Keiji Shono

Origin of Negative Differential Resistance Observed on Bipolar Resistance Switching Device with Ti/Pr0.7Ca0.3MnO3/Pt Structure

Bipolar resistance switching was investigated on sputtered Pr0.7Ca0.3MnO3 (PCMO) sandwiched by Pt- and Ti-electrodes. Based on electrical conductivity measurements and a combination of electron energy loss spectroscopy analysis and transmission electron microscopy observation, we found that the negative differential resistance observed in the forming process originates from the motion of oxygen ions at the Ti/PCMO interface. We propose that the observed resistance switching is caused by an oxidation/reduction reaction at the interface.

Enhancement of Switching Capability on Bipolar Resistance Switching Device with Ta/Pr0.7Ca0.3MnO3/Pt Structure

We found that Pr0.7Ca0.3MnO3 (PCMO) film sandwiched by a Ta top electrode (TE) and a Pt bottom electrode (BE) exhibited bipolar resistance switching similar to that of Ti(TE)/PCMO/Pt(BE). The switching capability of Ta/PCMO/Pt was greatly improved by pulse-forming compared to dc-forming, which are both pre-treatments to enable resistance switching by pulsed voltage. Switching speeds faster than 100 ns and rewrite cycles of more than 10,000 were obtained while maintaining a ratio of resistance change larger than 1,000%. The mechanism of resistance switching was explained by an oxidation/reduction reaction at the Ta/PCMO interface, as previously proposed for Ti/PCMO/Pt.

Microstructure of sputtered garnet films for magneto‐optical recording media

Sputtered garnet films have a high potential for use as a magneto‐optical recording medium. However, the medium noise is thought to be large because of its crystal boundaries. We investigated the microstructure of films on GGG substrate, using TEM and SEM. The film crystallized by postannealing shows a rectangular hysteresis loop with high coercivity, and has an uneven surface. This is because the direction and size of each crystallite is not uniform. However, the film crystallized during deposition has quite a smooth surface, but does not have magnetic properties suitable for use in magneto‐optical recording. This film has a mosaiclike structure, in which all crystallites are uniformly oriented. We found that ion implantation of Ne+ and successive annealing of the film crystallized during deposition improved the magnetic properties without roughening the surface.Sputtered garnet films have a high potential for use as a magneto‐optical recording medium. However, the medium noise is thought to be large because of its crystal boundaries. We investigated the microstructure of films on GGG substrate, using TEM and SEM. The film crystallized by postannealing shows a rectangular hysteresis loop with high coercivity, and has an uneven surface. This is because the direction and size of each crystallite is not uniform. However, the film crystallized during deposition has quite a smooth surface, but does not have magnetic properties suitable for use in magneto‐optical recording. This film has a mosaiclike structure, in which all crystallites are uniformly oriented. We found that ion implantation of Ne+ and successive annealing of the film crystallized during deposition improved the magnetic properties without roughening the surface.

Magneto-optical recording of sputtered garnet films using laser diode

Sputtered garnet film is so transparent in the near-infrared region that recording using a laser diode was thought to be impossible. However, we have found that it is possible if Cr is applied to a thin garnet film. We investigated the sensitivity for static recording. The optical absorption of Cr film is 60% in the 0.8 μm wavelength range, and temperature elevation ocurrs easily. Recording becomes possible through the heat transfer from the Cr to the garnet film. The sensitivity depends mainly on the thickness of the Cr and garnet films. The optical absorption of the garnet film makes little contribution to the sensitivity. Recording needs only 6mW at a pulse width of 0.15 μs for a 0.1 μm thick garnet film by applying a bias field of 150 Oe. This sensitivity is considered to be sufficient for an optical disk mediun. Furthermore, dynamic recording was carried out using a 5-inch glass disk. We obtained a rather low C/N value of 30 dB. The reason for this low C/N value is due to both the irregular bit shape and the medium noise, resulting from the poor morphology of garnet film.

