Koji Takei

Compositional inhomogeneities in Co-Cr-Ta/Cr films for longitudinal magnetic recording

The change in compositional distribution of sputtered Co-12at.%Cr-2at.%Ta films deposited on Cr underlayers was investigated as a function of substrate temperature, using spin-echo /sup 59/Co nuclear magnetic resonance. It was observed that a highly Co-rich component with an estimated Co content of about 97 at.% precipitates as the substrate temperature increases from 25 to 250 degrees C. The compositional microstructure observation suggests compositional separation within grains having Cr-enriched grain boundaries. The precipitation is in accordance with an increase in coercivity and a decrease in coercive squareness and media noise. The compositional inhomogeneities observed in Co-Cr-based alloy films for either longitudinal or perpendicular recording media are thought to be effective in promoting the change from a continuous structure to a particulate structure suitable for high-density magnetic recording. >

NMR Study of Compositional Inhomogeneities in Sputtered Co–Cr Films

The development of compositional inhomogeneities in sputtered Co-22 at.% Cr films is studied as a function of substrate temperature using the spin-echo 59Co-NMR technique. It is observed that a highly Co-rich component grows as the substrate temperature increases from 30°C to 400°C. The Cr content of the Co-rich part in the film deposited at 400°C is estimated to be as low as 5-7 at.%. The results are consistent with the appearance of a clearly striped compositional microstructure which suggests compositional separation.

Ar ion bombardment effects of NbN/Pb Josephson junctions with plasma oxidized barriers

Details of the relation between Ar ion bombardment effects on NbN surface and I‐V characteristics of NbN/Pb junctions with plasma oxidized barriers have been studied. NbN base electrodes, which behave as a strong coupling superconductor (Tc>15 K, 2Δ/kTc=4.2–4.3), were prepared onto substrates at 300 °C by reactive dc magnetron sputtering. In the NbN/Pb junction fabrication, cathode self‐bias voltage VCSB during Ar sputter cleaning strongly influences the subgap leakage current value, which increases with VCSB . The VCSB also influences normalized normal tunnel resistance ARNN. It is found, in reflection electron diffraction and x‐ray photoelectron spectroscopy studies, that the Ar ion bombardment at VCSB> 200 V on a NbN surface produces an amorphous structure in the surface layer. It is also found that the thickness of amorphous layer increases with Ar ion energy. Based on the experimental results, low subgap leakage current junctions, where Vm exceeds 100 mV at 4.2 K, were fabricated under low energy pla...

Application of Co‐based amorphous ribbon to a noise filter and a shielded cable

This paper describes the application of a Co‐based amorphous alloy ribbon to a common‐mode noise filter in the 1 MHz–1 GHz frequency range. Its saturation magnetization, and relative permeability were 6000 G and 1450, respectively. The 10‐μm‐thick amorphous ribbon exhibits high loss characteristics. The resistance of a 2‐mm‐diam roll of amorphous ribbon (12.5 mm wide and 30 cm long) increases from 3 Ω at 1 MHz to 50 Ω at 350 MHz. It is confirmed that a filter 1/17th the volume of a conventional (ferrite core) filter exhibits the same noise attenuation. Moreover, the noise emission from a twisted pair cable is greatly reduced without any increase in the cable volume by employing the amorphous ribbon as a shielding material.

Micromagnetic domain observations of Co-Cr films by neutron scattering

Magnetic microstructures revealed by neutron small angle scattering are correlated with the compositional separation (CS) in sputtered Co-22at%Cr thin films for high density magnetic recording. The CS is confirmed to be strongest for deposition temperatures from 150/spl deg/C to 400/spl deg/C, using nuclear magnetic resonance. The finest magnetic particles, less than 10 nm in diameter, are detected in a film deposited at 200/spl deg/C, in good agreement with the smallest features observed in the etched microstructure by transmission electron microscopy. This indicates that CS produces magnetic grains much smaller than the crystal grains which can, therefore, be applied to synthesize Co-Cr thin film media for ultra high density magnetic recording. >

Investigations of compositional separation in Co-Cr-Ta/Cr thin-film recording media

Abstract The compositional distribution in Co-Cr-Ta thin films is investigated as a function of Cr underlayer texture and substrate temperature ( T s ) using spin-echo nuclear magnetic resonance and preferential chemical etching. Both the extent and pattern of compositional separation (CS) show no variation with Cr underlayer texture. Elevated T s is observed to promote both CS and grain boundary Cr enrichment. The magnetic anisotropy fields of Co-enriched components in the films are observed to increase with T s , explaining the T s dependence of H c . Ta addition is observed to control the distribution of Co and Cr, leading to a unique compositional microstructure which may enhance the magnetic and recording properties.

The dependence of compositional separation on film thickness for Co-Cr and Co-Cr-Ta magnetic recording media

The dependence of the compositional distribution on film thickness was investigated for Co/sub 78/Cr/sub 22/ and Co/sub 86/Cr/sub 12/Ta/sub 2/ films. Spin-echo NMR and preferential chemical etching revealed that the extent of compositional separation was greatest in thinner films and decreased during film growth for both Co-Cr and Co-Cr-Ta. Thinner films exhibited enhanced Ms and a higher density of smaller Co-enriched regions. Ta addition was observed to change the compositional distribution, promoting a finer pattern of Co-enriched regions. >

Detection of Compositional Separation in Co-Ru Alloy Magnetic Films

We examined the possible occurrence of compositional separation (CS) in hcp Co-Ru alloy magnetic films. Using spin echo 59Co NMR and transmisson electron microscopy, we found that CS, which produces a Co-rich component (≈10 at.%Ru), occurs in sputtered Co-≈33 at.%Ru films when substrate temperature is elevated to 400°C. The results suggest that the origin of CS lies in magnetically induced phase separation during film growth.

Compositional microstructures in Co-Cr films for magnetic recording

Abstract Using spin echo 59 Co nuclear magnetic resonance and preferential chemical etching, we have examined compositional inhomogeneities in Co-Cr based alloy thin film media for magnetic recording. We found that drastic compositional separation (CS), which produces highly Co-enriched regions within grains, occurs in both perpendicular and longitudinal recording media. CS is expected to promote the change from a continuous to a particulate-type magnetic microstructure suitable for high density magnetic recording.

Fabrication of High Quality NbN/Pb Josephson Tunnel Junctions with Plasma Oxidized Barriers

Extremely low subgap leakage current NbN/Pb tunnel junctions, in which Vms exceed 100 mV, were fabricated using a low energy rf plasma oxidation technique. This was done under conditions where cathode self bias voltages VCSB were below 200 V. XPS study shows that NbN surface oxide layer thickness increases from 1.5 nm to 4.5 nm after Ar ions bombarded the NbN surface at VCSBs above 200V. Ar ion bombardment at VCSBs above 200V produces a damaged layer on the NbN surface. Artifical initial oxide layer formation subsequent to NbN film preparation is effective in reducing subgap current under lower VCSB conditions for cleaning and oxidation.