Keiichiro Yusu

High-density recording capability of granular media composed of Co-Pt grains and SiO/sub 2/ matrix

Co-Pt and SiO/sub 2/ granular films were prepared on a Cr seed layer. Co-Pt grains dispersed in the SiO/sub 2/ matrix discretely. Conical grains grew in contact with the Cr layer and spherical grains grew between the conical ones. Coercivity exceeded 2 kOe and coercive squareness reached about 0.7 at a remanent magnetic moment of 0.4 memu/cm/sup 2/. Activating magnetic moment was 10/sup -15/ emu and the index of the thermal stability was about 100. Bit error rate was held below 10/sup -5/ up to a linear density of 280 kfci. The recording pattern at the track width of 1 /spl mu/m was clearly visible. The Co-Pt and SiO/sub 2/ granular media have potential for 5 Gb/in/sup 2/ recording.

Recording Characteristics of Ge Doped Eutectic SbTe Phase Change Discs with Various Compositions and Its Potential for High Density Recording

We describe the recording characteristics of discs having Ge doped eutectic SbTe alloy recording films with various compositions. The recording characteristics are fairly sensitive on the film composition. For our recording condition, the Sb/Te ratio of around 4 and the Ge content of about 5 at.% were preferable to obtain good carrier to noise ratio (CNR). A disc with eutectic recording film of that composition is compared with one with a GeTe–Sb2Te3 pseudo-binary alloy recording film, varying the mark length and the track pitch. The eutectic disc showed better CNR for the shorter mark lengths than the pseudo-binary disc. The cross-erase ratio of the eutectic disc is substantially higher than that of the pseudo-binary disc. The result suggests that the eutectic disc is suitable for higher linear bit density and the pseudo-binary disc is suitable for higher track density.

Spin-Dependent Tunneling between a Soft Ferromagnetic Layer and Hard Magnetic Nanosize Particles

Spin-dependent tunneling has been investigated for ferromagnetic tunnel junctions with a new structure, which consists of a hard ferromagnetic insulating granular thin film sandwiched between two soft ferromagnetic layers, or soft ferromagnetic and non-ferromagnetic layers. The granular film, consisting of ferromagnetic Co80Pt20 nanosize particles embedded in a SiO2 matrix with 13 nm thickness, has a high coercive force. The film was prepared by co-sputtering of Co80Pt20 and SiO2 targets on a glass substrate with a Cr or a Co80Pt20/Fe underlayer as a bottom electrode. A ferromagnetic Co9Fe layer as a top electrode was deposited over the granular film to form cross pattern junctions of 0.01 mm2 junction area through a metal mask. The tunneling magnetoresistance up to 4.5% at RT was observed for a Co9Fe/Co80Pt20–SiO2/Co80Pt20/Fe junction at a low field.

Dual-layer rewritable phase-change recording media for HD DVD system

We have investigated the recording characteristics of the dual-layer phase-change recording media for a system that uses the blue-violet laser (wavelength of 405 nm), with the numerical aperture of 0.65 and light incidence on 0.6-mm-thick substrate having the land and groove format (HD DVD). In order to achieve higher storage density for its beam spot size, a number of technologies have been incorporated into the media, such as the bismuth-substituted pseudobinary GeSbTe alloy recording film and hafnium oxide-based interface layer material. The feasibility of the user capacity of 36 GB has been successfully demonstrated.

Two Different Types of Antiferromagnetic Couplings and Magnetoresistances in Fe/Si Multilayers

Two different types of antiferromagnetic (AF) interlayer couplings as a function of Si layer thickness, t Si, have been observed in a series of (2.6 nm Fe/ tSi nm Si)22 multilayers. One of the AF-couplings was observed at around t Si=1.2 nm at room temperature (RT) and changed to ferromagnetic (F) coupling at a low temperature. The other AF coupling was observed for t Si thicker than 1.5 nm with a minimum around t Si=2.5 nm, and was almost temperature independent. Negative magnetoresistance has been observed in the multilayers with the AF coupling, and has similar temperature dependence as that of the AF coupling.

