Akikazu Odawara

Micro-imaging system using scanning DC-SQUID microscope

A micro-imaging system in a low temperature environment has been developed for the study of superconducting films and magnetic films and for the inspection of superconducting integrated circuits. The system consists of a micro DC-SQUID, a cryostat, a precise scanning stage, and a computer. Two different types of micro DC-SQUIDs were designed. One was a magnetometer (Bz) with a one-turn pick-up coil with a diameter of 10 /spl mu/m, and the other was a gradiometer (dBz/dx) with a planar first-order derivative pick-up coil. Each micro DC-SQUID was integrated on a 3 mm /spl times/3 mm Si chip using thin Nb film fabrication technology. Preliminary experiments were made using the system and several magnetic images were obtained. We present observations of a thin superconducting Nb film pattern by applying the Meissner screening and the magnetic domains of a thin garnet ((YBi)/sub 3/(FeAl)/sub 5/O/sub 12/) film.

Observation of Superconducting Device Using Magnetic Imaging System with a Micro-DC Superconducting Quantum Interference Device Magnetometer

A magnetic imaging system with a micro-DC superconducting quantum interference device magnetometer has been developed to evaluate superconducting films and to inspect a superconducting device with a high spatial resolution. A superconducting device, i.e., a DC-SQUID made of a superconducting Nb thin film, was observed using the system. The shape of the device was identified by applying the Meissner effect. The trapped fluxes in the device were clearly visible. In addition, the input current and the shielding current flowing on the device were detected by feeding a current to the device. It was confirmed experimentally that the magnetic imaging system was effective in studying superconducting devices.

Nondestructive detection of dislocations in steel using a SQUID gradiometer

We have developed a magnetic flux imaging system. The magnetic flux image (MFI) shows the distribution of magnetic flux density over a plate like specimen. The change of the pattern in the MFI by tensile deformation has been investigated in a carbon steel without applying magnetic field using the system. A clear striped pattern appeared after deformation by fine plastic strain between 0.2 and 1.0% before strain-hardening. Luders bands, which are groups of slip bands, appeared on the plate surface and were observed at the corresponding locations of the stripes in the MFIs. This shows that the fine deformation is nondestructively detected using the MFI.

Development of Integrated Direct Current Superconducting Quantum Interference Device Gradiometer for Nondestructive Evaluation

A Direct Current Superconducting Quantum Interference Device (DC-SQUID) gradiometer for nondestructive evaluation was developed. A pick-up coil and a DC-SQUID were integrated on a 3×3 mm2 Si chip. In order to raise the spatial resolution of the gradiometer, a coplanar concentric second-order derivative coil was adopted as a pick-up coil. The pick-up coil consisted of a one-turn outer coil and a four-turn inner coil connected in series and counterclockwise to each other. The diameters of the outer coil and the inner coil were 2 mm and 1 mm, respectively. The size of the pick-up coil was determined by calculating the spatial resolution in relation to the bottom thickness of the cryostat. The high spatial resolution of the DC-SQUID gradiometer was experimentally confirmed.

Development of scanning SQUID microscope for studying superconducting films and devices

A micro-imaging system using a low-temperature superconductor (LTS) scanning SQUID microscope (SSM) has been developed for the study of superconducting films and devices. The system consists of a micro DC-SQUID, a cryostat, a precise scanning stage, and a control unit. The spatial resolution was improved by reducing the size of the pick-up coil of the micro DC-SQUID. A cryostat without liquid nitrogen makes the operation of the system easy and speedy. Preliminary experiments were made using the system and several magnetic images were obtained. We present observations of trapped fluxes in two types of superconducting thin film: a single-crystal-like Nb film grown epitaxially and a polycrystalline Nb film. We also present observations of a SQUID array, which consists of 50 DC-SQUIDs connected in series.

Scanning DC-SQUID system with high spatial resolution for NDE

We have developed a scanning DC-SQUID system with a high spatial resolution for NDE. The SQUID system is composed of a concentric multiloop DC-SQUID gradiometer using Nb/AlOx/Nb Josephson junction, a non-magnetic scanning stage, a cryostat and a computer for gathering data and controlling the system. We have developed the concentric multi-loop gradiometer (d/sup 2/Bz/dr/sup 2/, r/sup 2/=x/sup 2/+y/sup 2/) to have a higher spatial resolution and reduce background noise. The gradiometer was integrated on a single 3/spl times/3 mm/sup 2/ Si chip. To reduce the standoff distance between the specimen and the pick-up coil, a cryostat with a thin bottom space was developed. A scanning stage was made of nonmagnetic materials and was driven by air pressure using a rod-less cylinder to avoid making magnetic noise.

