Miyozo Maeda

Study on readout stability of TbFeCo magneto-optical disks

The authors studied the readout stability of TbFeCo magnetooptical disks by irradiating them with a readout laser beam and found that carrier-to-noise ratio decreases proportionally with the logarithm of time. Thus, the readout stability can be evaluated by determining the log t coefficient. They have found that readout stability is better in transition-metal-rich films than in rare-earth-rich films. Changes in the recorded bit length shows that the carrier-to-noise ratio decreases in proportion to the increase of variance in the recorded bit length. >

Selenium Alloy Film For New Erasable Optical Disk Media

Selenium film alloyed with indium and antimony has good characteristics for erasable optical disk media. Depending on the conditions of the irradiation directed to the film, it exibits two different reflectances and geometrical deformations,.and the change between these two states is reversible. The mechanism of the reversible changes is different from amorphous-to-crystalline phase transition. Both states are definitely crystalline, which have been confirmed by transmission electron microscope analysis. The film has a good durability of write-erase cycles and is chemically stable. The disk sample has been confirmed to be capable of writing and erasing at a speed of 600 rpm. The resolution is sufficient to record a bit of 1 pm length on the disk. Carrier-to-noise ratio is fairly good. This film also has a potential for higher signal transfer rates.

Direct overwriting capability of magneto-optical disks

Erasing magneto‐optical disks were studied using constant frequency pulses. The magnetic layer was Gd12Tb12Fe76. The linear velocity was 7.2 m/s and writing and rewriting were done at 1.8 MHz. The magnetic field was the same during writing and erasing. If the erasing power is too low, bits are incompletely erased, and if the erasing power is too high, erase pulses nucleate domains. Erasing performance can be improved by high frequencies and low duty cycles. Erasing performance decreases as the magnetic field in write direction becomes stronger. However, almost the same rewriting C/N as the original C/N can be obtained in a weak magnetic field. We obtained a rewriting C/N of 29 dB, while an original C/N was 31 dB at a magnetic field of +50 Oe.

DyFeCo magneto-optical disks with a Ce-SiO2 protective film

We developed a DyFeCo magneto-optical disk using a protective film made of Ce-SiO2. The disk structure, composition, and sample preparation conditions were optimized to provide adequate read-write characteristics and reliability. Since DyFeCo is considerably less expensive than TbFeCo, our disks are practical alternative.

Direct overwriting of magneto-optical disk

The possibility of direct overwriting in magneto-optical (MO) disk is demonstrated. By a modfied frequency modulation technique of laser pulse, erasing of recorded bits could be accomplished on the rotating disk. We measured the erasure characteristics for GdTbFe films prepared by RF magnetron sputtering, and found that erasure performance depended on the frequency and the power of the laser pulse and on the magnetic field. We also found that there was an optimum power for erasing the recorded bits. Our experiment produced almost the same rewriting carrier-to-noise ratio (CNR) as the original CNR under a weak constant magnetic field. We confirmed that direct overwriting in MO disk is possible by using a two-beam technique and that read-before-write scheme which makes a drive complex is unnecessary.

Magneto-optical recording enhanced by magnetic recording techniques

Front illumination is the fastest type of optical disk. Front illumination also allowed us to design a very small and light head with a beam size of 1 mm. It also allowed us to use an opaque aluminum substrate, which can rotate at high speeds. If optical disks are mounted in an enclosure like those for hard disks, they are protected from contamination and front illumination would be practical.

Bit-Error Reduction In Magneto-Optical Disks

The number of bit errors in magneto-optical disks were reduced and a bit-error rate of 10-6 was achieved even after an accelerated lifetime test. To reduce the number of bit errors, we performed error detection and classification and found that the substrate was the dominant cause of errors. The substrate we used was a photopolymer(2P)-glass substrate, and we found two types of defect origin. One of them was due to 2P residues on the stamper, and the other was due to dust adhering to the substrate. We reduced the initial bit-error rate by removing these defects. We also found that 2P impurities considerably increase the bit-error rate during the accelerated lifetime test. By purifying the 2P, we made our disks more stable, and estimated that their lifetime would exceed 20 years at 40°C and 90% RH.