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We show how to design the position of a Josephson junction (JJ) and holes to prevent trapping of magnetic fluxes on the JJ in the superconducting device when the strength of magnetic field is determined in cooling from normal to superconducting state. The number of magnetic fluxes ( N MF ) is determined as N = α ( B − B 0 ), where B is the external field in cooling and B 0 is the threshold of external field that one magnetic flux is trapped, α is the coefficients that might depend on the device size and structure. If the number of holes ( N H ) is larger than N MF , the magnetic fluxes are trapped into the holes. The holes should be arranged regularly so that all the magnetic fluxes are trapped into the holes. If N MF is larger than N H , the magnetic fluxes occupy the holes initially, and then, the excess magnetic fluxes are trapped around the position of the potential wells arising from the interaction among the previously trapped fluxes. The position of JJ must be designed to avoid the position of a potential well. The cooling speed from normal to superconducting state should be as slow as possible to prevent non-uniformity of temperature distribution that would cause formation of unexpected potential wells outside the designed positions.

Recently, vortices confined into micro-scale superconductors with shapes like a disk, triangle, square, etc., have attracted much attention because of the quantum phase transition of the self-organized vortex arrangement occurring within such geometrical constraints. Such a transition can be observed using a scanning SQUID microscope with high spatial resolution. We have successfully improved spatial resolution by incorporating a microfabrication technique that reduces both the size of the pick-up coil of the micro DC-SQUID and the standoff distance between the pick-up coil and the sample surface. Using this microscope, we have studied vortex arrangements in micro-scale superconductors made of Nb and YBa2Cu3O7−δ films with various sizes and geometrical shapes. A peculiar oscillating behavior of diamagnetic magnetization corresponding to the particular vortex state was observed.

We have investigated the magnetic properties of a triangular micro-hole lattice on Pb film (TriMHoLP) by means of a SQUID magnetometer and a SQUID microscope. A matching field H φ of TriMHoLP is 0.425 G. The zero-field-cooled magnetization curves are in accordance with those expected for typical superconductors. However, the curves taken in a field-cooling mode upon cooling are always negative irrelevant to the field polarity. Even in the zero field, we have observed an appreciable amount of vortices and anti-vortices by the SQUID microscope. These strongly suggest that there must be a spontaneous magnetization at temperatures near T c . A clear vortex density wave has been found for a half-filling state of TriMHoLP.

A cryogen-free energy-dispersive spectrometer (EDS) using a transition edge sensor (TES) was developed for material analysis. This system can maintain a temperature at 130 mK within 30 μK, and has good energy resolution (19 eV for Mn-Kα) for long-time measurement with a drift in the DC level of less than 0.02 eV/min. This system utilizes a dilution refrigerator (φ 272 mm×height 572 mm) and has a snout (370 mm long and φ25 mm) similar to that in a conventional EDS system. The dilution refrigerator is pre-cooled by a GM refrigerator. A flexible tube between the dilution refrigerator and GM refrigerator damps the mechanical vibration of the GM refrigerator. Two shields (4 and 80 K) thermally protect the Cu rod (φ8 mm) cooled to be 100 mK. Windows composed of polyimide+Al film allow X-ray detection above the C-Kα line. A TES (6 mm×6 mm) and array SQUID amplifier (1.5 mm×3 mm) are mounted on top of the Cu rod. For Mn-Kα, the pulse height is 5.5 μA and decay time (τeff) is 90 μs. The maximum count rate (1/20 τeff) is estimated at about 500 cps.

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.

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

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.