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.

A low-refractive-index film plays an important role in attaining a high density of rewritable HD DVD optical recording media, for which the blue-violet laser of 405 nm wavelength incident on a 0.6-mm-thick Substrate is used. We have developed a new low-refractive-index material, SiOC, for the medium, which can be sputter-deposited at. much higher rates than the conventional SiO2 film. The SiOC film is formed using a SiC target with an Ar and O-2 gas mixture using either a RF or DC power supply. The film exhibited similar optical constants as SiO2, and DC sputtering in particular showed very low substrate temperature rise. The resultant deposition rate per sputter power is about three times higher than the rate of deposition using a SiO2 target. The obtained thin film consists of Si, O, and C. Although the film includes a substantial amount of carbon, it showed almost the same optical characteristics as SiO2. We demonstrated that the single-layer HD DVD rewritable medium with 20GB capacity using SiOC film had satisfactory recording characteristics. We believe that this media can be manufactured at a cost similar to that of conventional rewritable DVDs in mass production.

A signal enhancing technique for a reflection-mode near-field scanning optical microscope (NSOM) is proposed. Optical interference between the signal light, from an aperture at the tip of a tapered optical fiber, and the reflected light, from a metallic coating around the aperture, enhances the signal intensity. We used a rewritable high-definition digital versatile disc (HD DVD) with dual recording layers as a sample medium, and demonstrated observation of amorphous recording marks on the semitransparent (the first) recording layer. In spite of low optical contrast between the crystal region and the amorphous region on this layer,,we successfully observed recording marks with good contrast.

Thermal conductivities and boundary thermal resistances of thin films having the thickness of the order of ten nanometers were measured by using the thermo-reflectance method at room temperature. A thermal simulation of HD DVD-ARW (the next-generation advanced rewritable DVD) media was carried out to clarify the effect of boundary thermal resistance at the interface of those films. The thermal conductivity of thin films greatly depends on film thickness. The result of the thermal simulation depends significantly on whether the boundary thermal resistance is considered or not. Thus it is important to consider the boundary thermal resistances and using thermal properties of thin films to perform more accurate calculation for the phase change recording media. The results of the thermal simulation also suggested that the boundary thermal resistances dominate the thermal diffusion and response of the medium.

We have investigated the recording characteristics of the dual-layer phase change recording media for the system with the NA of 0.65, the wavelength of 405 nm, and the light incidence on 0.6-mm-thick substrate having the land and groove format. For both L0 and L1 of the dual-layer disc, we have adopted bismuth substituted pseudo-binary GeSbTe alloy film as the recording layer and a novel interface layer material in order to improve overwriting characteristics. Bit error rate measurements have successfully demonstrated the feasibility of the user capacity of 36GB and confirmed 30GB with tilt margins.

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.

We developed a Finite-Difference Time Domain (FDTD) Method based simulator that can analyze Dual-Layered phase-change disc structure. Because of a substrate thickness error, it is necessary to consider spherical aberration. The electric field distributions have been calcuated on Single-Layered and Dual-Layered phase-change discs. We found differences between the electic distributions focused on a groove and on a land. These calculation results can explain the corresponding experimental results clearly.

In order to increase the recording density and decrease the track pitch of the digital versatile discs (DVD) media using the land-and-groove recording format, it is essential to suppress the magnitude of the cross-erasure of the recorded marks. A computational analysis to estimate the mark and erasure shapes was carried out, and comparisons with the experimental results have shown good agreement. The cross-erasure was reduced under the conditions given in this paper by increasing the duty ratio of land width to groove width (L/G) while keeping the groove pitch constant.

Advanced phase change medium of over 30GB data capacity has been proposed and the recording characteristics of the medium have been described. The medium has L-H polarity, Bi-substituted GeTe-rich pseudo-binary alloy film as the recording layer, and thin metal absorption control layer. The recording measurement was carried out by using a blue laser light source with the wavelength of 405nm and an objective lens with NA of 0.85. Both CNR and ER have been improved by the use of the Bi-substituted GeTe-rich composition. XE has been reduced by the use of the absorption control layer. The bER measurement demonstrated its capability for over 30GB data capacity.

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.

The possibility of an optical disk capacity of 28GB/side is studied. The read-write characteristics of a phase-change optical disk are examined using a thin cover layer, a blue laser diode, high NA objective lens and the PRML method. The wide tilt margins of high-density recording are achieved.

