Seiya Ogawa

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.

Longitudinal recording on a rigid disk using Ba-ferrite powder

We propose a new concept for a Ba-ferrite coated disk, in which the Ba-ferrite particles are longitudinally oriented and to which acicular Co-γFe 2 O 3 powder is also added, to achieve high density recording and low noise characteristics for rigid disk applications. By using a ring head, longitudinal orientation was found to be effective in obtaining an increased output and a symmetrical waveform. Acicular powder is added to reduce pinholes upon longitudinal orientation. Although the critical bit density, D 50 , decreases slightly, practical recording density, judged from the degree of peak shift, is larger for longitudinal orientation than for perpendicular orientation. The new disk has great promise for applications of more than 40 kFRPI, without any change to conventional disk drives and ring-shaped heads.

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.

A new protective film for magneto‐optical TbFeCo media

A new protective film, Tb‐SiO2, was developed to increase the lifetime of TbFeCo magneto‐optical disks. Since Tb, a rare‐earth metal, is quite easily oxidizable, an efficient protective layer is essential to assure the stability of magneto‐optical disks. Given that vacuum‐deposited SiO2 film acts as an oxidation‐inducing layer, unlike SiO2 on a thermally oxidized silicon wafer, we invented a new protective film, Tb‐SiO2, in which Tb is intended to trap active oxygen in the vacuum‐deposited SiO2. The results were excellent and we estimate the lifetime of TbFeCo disks with Tb‐SiO2 as an underlayer and overlayer, at more than 20 years at 40 °C, 90% relative humidity. The properties and protection effects of Tb‐SiO2 are described.

Dielectric Constant and Conductivity Measurement of Powder Samples by the Cavity Perturbation Method

The dielectric constant and conductivity of powder samples can be measured by a cavity perturbation method. A small amount of powder is placed at the point of maximum electric field intensity in a resonant cavity of TE103 mode at about 8935 MHz. Experimental data are analyzed by a theory which gives the complex conductivity of powders. The theory is based on the assumption that the quasistatic approximation is valid and there exists an equivalent powder depolarization factor. Coprecipitated Mn-Zn ferrite powder, 160 A in diameter, is used as a powder sample. The observed dielectric constant and conductivity are in the range 8~16 and 0.01~0.02 \mho/cm, respectively. A Grounded n-Ge sample is also measured. The theory and measuring technique developed herein are general and may be applicable to measurement of various powder samples.

Ion Diffusion and Disaccommodation in Ferrites

In various mixed ferrites, the relationship between disaccommodations (DA) and cation vacancy densities was investigated, the latter being estimated from the diffusion coefficients of isotopes. The relaxation time of DA–I observed at approximately 250°C was in agreement with an extrapolated value from the cation diffusion at elevated temperatures, and the two activation energies were nearly equal, 1.5 eV. The magnitude of DA–III observed at room temperature did not necessarily correspond to the vacancy density, i.e., an iron-poor Mn–Zn ferrite containing an appreciable amount of the vacancies did not exhibit DA–III.

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. >