Materials for Magneto-Optical Recording
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Materials for
Magneto-Optical Recording Masahiko Kaneko
Abstract This article describes the characteristics and development of materials for magnetooptical (MO) recording. Magneto-optical recording is derived from thermomagnetic recording and readout by the magneto-optical effect. Rare-earth/transition-metal alloys, mostly Tb-Fe-Co, have been used for recording materials in MO discs. MO discs were first put on the market in 1988. Magnetic-field modulation for recording was introduced in MiniDisc (MD) systems in 1992. MO discs have an advantage in durability and achievability, because recording by magnetization reversal is not accompanied by any atomic movement. Magnetically induced super-resolution (MSR), reported in 1991, enabled resolving powers that are higher than the optical limit. MO discs based on MSR were commercialized as 3.5-in. discs for data recording and as 2-in. discs for digital cameras. As extensions of MSR, DWDD (domain-wall displacement detection) and MAMMOS (magnetic amplifying magneto-optical system) have been proposed, in which the marks are expanded during readout for a large signal. DWDD technology has been used in Hi-MDs (high-density MiniDiscs), which were commercialized in 2004 with storage capacities of 1 Gbyte per 64-mm-diameter disc. Keywords: magnetic, magneto-optical, memory.
Introduction Magneto-optical (MO) recording was first reported in 1957 by Williams et al. using MnBi film as the recording material.1 After various advances in tracking and focusing servo systems, laser diodes, and recording materials, a magneto-optical recording system was announced by Imamura et al. in 1980.2 The first commercialization of an MO disc was in 1988 for data recording with a capacity of 650 Mbyte per two-sided 5.25-in. disc. This article describes the characteristics and development of materials for MO recording.
Magneto-Optical Disc Structure The most popular material for MO recording is Tb-Fe-Co, an amorphous alloy consisting of the rare-earth Tb and the transition-metal alloy Fe-Co. An example of a typical composition is Tb20Fe74Co6. In most cases, the material is doped with a small amount of Cr for protection from corrosion. When dielectric layers of Si-N
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(typically, Si3N4) are inserted on both sides of Tb-Fe-Co, the structure is very resistant to corrosion. A Tb-Fe-Co film, dielectric layers, and reflective Al alloy film are deposited on a polycarbonate substrate by sputtering in Ar gas. A typical four-layer structure used as the best balance between recording and readout characteristics in conventional MO discs is a polycarbonate substrate deposited with Si-N (100 nm)/ Tb-Fe-Co (20 nm)/Si-N (40 nm)/Al alloy (40 nm).3
state, as shown in Figure 1. Tb-Fe-Co has various advantages as the recording material: (1) a high perpendicular anisotropy constant Ku, typically several 106 erg/cm3, which originates from Tb, as Tb has a large magnetostriction coefficient and a large single-ion anisotropy; and (2) a rare-earth Tb spin directed opposite to the spin of the transition-metal Fe-Co, since
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