Magneto-Optical Storage Materials
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discovered amorphous GdCo with perpendicular magnetic anisotropy as a possible MO recording material. Today a m o r p h o u s t e r n a r y r a r e - e a r t h (RE) transition-metal (TM) alloys like GdTbFe and TbFeCo are the recording layers in the MO disks which appear on the market. Although thèse materials show good recording performance, they also exhibit some drawbacks, mainly caused by their susceptibility to oxidarion and corrosion. However, two new classes of suitable MO materials with good oxidation résistance are emerging. Recently, good recording results hâve been reported in
Laser
H c (kA/m)
300
(b) Figure 1. Principle of the thermomagnetic writing process. (a) A laser générâtes a température profile T(x) in the perpendicular magnetized layer M; consequently, the coercive field Hc decreases with température T as in Figure (b). The magnetization direction will be reversed locally by the applied field H, as in (b) at T = 150°C and H = 30 kA/m.
MRS BULLETIN/APRIL1990
garnets 4 and in Co/Pt multilayers. 5 This article will discuss the principles of magneto-optical recording. We will describe the issue of direct overwrite and then the requirements for materials suitable for MO data storage applications. Next, we présent the properties and performance of RE-TM alloys used today, as well as new and promising MO materials like oxides and Co/Pt or Co/Pd multilayers. Finally, we give an outlook on future média developments. Thèse are closely linked to future drive and Systems requirements. Magneto-Optical Recording Principles of Write and Read The common way to write data in an MO film is to heat the film locally by a focused diode laser beam and to employ the température dependence of the film's coercive field. This process is called thermomagnetic writing. Sometimes one distinguishes between Curie point writing and threshold writing, depending on whether the MO layer is heated above the Curie température Tc, where the coercivity, Hc, vanishes, or to a température below Tc where the effective field equals Hc. This effective field consists of the applied field and the internai demagnetizing field. During cooling, the magnetization will direct itself according to the effective field a n d will t h u s be locally reversed. The whole process is illustrated in Figure 1. The w r i t t e n marks typically hâve the diameter of the laser spot (1 /xm). Typical laser puise time is 100 ns. Domain patterns are read by detecting changes in the polarization direction of linearly polarized light upon reflection (Kerr effect) or transmission (Faraday effect). Plane-polarized light, when reflected frorn a magneto-optically active surface, will become elliptically polarized, with its major axis slightly rotated with respect to the original direction. The sign of the angle through which the p o l a r i z a t i o n is r o t a t e d d é p e n d s on whether the magnetization is directed up or down (Figure 2). In practice, the magnitude of this Kerr rotation is fairly small, typically 0.2-0.4 deg. If the Faraday effect is used, t
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