NONDESTRUCTIVE MAGNETIC DETECTION OF PLASTICIZED AREA USING SUPERCONDUCTING QUANTUM INTERFERENCE DEVICE

The relationship between the magnetic and structural changes induced by the application of a tensile load to a carbon steel specimen was investigated by correlating the change in the spatial second-order gradient component of the magnetic flux density d2Bz/dr2 with the appearance of the Luders bands on the specimen surface. The magnetic change was measured using a nondestructive evaluation (NDE) system with a concentric second-order superconducting quantum interference device (SQUID) gradiometer of low temperature superconductor. The area where peaks and folds appeared in the 3-dimensional map of d2Bz/dr2 corresponded to the area where Luders bands appeared on the specimen surface. The dimensions of the area where peaks and folds appeared had a strong correlation with the residual strain of the specimen. It was concluded that the magnetic NDE using SQUID has the ability to detect the area plasticized by fine deformation in ferromagnetic materials.

Development of mice biomagnetic measurement system using dc-SQUID magnetometer

A biomagnetic measurement system on mice using a dc superconducting quantum interference device magnetometer has been developed. We are going to use it for comparative magnetoencephalogram (MEG) and magnetocardiogram (MCG) studies of transgenic mice to clarify the functional influence of gene modification in a living body. The system has a low-T/sub c/ direct coupled dc SQUID. A focuser type pickup coil whose outside/inside diameter is 1 mm/100 /spl mu/m was adopted for detailed functional mapping of the brain or the heart of mice. The minimum lift-off distance was reduced to 700 /spl mu/m to achieve the sufficient spatial resolution and magnetic field sensitivity. The field sensitivity of this system is 1.3 pT/Hz/sup 1/2/ in the white-noise region (10 Hz-10 kHz). The MCG of a wild mouse at 9-months of age was measured and compared with the data of 3-month old mice. The spatial and time changes of MCG signals were similar in both ages of mice, while the amplitude of MCG signals of 9-month mice was less than 40% of that of 3-month mice. This decrease in MCG amplitude may be due to aging. The result suggested the capability of our system in evaluating the mice MCG signals.

Flux trapping in a superconducting thin film as revealed by simulations and scanning superconducting quantum interference device microscope observations

Penetration of magnetic fluxes into a superconducting thin film that contained holes was studied for fast and slow cooling rates (0.18 and 0.009 K/s, respectively) under an ambient magnetic field of about 3 μT. By using a scanning superconducting quantum interference device microscope, we observed trapped magnetic fluxes both inside and outside of holes prepared in a superconducting film. The trapped fluxes outside the holes appeared in a regular arrangement when the superconducting film was cooled at the slow rate. Observed arrangements of the trapped fluxes were compared with a simulation that was based on a theoretical model that considered the surface barrier and the interaction among fluxes. The simulation explained well the observed arrangements of magnetic fluxes.

Development of an energy dispersive spectrometer for a transmission electron microscope utilizing a TES microcalorimeter array

A high‐energy‐resolution energy dispersive spectrometer (EDS) utilizing a TES (transition edge sensor) microcalorimeter array is developed for a transmission electron microscope (TEM). The goals of the development are (1) an energy range of 0.3–10 keV, (2) an energy resolution of FWHM <10 eV, (3) a maximum counting rate of 3 kcps, and (4) a cryogen‐free cooling system. We adopted a dilution refrigerator (DR) pre‐cooled by a Gifford‐McMahon (GM) refrigerator to cool the TES microcalorimeter to ∼100 mK. In order to avoid micro phonics of GM fridge to propagate to the TEM, pressurized He gas is circulated between the DR and the GM to reject heat from the DR. The GM is mechanically well isolated from the TEM. In oder to obtain 3 kcps counting rate, we utilize a ten pixel TES array and read out the signals in parallel wtih ten analog signal channels from cryogenic to room temperature electronics. One of the pixels can be always irradiated by a radio isotope for energy calibration. As the first step, we have at...