Optimized structure of sputtered garnet disks

The optimum structure of the sputtered garnet magnetooptical disk was investigated by numerical calculations and experiments. From the Faraday rotation, Faraday ellipticity, refractive index, and extinction coefficient, the authors determined the dielectric tensor elements of a sputtered garnet film. Using these values, they calculated the thickness dependence of the magnetooptical rotation angle, the reflectivity, and the reproduced signal amplitude of the disk with a reflecting layer. These results were then compared with measurements. The optimum garnet thickness and reflector material for a sputtered garnet disk were found. The optimum structure has a carrier/noise ratio of 60 dB at a carrier frequency of 0.6 MHz and possesses good recording sensitivity. >

Ce‐substituted garnet media for magneto‐optic recording

The magnetic and magneto‐optical properties of crystallized as‐deposited Ce‐substituted sputtered garnet films were measured as a function of temperature. The films were found to exhibit large Faraday rotation and high coercivity at room temperature. The small grain size and high squareness ratios of these films suggests that this media holds promise as a potential magneto‐optical recording media candidate for use at laser diode wavelengths. Furthermore, it was found that depending upon the value of the sputtering system background pressure prior to film deposition the compensation temperature of these films varied from below room temperature to the Curie point. The temperature dependence of magnetization of these two types of films was found to be very different and ESCA analysis was performed in order to ascertain the origin of these differences.

Sputtered garnet media for magneto-optic data storage

Abstract In situ crystallized Bi,Ga:DyIG and Ce,Ga:DyIG films were deposited on GGG substrates by rf sputtering. The films had single-crystal-like columnar structure, which resulted in low media noise. The films showed square hysteresis loops with large coercivity, the perpendicular anisotropy being ascribed to the large negative magnetostriction of DyIG. The compensation temperature of Ce,Ga:DyIG films changed drastically depending on residual oxygen in the sputtering chamber, and this unique phenomenon was attributed to the site preference of Ga. Furthermore, it was found that the main contribution of Faraday rotation in the Ce-substituted samples was due to Fe in the tetrahedral sites. Dynamic recording was performed using an Ar ion laser, with CNRs of 57 and 48 dB being obtained for Bi,Ga:DyIG and Ce,Ga:DyIG, respectively. The problem of write noise still remains for Ce,Ga:DyIG.

Potential of Bi-substituted sputtered garnet media for high density recording using shorter wavelength lasers

Dynamic recording using Bi-substituted sputtered garnet media on a GGG (Gd/sub 3/Ga/sub 5/O/sub 12/) substrate was performed using several wavelengths of an Ar ion laser. Good writing sensitivity and high carrier-to-noise ratios (CNRs) were obtained in the blue region of the spectrum, indicating that a SHG (second harmonic generation) laser could be adopted as the light source for ultrahigh-density recording. A 785-nm laser-diode tracking servo was reliably performed for pregrooved substrates, producing a CNR of 48 dB. Overwrite experiments using magnetic field modulation were also performed. A CNR of 48 dB was obtained and complete erasure was confirmed. >

Very High-Density Recording on Exchange-Coupled Trilayer Magnetically Induced Super Resolution Media without Special Initializing Magnet.

We obtained a high carrier-to-noise ratio (CNR) of 47 dB and a low jitter of 8% for 0.33 µm mark length ( 0.66 µm mark pitch) on exchange-coupled trilayer magnetically induced super resolution (MSR) media. This media is composed of a readout layer, an intermediate layer, and a writing layer. The magneto-optical signal is read through an aperture at the focal point between a front mask and a rear mask. The double mask can be formed by applying a readout magnetic field smaller than 300 Oe.

Improvement of Recording Density of Amorphous TbFeCo Magnetic Recording Layer on FeC Soft Magnetic Back Layer

The effect of the surface morphology of an intermediate layer on recording density was studied for perpendicular magnetic recording media composed of an FeC soft magnetic back layer, a nonmagnetic intermediate layer and an amorphous TbFeCo perpendicular magnetic recording layer. We confirmed that the formation of a very thin intermediate layer with small grains enabled high-density recording on the TbFeCo recording layer. The intermediate layer was 3 nm thick and the grain diameter was about 13 nm. The intermediate layer had a multilayered structure, which consisted of a dielectric layer, two metallic layers, and a C layer. The D50 of the media with the multilayered intermediate layer was 100 kilo flux changes per inch (kFCI) higher than that of the media with a single-layer intermediate layer. In addition, the roll-off characteristic was considerably improved by differentiating the row read head response; D50 was 395 kFCI.