Magneto-optic Recording Materials
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for optical recording has already been demonstrated by several groups. Bluewavelength recording with a frequencydoubled laser (429 nm)8 has already been shown to increase the density up to 2.5 Gbits/in.2 with a data rate of a few Mbytes.1 Another example is the work reported using a green laser, which demonstrated a carrier-to-noise ratio (CNR) of 45 dB at a 0.4-^m mark size
— - Film ct(Tb 22 Fe 68 Co 1o ) — Film Y (Tb 28 Fe 60 Co 12 )
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Film p (Tb 27 Fe 61 Co 12 )
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80 120 160 Temperature (°C)
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80 120 160 Temperature (°C)
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Figure 1. Temperature dependences of magnetic properties (saturation magnetization M s and coercivityHc) of so-called 1X (film a), 2X (film (3), and 3X (film y) media where H w is the initial magnetic field.mM
with a track pitch of 0.9 /nm.9 For shortwavelength recording, media must exhibit high polar Kerr magneto-optic activity. In going to a shorter wavelength on the other hand, the efficiency of the detector is reduced. Thus an increase of the optical power on the detectors is necessary to maintain the same read-signal level if media-noise-limited performance is to be maintained. This requires the desensitization of media, leading to a higher read power or to a higher amount of light reflected by the polarization beam splitter to the detector channel.1 Increases in writing power can cause more rapid degradation of the medium, thus shortening the lifetime. Using a high numerical aperature (NA) objective lens can also reduce the spot size. One example is the use of a solid immersion lens,6 in which case a spot size as small as 360 nm was achieved using a diode laser of A = 830 nm. A CNR of 50 dB at 1 MHz was obtained using this technique in TbFeCo disks. Pulse width modulation is a scheme already used in some write-once optical and 3X magneto-optical recording drives.10 Since the information is stored at the edge of a written mark instead of in the mark itself, the density is doubled in comparison to the conventional pulse position modulation (PPM). The issue with this approach is to precisely place the edge of written marks under all variations of writing conditions without causing high jitter in the readout signal. Jitter as small as 100 A is significant. In addition the mark edge has to be extremely stable with repeated reads after writing as this contributes to the jitter (noise) as well. Magneto-optic Recording Media The magneto-optic recording media used in the ISO standard IX and 2X drives are amorphous RFeCo films (R = Tb, Gd, and Dy). These media exhibit high perpendicular magnetic anisotropy (>106 erg/cm 3 ), high squareness, and high coercivity, in addition to a relatively low Curie temperature (~250300°C). Also the compensation point where the net magnetization is zero can be adjusted by the contents of R as well as of Fe and Co, thus leading to various dependences of saturation magnetization on temperature11 (Figure 1). While the polar Kerr activity of these RFeC
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