Site: Sanyo Corporation
Hypermedia Research Center
Optical Recording Technology
180, Ohmori, Anpachi-cho
Anpachi-gun, Gifu 503-01, Japan
Date Visited: 3 April 1998
WTEC: M. Mansuripur (report author)
This group is involved only in magneto-optical (MO) recording research, although Sanyo manufactures CD, CD-ROM, MiniDisc, pickups, and lasers for the optical data storage market. Advanced storage magneto-optics (ASMO) technology seems to be one focus of research at Sanyo. The current plans call for the 6 GB ASMO disk on a 12 cm platter (single-sided), followed by the second-generation disk at 12 GB using the MAMMOS technology, and leading up to the 30 GB disk in the third generation.
According to the Sanyo researchers, magnetic-field modulation (MFM) recording is attractive because it offers the possibility of recording domains as small as 0.1 microns in diameter. Although the minimum required data rate is currently around 4 Mb/s, they foresee the potential of 50 Mb/s data rates in the next 2-3 years using the MFM technique. They also believe that in the near future access times will reach below the current value of 60 msec.
The substrate thickness for ASMO disks is 0.6 mm in the data area (1.2 mm in the central hub area). The tilt of the disk is not considered a serious concern here because MFM recording and the technique of magnetic super resolution (MSR) readout used in ASMO disks are not tilt-sensitive. Therefore high-numerical aperture objectives can be used in conjunction with the 0.6 mm substrate without much concern for tilt-induced coma. The preferred mode of MSR seems to be CAD-MSR. The GdFeCo layer used as the readout layer has a large Kerr signal, even at short (blue) wavelengths. As for future-generation devices using blue laser diodes, the Sanyo researchers mentioned that they had researched superlattice materials such as PtCo, but they also felt confident that, using the MAMMOS technology, TbFeCo and GdFeCo media would offer sufficient sensitivity and acceptable levels of signal-to-noise ratio (SNR) at short wavelengths.
It was mentioned that the density of MO media is limited only by the width of magnetic domain walls in amorphous RE-TM alloys (on the order of 10-20 nm), and that data transfer rates are limited perhaps by the 10 psec time constant for magnetization reversal in these media.
Concerning the competition between hard disk drives (HDD) and optical disk drives (ODD), the hosts expressed the view that both technologies will coexist in the future. While HDD is superior in terms of cost-performance and data rate, ODD has the advantage of removability and mass-reproducibility. It was mentioned that the rapid growth and the pace of change in HDD technologies is perhaps the reason why the Japanese companies are not leading in this area. Slow decisionmaking processes were blamed for the slow pace of change in Japan. The hosts emphasized that while the hard disk drives are made in the United States, many parts and components come from Japan. The focus of magnetic recording in Japan seems to be on magnetic tapes for VCRs.
The Sanyo researchers felt that the MO technology is superior to phase-change (PC), even in terms of compatibility with DVD-ROM. The wollaston prism, for example, costs under a dollar in large quantities, and silicon photodiodes are very cheap; in fact the split detectors needed in MO drives are about the same price as the single-detectors used in PC drives. Some versions of MAMMOS operate without a magnetic field whatsoever. So all the talk of MO being more expensive than PC, they contend, is unrealistic.
It was mentioned that the growth path for LIMDOW technology is through MSR and blue lasers, although the researchers felt that the commercialization of high-power blue lasers before the end of 1998 was not likely. They also emphasized the importance of the PRML technique for high-density recording. As for the use of liquid crystal (LC) devices in optical disk drives, they mentioned that Sanyo currently uses LCs in its DVD-ROM drives to achieve compatibility with CD-ROMs, but they said that the cost of these elements must come down before their use becomes widespread.