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A recording density of 1.5 Pb m−2 using heat-assisted magnetic recording in a bit-patterned media is demonstrated. This represents a dramatic improvement in track width and optical efficiency over continuous media, owing largely to advantageous near-field optical effects.

To reduce interlayer crosstalk caused by the ghost spot which appears in a multilayer optical disk with more than three information layers, a multilayer disk structure which reduces interlayer crosstalk with a wide disk-fabrication margin was proposed in which the backward reflectivity of the information layers is sufficiently low. It was confirmed that the interlayer crosstalk caused by the ghost spot was reduced to less than the crosstalk from the adjacent layer by controlling backward reflectivity. The wide disk-fabrication margin of the proposed disk structure was indicated by experimentally confirming that the tolerance of the maximum deviation of the spacer-layer thickness is four times larger than that in the previous multilayer disk.

To obtain minimum overwrite jitter after 104 overwrites, the cause of degradation of readout waveforms after 104 overwrites was investigated by transmittance electron microscopy (TEM). To decrease the temperature of the recording film at the track center during recording, an Al-alloy double-reflective-layer structure is proposed. Two relationships, one between the thermal conductivity of the first reflective layer and the temperature of the recording film during recording and the other between the thermal conductivity and the readout waveforms after overwrites of more than 105 times, were investigated using thermal simulation and a digital versatile disk (DVD) disk tester. The optimized disk has a minimum overwrite jitter after 105 overwrites at a linear velocity of 6 m/s.

A Ag–Ge–Sb–Te recording film was developed, and its chemical composition was optimized to minumize overwrite jitter. The cause of the jitter was investigated by measuring the crystallization ratios and by transmission electron microscopy. It was found that the overwrite jitter increased with a shortage of Ge or Ag because the crystallization speed increased when these elements were added, while it increased with an excess of Ge or Ag because the crystallization speed decreased when these elements were added. A model based on the atomic bond distance and the strength of interatomic interactions in the crystal was devised. The model can be used to explain the mechanism by which a slight change in the chemical composition of recording films changes the crystallization speed. It was confirmed that the proposed disk with recording film having a small amount of Ag had the minimum overwrite jitter (under 10%).

To realize optical discs with the sub-terabyte data capacity, we propose the three-dimensional pit selection (3DPS) method where a single data pit to be read out in a multi-layer disc is selected three-dimensionally to obtain super-resolution in the disc plane and to reduce layer cross-talk. To examine the feasibility of this method, the phase-change pit capsule method was tested where the data pits consist of a phase-change material which melts during readout. The super-resolution effect was observed for both layers of a dual-layer disc. It was shown that a quadric-layer disc can be designed because of the high transmittance of each layer. Thus, 3DPS is considered to have the potential for a data capacity of hundreds of gigabytes with a conventional optical system.

We have introduced a tungsten oxide film to a layer-selection-type recordable optical disk (LS-R) as an inorganic electrochromic (EC) film. The EC film showed a large transmittance change in a wide wavelength range. Layer selection was demonstrated with a dual-layer disk whose spacing was 0.3 µm, and the capacity of a 20-layer LS-R was estimated to be 1 TB.

The signals obtained from dual-layer optical disks were analyzed experimentally and theoretically with emphasis on reducing layer cross talk (LCT) and spherical aberration (SA). Using the dual-layer phase change optical disk, the DC element of the LCT was measured and phase change marks were written on both layers. Diffraction calculations were used to explain the experimental results and to determine the amount of LCT and signal degradation caused by SA as functions of spacer layer thickness and optical detector size. Analytical methods to estimate LCT and SA were proposed, and their predictions turned out to agree well with the experimental and diffraction calculation results. These findings indicate that for a blue laser, the conventional techniques cannot simultaneously keep the LCT and SA to within practical values. Accordingly, designs for disks and drives for dual-layer optical disks are discussed.

The effect of the magnetic and structural properties on recording performance was investigated. Well grain-isolated media were obtained by adding boron and oxygen into the [Co/Pd]n multilayer and cooling the substrate. We found that the recorded bit size could be decreased by reducing the magnetic cluster size. Twenty-flve-nm bits were successfully recorded on the improved media with a magnetic cluster size of about 30 nm.

