Masatoshi Takao

Rapid‐phase transitions of GeTe‐Sb2Te3 pseudobinary amorphous thin films for an optical disk memory

Amorphous films having a component of the stoichiometric GeTe‐Sb2Te3 pseudobinary alloy system, GeSb2Te4 or Ge2Sb2Te5 representatively, were found to have featuring characteristics for optical memory material presenting a large optical change and enabling high‐speed one‐beam data rewriting. The material films being sandwiched by heat‐conductive ZnS layers can be crystallized (low power) or reamorphized (high power) by laser irradiation of very short duration, less than 50 ns. The cooling speed of the sandwiched film is extremely high: more than 1010 deg/s, which permits the molten material to convert to the amorphous state spontaneously; whereas, a low‐power pulse irradiation of the same duration changed the exposed portion into the crystalline state. The optical constant changes between the amorphous state and the crystalline state of them were measured to be large: from 4.7+i1.3 to 6.9+i2.6 and from 5.0+i1.3 to 6.5+i3.5, respectively. The crystallized portion was known to have a GeTe‐like fcc structure ...

High Speed Overwritable Phase Change Optical Disk Material

It was found that GeTe-Sb2Te3 pseud-binary amorphous alloy films showed remarkably fast switching properties to laser irradiation. By the static laser irradiation test, the film whose composition corresponded to stoichiometric compound of GeSb2Te4 were crystallized within 50ns of pulse duration at power of 8mW, whilst they could be amorphized with the same pulse duration at power of 20mW. Direct overwriting cycle test was performed on the revolving disk system for 105 times using single laser beam. CNR of more than 50dB and erasability of -22dB were obtained for linear velocity of 22m/s and overwriting frequencies of 5 and 7 MHz. The laser powers were 22 mW for recording and 10 mW for erasing. These materials will be applicable to high data rate direct overwritable disk media.

Magnetic Properties of Ferromagnetic Metal Alloy Fine Particles Prepared by Evaporation in Inert Gasses. II.

Single domain particles of Fe alloys (Fe versus Ni, Cu, Si, Cr, Gd and Ho) were prepared by the evaporation method. The saturation magnetization, remanent magnetization and coercive force of these particles were measured by an automatic magnetic balance. The dispersion of the anisotropy of Fe, Co and Ni particles were determined by a torque meter. The characteristics of these alloy particles were discussed in terms of their applicability to industrial use. The stability for oxydation and the magnetic orientation of particles have already been reported in the previous paper of this series. By choosing a suitable composition of alloys, the coercivity of the prepared particles can be controlled in the range of 200 to 1800 Oe. It was found that the anisotropy dispersion of Fe-Ni alloy was smaller than that of a commercial CrO2 tape.

Te-Ge-Sn-Au Phase Change Recording Film For Optical Disk

Te-Ge-Sn-Au thin films were studied for phase change type rewritable disk media, in order to obtain the fast crystallization speed and the thermal stability. Films were prepared by co-evaporation method. It was found that through static record/erase measurements the threshold crystallizing pulse duration of Te60Ge4SnllAu25 film was only 1 usec at 2 mW of laser power; that is less than one tenth compared with that of Te80Ge5Sn15 film, while the threshold amorphizing laser power of them were almost the same, 6 mW at 0.2 psec of pulse duration regardless of Au concentration. Its crystallizaton temperature of about 130°C was enough to maintain the good thermal stability. Through DSC, X-ray and electron diffraction studies, the first appeared crystalline state in crystallization process, corresponding to the drastic change in optical property, showed only one phase of metastable simple cubic(SC) structure. It was concluded that the appearance of the SC structure made the crystallization speed higher. The obtained thin film was a candidate for the simultaneously erasable and recordable material.

Insoluble binary system thin film

Abstract Insoluble binary amorphous Fe-Ag and FeCo-Ag or slightly soluble crystalline Fe-Cu films were co-deposited. It was established that they were crystallized or segregated after heat treatment. During the treatment, electrical resistivity drastically decreased at 250°C and coercivity surprizingly increased up to 840 Oe as a result of the shape anisotropy.

TeGeSnAu Alloys for Phase Change Type Optical Disk Memories

TeGeSnAu thin films were studied for phase change type rewritable optical disk material. Addition of Au to TeGeSn thin film shortened crystallization time. The crystallization time of Te60Ge4Sn11Au25 thin film was about 0.3 µs and was less than one 100th of that of Te80Ge5Sn15 thin film. Formation of a simple cubic structure with no atomic diffusions had caused the fast crystallization. The obtained optical disks showed high recording/erasing sensitivity, cyclability and thermal stability. The TeGeSnAu alloys will be applicable to rewritable optical disks with high data transmission rates.

Recent Advances In Erasable Phase-Change Optical Disks

Our recent studies of phase-change type erasable optical disk are reviewed. Two types of disk systems have been developed. The first one is a inherent overwrite system using single circular shaped laser beam. The recording materials for this case are Te-Ge-Sb ternary compositions corresponding to the stoichiometric compounds. They showed remarkable short switching time of less than 100 ns. The second type is a dual laser beam system for simultaneous erase/record in the single pass. One laser beam is circular shaped for both recording and reading. The other is elliptical shaped for erasing. The recording materials of Ge-Sb-Te-Se quaternary alloy, for this case, showed rather long erasing time about 500 ns but high sensitivity. The shape of erasing beam was modified for a successive thermal process of a homogenizing melt and a crystallizing anneal of the recording layer. The obtained erasability was less than -45dB, whilst it was about -30dB for the ordinary elongated beam by only annealing without melting.

Mössbauer study of insoluble Fe‐Ag coevaporated thin film

Structural change and accompanied magnetic properties corresponding to the annealing of insoluble Fe‐Ag coevaporated thin films were studied with Mossbauer spectroscopy. The observed linewidth of the absorption peaks of the spectra was remarkably large and implies that the hyperfine field of the Fe atom was largely distributed for as‐deposited Fe49Ag51 film. After the successive heat treatment, crystallization of the thin film gradually proceeded up to 150 °C annealing and finished at 200 °C annealing. The coercive force of the film was several oersteds for as‐deposited film and increased rapidly with an increase of annealing temperature. It suggests the growth of the segregated particles.

Magnetic Properties of Ferromagnetic Thin Films Prepared by Ion Plating

Thin films of Fe, Co and their alloys were prepared by ion plating. Metals were evaporated in the argon gas atmosphere of about 0.01 Torr, by applying a dc electric field of 2 kV/15 cm between the substrate and the evaporation source. Since evaporated metal atoms, which were given large kinetic energies by the gas plasma, impinged on the surface of the substrate, a strong adhesion between the ferromagnetic thin films and the substrate could be obtained. Due to the granular structure of the thin film, a higher coercive force and the squareness ratio could be obtained: i.e. by changing the Ar gas pressure, the coercive force and the squareness ratio could be controlled up to 500 Oe and 0.6, respectively, and also they were controlled by choosing the constituents of the mother alloys.