional optical compact discs. Land-groove track formats and various servo-correction schemes compensate for disk runout, maintaining track-following accuracy to within tens of nanometers. Magneto-optical data storage as described in Communications of the ACM places the read-out physics in the context of competitive storage technologies.

Recording Materials and Areal Density

Commercially deployed magneto-optic disks use amorphous TbFeCo or GdFeCo alloy films deposited by sputtering on a polycarbonate or glass substrate, with the magnetic layer sandwiched between dielectric layers that serve simultaneously as antireflection coatings and protective barriers. Domain sizes are limited by the diffraction-limited laser spot, typically 0.5 to 1 micrometer with 680 nm or 780 nm diode lasers, yielding areal densities of several hundred megabytes per disk side in early generations. The Super Resolution and Magnetic Super Resolution techniques, developed in Japan in the 1990s, enabled reading of domains smaller than the diffraction limit by exploiting optically induced magnetic transitions in a sacrificial layer above the recording layer. Research on present and future magneto-optic recording materials and technology from the late 1990s explored L10-ordered FePt and other high-anisotropy materials as paths toward gigabit-per-square-inch densities.

Applications

Magnetooptic recording has applications in a range of fields, including:

• Rewritable removable data storage in professional workstations and archival applications during the 1990s and 2000s
• Medical and scientific image archival, where WORM (write once, read many) variants provided tamper-evident long-term storage
• High-resolution digital imaging workflows in pre-press and broadcast production
• Laboratory magneto-optic disk drives as test beds for thin-film magnetic materials research
• Historical reference for understanding the physics of thermomagnetic recording, which informs modern heat-assisted magnetic recording (HAMR) in hard disk drives