The formation and observation, with reflected light, of 60‐nm‐diam phase‐changed domains in a thin GeSbTe film using a scanning near‐field optical microscope with a 785 nm wavelength laser diode is demonstrated. The dependence of the domain size on incident laser power was obtained, and the size changed from 150 to 60 nm in diameter with incident power of 8.4–7.3 mW in the probe. At the threshold power of 7.3 mW, the film temperature rose to around 180 °C to partially phase change the local area of the film from amorphous to crystalline. A detected reflectivity increase due to phase change in the formed domain was 8%–2%. The observing (reading) was performed with an incident laser power of 0.2 mW, which corresponds to 10−2–10−3 times less than in a magneto‐optical recording. The incident laser power shows that the phase change reading using the reflection scanning near‐field optical microscope has the potential to read the recorded bit at a speed over 10 MHz.

We demonstrate 80 nm diameter bit recording for the first time using a phase change recording film and a reflection scanning near-field optical microscope with a 785 nm wavelength laser diode. The sample structure was a 20 nm thick ZnS-SiO 2 protection layer/30 nm thick Ge 2 Sb 2 Te 5 recording film/150 nm thick ZnS-SiO 2 protection layer/polycarbonate substrate. Writing was performed with pulsed laser light of 8.4 mW for 5 ms and 0.5 ms, and 8.0 mW for 5 ms. Written bits were observed in reflection by illuminating a small light of 0.2 mW. In this form of recording, a formation of phase change domains of about 50 nm in diameter is expected if the surface deformation is suppressed. Our results indicate the possibility to achieve an ultra-high recording density of more than 100 Gb in -2 .

Scanning near-field optical microscopy (SNOM) was applied to phase-change (PC) mark writing to investigate the potential of ultrahigh density data storage. GeSbTe, one of the typical PC media, was used as a recording layer. Using a laser diode (LD) with a wavelength of 785 nm as light source, crystalline marks with a minimum size of ∼ 80 nm in diameter were successfully written in the amorphous GeSbTe film with pulse conditions of 5 ms and 0.5 ms and 8.4 mW inside the probe. The detected power in observation of the marks was 10 2 –10 3 times as high as that in magneto-optical (MO) observation.

Three-dimensional pit selection (3DPS) is proposed for optical discs with the capacity of hundreds of gigabytes, where the super-resolution and multi-layer techniques are combined. In this method, a high transmittance of each layer is obtained by discrete opaque data pits made of a super-resolution material. The dual-layer disc was fabricated using a phase-change material as a pit material. The experimental data show that the super-resolution effect is obtained for both layers. The additional advantages and the future expandability are discussed

We introduced an inorganic electrochromatic layer for layer selection recordable optical disk (LS-R) by applying voltage. The material showed a large transmittance change in a wide wavelength range. The cross-talk between layers was sufficiently low.

Characteristics of a phase‐change optical disk using a (Ge–Sb–Te)‐(Cr–Te) recording film containing a high‐melting‐point component Cr–Te are studied. The high‐melting‐point component Cr–Te precipitates in the recording film, and prevents a material flow of the recording film during repeated rewrites in which the recording film is melted. There is no drawback such as noise increase by the addition of Cr–Te. Thus the phase‐change optical disk using this recording film shows very small distortion in the reproduced signal wave form even after 2×104 rewrites of high‐density recording signals that are liable to cause the material flow by forming long and short recording marks on the disk. With a phase‐change optical disk, it is easy to overwrite (rewrite without prior erasure) information, which makes it highly suitable for digital recording of motion pictures and sound. They are expected to be used as rewritable compact disks, rewritable CD‐ROM disks, rewritable video disks, and small recording media for personal computers.

Designs of drives and disks for dual-layer optical disks are discussed based on a newly proposed theoretical model to estimate the layer cross talk approximately.

Phase change mark simulator is investigated. Nucleation, crystal growth, and re-crystallization at amorphous/crystal boundary are taken into consideration. Crystallization is evaluated with 3-dimensional thermal analysis. Mark shapes of experiments and simulation are in good agreement. Amorphous mark forming process was analyzed with the simulator.