Media Technologies of 20 GB Single-Layer Rewritable Phase-Change Disc for HD DVD System

Sufficient signal amplitude, erase ratio and low amount of cross-erase must be attained to achieve higher recording capacity of rewritable phase-change media. We have developed new media technologies, such as the GeTe-rich GeTe-Sb2Te3 pseudobinary alloy recording film partially substituted with bismuth and an additional layer to control thermal diffusion. The recording layer material enables a high erase ratio with a high signal amplitude. The additional layer also reduces the amount of cross-erase while maintaining a sufficient erase ratio. We have successfully demonstrated rewritable phase-change media having 20 GB capacity by using these media technologies for the HD DVD system with a blue-violet laser diode having the wavelength of 405 nm, a numerical aperture of the objective lens of 0.65 and light incident on a 0.6-mm-thick substrate. In this paper, we describe our new media technologies and the evaluation results, such as tilt margins, uniformity, overwriting cyclability and life durability.

Transition from Antiferromagnetic Coupling to Biquadratic Coupling in Fe/FeSi Multilayers

The detailed temperature dependence of interlayer exchange coupling in Fe/FeSi multilayers prepared by ion beam sputtering was investigated and compared with the data for Fe/Si multilayers. Significant differences between interlayer exchange coupling in Fe/FeSi and Fe/Si multilayers were observed. The coupling between Fe layers in Fe/FeSi multilayers showed a transition from antiferromagnetic to 90° coupling with decreasing temperature. This was due to the strong temperature dependence of J2 (biquadratic coupling), which outweighs that of J1 (collinear coupling) at low temperatures.

Advanced phase change media for blue laser recording of 18 GB capacity for 0.65 numerical aperture and 30 GB capacity for 0.85 numerical aperture

We attempted to improve phase change media to achieve larger capacity for systems with blue lasers using numerical apertures (NA) of both 0.85 and 0.65. It was found that the Bi-added GeTe-rich pseudo-binary composition promised high erase ratio even when we did not use any interface layers on a recording layer. The use of an absorption control layer (ACL) was also attempted to suppress the cross erase (XE). It was found that the ACL was very effective in suppressing the XE. The advanced phase change media that used the above novel technologies showed good analog performance. The advanced media also showed bER of less than 1.0×10-4, promising 18 GB capacity for an NA of 0.65 and 30 GB capacity for an NA of 0.85.

High-Density Recording Capability of Five-Layered Phase-Change Optical Disc

High-density recording capability of the five-layered phase-change disc has been examined from the viewpoints of optical simulation and disc performance. The optical simulation showed that the temperature rise in the amorphous state balanced that in the crystal state for the case when the thickness of the lower protective layer was adjusted from 70 nm to 120 nm. Carrier-to-noise ratio (CNR), overwriting erase ratio and jitter were measured by the optical recording system using a laser diode with a wavelength of 636 nm and an objective lens of 0.6 numerical aperture. Track-pitch was fixed at 0.6 µm and bit length was varied at a linear velocity of 6 m/s. CNR of 52 dB, overwriting erase ratio of more than 35 dB, and overwriting jitter of less than 10% were obtained on both land and groove at a bit length of 0.34 µm/bit.

High-Speed Recording Media for HD DVD Rewritable System

We have developed optical recording media for high-speed operation on the HD DVD rewritable (HD DVD-R) format, which uses a laser wavelength of 405 nm, a numerical aperture of the objective lens of 0.65 and a cover layer thickness of 0.6 mm. In this study, we optimized the amount of bismuth substitution for antimony in pseudo-binary GeTe–Sb2Te3 alloy recording film and inserted interface layers to achieve high-speed recording and sufficient thermal stability in our media. As the amount of bismuth substitution increased, the erase ratio of the media improved. The rewritable media in which all the antimony is substituted by bismuth, i.e., the media having GeTe–Bi2Te3 alloy recording film achieved a sufficient erase ratio at a linear velocity 2.4 times that of the previous study without sacrificing the good thermal stability of amorphous marks. We demonstrate the operation of the media at data transfer rates ranging from 36.55 to 87.72 Mbps.