Hisanao Tsuge

Reactive ion beam etching of Y‐Ba‐Cu‐O superconductors

It has been found that a reactive ion beam etching (RIBE) using Cl2 gas is useful for microfabrication of Y‐Ba‐Cu‐O superconductors. The etching yield enhancement of Y‐Ba‐Cu‐O is observed for Cl2 RIBE. The etching yield of Y‐Ba‐Cu‐O at 400 V accelerating voltage, 2×10−3 Torr Cl2 pressure for Cl2 RIBE is 2, which is twice that for Ar ion beam etching. Y‐Ba‐Cu‐O submicron patterns have been fabricated by focused ion beam lithography and Cl2 RIBE. Moreover, a Y‐Ba‐Cu‐O superconducting line with a 4‐μm linewidth has been fabricated by annealing an as‐sputtered Y‐Ba‐Cu‐O line pattern.

MAGNETIC TUNNEL JUNCTIONS WITH IN SITU NATURALLY-OXIDIZED TUNNEL BARRIER

Al/Fe/Al2O3/CoFe/Al junctions with dimensions of 2×2–40×40 μm2 were prepared on a 2 in. Si wafer using conventional photolithography and ion-beam etching. The junction trilayers were deposited sequentially without breaking vacuum and a tunnel barrier was in situ naturally oxidized. The resultant junction resistance scaled linearly with the junction area over all dimensions used. Normalized resistance of less than 1.5×10−5 Ω cm2 was obtained in maintaining a magnetoresistance (MR) ratio of about 5%. The resistance values are much smaller than ever reported and close to those required for an MR head device. The MR ratio exhibited no significant change up to at least 1×103 A/cm2 with increasing junction current density.

Low-resistance tunnel magnetoresistive head

A tunnel magnetoresistive (TMR) head with a low resistance of about 30 /spl Omega/ and effective track width of 1.4 /spl mu/m was fabricated using an in situ natural oxidation (ISNO) technique. Its read-output was almost the same as that expected from test elements at the wafer level. We found no large difference in noise voltages between TMR head and GMR head when their resistance was about 30 /spl Omega/. A very low-resistivity TMR element with a resistance-area product of 14 /spl Omega//spl middot//spl mu/m/sup 2/ and a fairly high /spl Delta/R/R of 14% was also developed using ISNO. A signal-to-noise ratio consideration suggests that such low resistance is a key to TMR heads for high recording densities.

A 4-kbit Josephson nondestructive read-out RAM operated at 580 psec and 6.7 mW

A fully decoded 4-kb Josephson nondestructive readout high-speed RAM with vortex transitional memory cells was designed and operated successfully. The 4-kb Josephson RAM is composed of 64-b*64-b cells, polarity-convertible drivers, address decoders using resistor coupled Josephson logic (RCJL) gates, and a resistively loaded sense circuit. The memory cells use vortex transitions in their superconducting loops for writing and reading data. The cells are activated by two control signals without timing control, while all peripheral circuits are activated by an AC power supply. This memory configuration eliminates the timing sequence needed for memory operations, resulting in a decrease in the memory operation time for an actual memory chip. The 4-kb Josephson high-speed RAM was fabricated using niobium planarization technique with a 1.5- mu m design rule. The RAM circuit size is 4.8*4.8 mm/sup 2/ and the memory cell is 55*55 mu m/sup 2/. More than 25000 Nb-AlO/sub x/-Nb Josephson junctions with approximately 1200 A/cm/sup 2/ critical current density are contained in the RAM chip. An access time of 580 ps and a power consumption of 6.7 mW are obtained for the nondestructive memory operation.

Interface structures and magnetoresistance in magnetic tunnel junctions

The interface structures of magnetic tunnel junctions were studied using x-ray photoelectron spectroscopy (XPS). The structures were correlated with magnetoresistance (MR) characteristics. For MR measurements, Fe(50 nm)/AlOx/CoFe(30 nm) junctions with an in situ naturally oxidized Al tunnel barrier were fabricated. The thickness of the Al layer, an important parameter in MR characteristics, was varied from 0 to 5 nm. MR curves showed that the largest MR ratio occurred when the Al layers were 2–3 nm in thickness. XPS analysis showed that an Al layer greater than 1 nm thick covers the entire surface of the Fe underlayer. However, if the Al layer is more than 1 nm thick, the unoxidized Al remaining after the oxidation process increases as the thickness is increased. For Al layers that are greater than 3 nm thick, the MR ratio is strongly affected by unoxidized Al, probably due to the decrease in spin polarization at the surface of an Fe/Al electrode. On the other hand, the hysteresis loops indicate that the ...