L10-ordered FePtAg-C granular media capped with an FePtAg-SiO2 layer were fabricated in order to obtain stable flyability performance on a spin-stand tester. The post-annealed FePtAg-SiO2 was more suitable for the cap layer than an FePt alloy without segregants, since it formed a continuous structure on the well-isolated FePtAg-C layer. By using the optimized post-annealed FePtAg-SiO2 capped medium, the surface roughness (Ra) was reduced to 0.45 nm, thereby enabling stable flying at a glide height of 6 nm.

High-speed recording and read-out were carried out under the condition of numerical aperture (NA) 0.65 and substrate thickness 0.6 mm using a blue-violet laser of 405 nm. Using a GeSbTe phase change material of the crystal nucleus generation dominant type, we obtained a carrier-to-noise ratio (CNR) of 50 dB with 0.24 µm mark and a DC erase ratio of 30 dB at a velocity of 18 m/s. It was proven that the realization of the 100 Mbps data transfer rate was possible by optimizing record compensation.

We experimentally studied an optical disk with a large capacity and high data-transfer rate that can record high-definition television signals. A high rotation speed was used to achieve the high data-transfer rate. Using Ge–Sb–Te phase change material, a carrier-to-noise ratio of about 50 dB was achieved at a linear velocity of 15–30 m/s with a 0.4-µm-long mark. High-speed crystallization was necessary to erase recording marks. Therefore, by appropriately designing layers that assist nucleation, we obtained a DC erase ratio of 29 dB with a 0.4-µm mark and 1-mW power margin for over-25-dB erase ratio at a linear velocity of 18 m/s. Furthermore, the 0.4-µm mark-space repetition jitter was less than 7% even after 1000 cycles. A data transfer rate of 60 Mbps was obtained by optimizing the write strategy using a multi-pulse with an optical system equivalent to that of current digital versatile disk (DVD).

We present for the first time a nanometer-sized phase-change recording using a scanning near-field optical microscope (PC-SNOM recording). The recording experiments were performed with a SNOM using a 785-nm-wavelength semiconductor laser diode, shear force detection for gap control and reflected light detection for observing the domains (reading). The recording media of ZnS·SiO2(20 nm)/GeSbTe(30 nm)/ZnS·SiO2(150 nm)/polycarbonate substrate were used. The writings were done at laser powers of 8.4–7.3 mW in the probe for pulse widths of 5 or 0.5 ms. As a result, we obtained a minimum recorded domain size of 60 nm in diameter. This size shows a potential to achieve an ultrahigh density PC-SNOM recording with about 170 Gb/in2. A possibility of achieving high speed readout for the future data storage is also discussed.

Double-reflective-layer structure disks with recording film containing a high-melting-point component that does not melt during recording are studied. Cr–Te is chosen for the high-melting-point component to be added to Ge–Sb–Te recording film to suppress a recording film material flow. A Si–Al double-reflective-layer structure has the advantage of increasing the material flow stopping power margin. The structure is also effective in preventing deformation of the recording marks due to the difference in light absorption between the crystallized region and the amorphized recording mark. A Si–Al double-reflective-layer-structure disk with Cr–Ge–Sb–Te recording film shows very little distortion in the reproduced signal waveform even after 104 overwrites of pulse-width modulation (PWM). This makes it suitable for high density recording as a rewritable digital video disk (DVD).

Thermal extraction behavior of He and D from He+-implanted yttrium iron garnet (Y3Fe5O12 or YIG) thin films annealed in D2 and activation energies of He and D in each trapping state have been investigated by means of a thermal desorption mass spectrometric (TDS) technique in order to clarify the effect of hydrogen on the effective anisotropy field change induced by ion implantation followed by D2 annealing. Four trapping states of He in He+-implanted YIG were observed when it was annealed under T a=320°C at a constant pressure of 760 Torr. The residual He atoms shifted to stabler states when it was annealed at above T a=350°C. Two trapping states of diffused D atoms which were extracted as a form of D2O were observed for two kinds of YIG samples annealed in D2 atmosphere. The relationship between effective anisotropy field change induced by ion implantation and the thermal extraction behavior of He and D was discussed against the annealing temperature.