Recording marks in digital versatile disc-random access memory (DVD-RAM) were examined using a scanning near-field optical microscope (SNOM) incorporating an improved probe tip design. The marks were recorded on an experimental DVD-RAM medium with a track-pitch of 0.6 µm. The tip size of the SNOM was made wider than the track width in order to avoid optical ghosting arising from the track edge. The recording power was varied in order to investigate the dependence of the mark shape on power and it was found that the mark shape correlated well with the reproduced signal. The SNOM is considered to be a good candidate for a practical characterization tool for optical recording media.

High-density recording capability has been demonstrated in a modified low-to-high type phase-change disc which consists of multilayered lower dielectric film and an AlN upper protective layer. The thermal simulation shows that the disc is effective for decreasing thermal interference. A carrier to noise ratio (CNR) of more than 52 dB and an overwriting erase ratio of more than 30 dB were obtained at a bit length of 0.28 µm and a track pitch of 0.6 µm on both land and groove at a linear velocity of 8.2 m/s.

The temperature dependence of the coercivity H, was measured for Copt-based thin film, CoPt-Si02 granular film and CoCr-based thin film recording media. All the samples showed a linear decrease in H, with temperature, -dHJdT. It is found that Copt-based media have smaller -dHJdT values than CoCr-based one. This means that Copt-based media have good stability for archival and readwrite characteristics at elevated temperatures. From a theoretical discussion, it is also found that Copt-based thin film and granular media with small activating magnetic moment value are expected to have smaller recording coercivity than CoCr-based media. Index Terms-coercivity, magnetic recording medium, recording coercivity, thermal stability I. INTRODUCTION It is important for a high density magnetic recording medium to know about the temperature dependence of coercivity (H,) since recorded information or read/write characteristics might change under various ambient temperatures. The temporal characteristics of H, is also important since H, under a high frequency switching field is related to the recording coercivity. It is known that magnetization reversal is a thermal activation process ( 11. This means that a temporal scale is equivalent to a thermal scale for a specific magnetization reversal process. Since H, is measured through a magnetization reversal process, it is possible to obtain the thermal and temporal characteristics of H, from the H, measurement at various temperature. We measured the temperature dependence of H, for Copt and CoCr-based high density magnetic recording media. In this paper, discussion of the change of H, with temperature and recording coercivity for these media is presented. 11. EXPERIMENT The temperature dependence of H, and magnetization were measured for Copt-SiOz granular media (2), Copt-based thin film media (CoPtCr) and CoCr-based thin film media (CoCrTa) on glass substrates. The values of H,, M,t (M, : remanent magnetization, t : film thickness) and the activating magnetic moment VI,B (V : unit volume of magnetization reversal, IsB : bulk saturation magnetization) at room temperature are listed in Table I. VI,B was obtained form a time decay of the magnetization undei various external magnetic fields 131. Differences in M,t and VIsB were due to differences in film thickness. Magnetization and coercivity were measured with a vibrating sample magnetometer with a maximum applied field

The disk characteristics of granular media comprising Co-Pt and SiO2 have been examined to demonstrate their high-density recording capabilities. The grain size increased when RF-bias was applied during sputtering. A coercivity was 2 kOe at a magnetic moment (Mrt) of 0.4 memu/cm2 and a Co-Pt content of 50vol.% when a Cr seed layer used. The mean grain size was less than 10 nm and the grains were thoroughly isolated in the SiO2 matrix. The normalized media noise was less than 10-2μm1/2μVrms/μVpp up to 200 kfci. Transition patterns of more than 200 kfci were clear at a track-width of 1μm. The surface roughness was similar to that of a bare glass substrate. No degradation in magnetic properties was observed after 1000 hrs. in an 80°C-75%R.H. environment.

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.

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.

The temperature dependence of the coercivity H/sub c/ was measured for CoPt-based thin film, CoPt-SiO/sub 2/ granular film and CoCr-based thin film recording media. All the samples showed a linear decrease in H/sub c/ with temperature, -dH/sub c//dT. It is found that CoPt-based media have smaller -dH/sub c//dT values than CoCr-based one. This means that CoPt-based media have good stability for archival and read/write characteristics at elevated temperatures. From a theoretical discussion, it is also found that CoPt-based thin film and granular media with small activating magnetic moment value are expected to have smaller recording coercivity than CoCr-based media.

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.