Resonant tunneling transport in YBaCuO/PrBaCuO/YBaCuO edge-type Josephson junctions

YBaCuO/PrBaCuO/YBaCuO edge-type Josephson junctions were fabricated and their transport properties were studied. Josephson effect in these structure was confirmed by the observation of microwave induced Shapiro steps and periodic magnetic field dependence of the critical current. Normal electron transport in the junctions was well explained by Glazman and Matveevs theory for hopping transport via localized states. In addition, the supercurrent transport in PrBaCuO is possibly supported by the resonant tunneling via the localized states. We estimated the density of the localized states to be 10/sup 16/ cm/sup -3/.<<ETX>>

Characteristics of DC SQUIDs Patterned on Highly Granular Y–Ba–Cu–O Thin Films

Measurements have been made of the properties of thin granular film DC SQUIDs fabricated from the high Tc superconductor Y–Ba–Cu–O, paying special attention to peculiar effects arising from the unique granularity of the film. The output voltage modulation of the SQUIDs was found to be very much lower than expected from the theory, and we present here several possible explanations for it, based on such factors as the asymmetry of the junctions and high kinetic inductance of the film. Multiple periodicities and phase jumps on VΦ-characteristics were observed. Normal operations of the SQUIDs were observed till ~65 K. Estimating from the best data, the equivalent flux noise was below 3×10-4 Φ0/Hz1/2 in the frequency range above 600 Hz at around 50 K.

Exchange-biased magnetic tunnel junctions fabricated with in situ natural oxidation

Exchange-biased magnetic tunnel junctions with a Ta/NiFe/FeMn/NiFe/Al–oxide/NiFe/Ta structure have been fabricated. The tunnel barrier was formed by the in situ natural oxidation of an Al metal layer under controlled oxygen pressure. Photolithography and ion milling were used to pattern the multilayer into junction structures of 2×2 μm2–20×20 μm2 dimensions. Magnetoresistance (MR) curves show spin-valve-like characteristics, in which an antiparallel configuration of magnetizations in both ferromagnetic layers is observed between 50 and 240 Oe, and the hysteresis loops for both the free and pinned layers exhibit sufficient separation. An evaluation of the MR curves shows the exchange-bias field to be 340 Oe and coercivity levels in the free layer to become as low as 13 Oe. At room temperature normalized junction resistance is 2×10−5 Ω cm2, with MR ratios still being maintained at 13%. This resistance value is much lower than previously reported values for junctions produced either with plasma oxidation or ...

Focused ion beam processes for high‐Tc superconductors

Focused ion beam (FIB) processes have been developed for Y–Ba–Cu–O superconductor films. A Y–Cu liquid metal ion source has been fabricated, using a Y67 –Cu33 eutectic alloy as the ion source. As‐sputtered Y–Ba–Cu–O film etch rate ratios to GaAs(100) and Si(100) substrates are 0.28 and 1.4 for 130‐keV Au+ FIB ion etching, respectively. Y–Ba–Cu–O submicron patterns have been demonstrated by using FIB lithography and Cl2 reactive ion beam etching. Moreover, a Y–Ba–Cu–O superconducting line with 4‐μm linewidth has been fabricated by annealing an as‐sputtered Y–Ba–Cu–O line pattern. Tc control of Y–Ba–Cu–O film has been achieved by 200‐keV Ne+, using conventional ion implantation and 300 keV Si++ FIB ion implantation.

In‐plane anisotropic transport properties observed in epitaxial Bi2(Sr,Ca)3Cu2Ox films grown on tilted (001)SrTiO3 substrate

We report on large in‐plane anisotropic transport properties observed in epitaxial Bi2(Sr,Ca)3Cu2Ox films grown on tilted (001)SrTiO3 substrate. By tilting the surface normal axis about 4° toward [111]SrTiO3, regular steps and terraces were formed on the substrate surface and perfect alignment of film b axis with incommensurate modulation along [110]SrTiO3 was realized. The film c axis was perpendicular to the (001)SrTiO3 terrace, thus the film c axis grew tilted 4° toward [111] from surface normal due to the surface inclination. In those epitaxial configurations, the in‐plane resistivity along the step direction(ρ[110]) involved the contribution from the c‐axis(ρc) component, and we observed the large resistivity anisotropy between a‐ and b‐ direction of the film. The transport along a axis(ρa) showed a low resistivity with metallic temperature dependence while the resistivity along the step direction(ρ[110]) was higher and semiconductive. The ratio of ρc/ρa ≊ 104 estimated in this experiment agrees well...