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We applied a spatial scanning measurement technique to neutron diffraction on a powder diffractometer, and performed scanning wide-angle neutron diffraction (SWND) experiments to investigate local structures of materials. The SWND technique has an advantage that one can observe a diffraction pattern over a wide scattering angle range, so that the positional dependence of local crystal structures can be measured for practical durations. Moreover, because neutrons have a larger penetration depth, on the order of a few cm, than X-rays, the SWND technique provides structural information even in internal regions of materials. We succeeded in observing positional dependence of diffraction patterns from small areas (approximately 2 ×5 mm2) of a type-304 stainless steel plate with a dimension of 19.5 ×50 ×4.7 mm3 subjected to a mechanical tensile strain of 90% of the 0.2% offset yield strength. In the SWND experiments, we confirmed that the crystallographic circumstance in the strain-loaded type-304 stainless steel is not homogeneous. For instance, we observed positional deviation in the grain direction of the master-alloy austenite phase, and local distribution of the stress induced martensite phase. The SWND technique is a particularly important probe for estimating the stress-induced phase, because it non-destructively observes even the inside of bulk materials.

The flux noise in a long superconducting quantum interference device (SQUID) array consisting of at least 100 dc-SQUID elements connected in series was characterized analytically, and then compared with measurements. In the analysis, noise sources in the array were shunt resistors for Josephson junctions, a damping resistor for a SQUID loop, the dynamic resistance of the SQUID array, and the noise of electronics at room temperature. The flux noise of the array was calculated analytically as a function of the number of dc-SQUID elements. The results reveal that the flux noise is significantly influenced by the dynamic resistance of the array, although the influence by the dynamic resistance for a single SQUID is negligibly small. The dynamic resistance becomes dominant for the flux noise when the number of SQUID elements in the array exceeds 100. These results agree well with the measured flux noise for a long SQUID array constructed using a Nb/AlOx/Nb fabrication process.

Josephson vortices trapped in cross-sectional edge surfaces of Pb0.6Bi1.4Sr2CaCu2Oy has been directly observed by using a scanning superconducting quantum interference device (SQUID) microscope. The magnetic field distribution Bz around each vortex is substantially anisotropic, compared with the usual vortex in the ab-plane, and is extended over 100 µm toward the in-plane direction. By fitting a theoretical Bz function to experimental ones, c-axis penetration depth λc was estimated to be 11.2 ±0.7 µm, which is in good agreement with the literature value, 12.6 µm, obtained from the Josephson plasma edge frequency.

In order to visualize spatial distributions of vortices strongly pinned in La1.8Sr0.2CuO4, we have performed scanning superconducting quantum interference device (SQUID) microscopic (SSM) measurements near Tc. The obtained SSM images have revealed that stripe-shaped regions with higher Tc run in an arbitrary direction with respect to crystallographic axes. Vortices tend to align in a one-dimensional fashion, negotiating around the higher Tc regions. High-resolution electron probe micro-analysis confirmed that the spatial variation in Tc corresponds well to the modulation of Sr/La composition. We also found that small-angle grain boundaries with tilt angles of 1–3 degrees are less effective in trapping vortices, indicating that superconductivity is not degraded very much.

A high Tc superconducting quantum interference device (SQUID) magnetometer utilizing a copper pickup coil cooled to T=77 K and a resonant coupling circuit was developed. Using the resonant coupling circuit, we can greatly increase the signal flux transferred to the SQUID. Cooling of the pickup coil is useful for increasing the signal flux transferred to the SQUID and for decreasing the noise of the pickup coil. As a result, we obtained a magnetic field noise of 34.8 fT/Hz1/2 at a resonant frequency of 6.37 kHz when the pickup coil was made of 200 µm- Cu wire with the number of turns n of 150 and radius of the coil r of 15 mm.

The detection of the fractional vortices at asymmetric bicrystal boundaries in Bi2Sr2CaCu2O8+y (BSCCO) and YBa2Cu3O7-y (YBCO) thin films using a scanning superconducting quantum interference device (SQUID) microscope (SSM) is reported. The measurements are performed from 3 K to 90 K and the unconventional fractional vortices are found to exist up to the local critical temperature TJc at the Josephson boundary. The results provide the evidence of dx2-y2 symmetry of superconducting order parameter up to TJc , and the observed fractional vortices under different bicrystal geometries are explained by the Sigrist-Rice Josephson current formula based on the dx2-y2 -wave pairing symmetry.

Magnetic images of high-Tc La1.85Sr0.15CuO4 (LSCO) and YBa2Cu3O7-y (YBCO) thin films near optimal doping observed by scanning superconducting quantum interference device (SQUID) microscopy are presented in the temperature range both below and above the transition temperature Tc. While the quantized vortices were observable below Tc, local diamagnetic domains that led to the Meissner state were found in the narrow limited temperature range above Tc. The result provides evidence that local diamagnetic domains are closely related to the pseudogap state and the d-wave superconductivity.

An ultrahigh current resolution of 1 pA/√Hz was achieved by a superconducting quantum interference device (SQUID) amplifier, which consisted of 128 repeated dc-SQUID elements. Each element had a 7-turn input coil. The SQUID amplifier was fabricated using a Nb/AlOx/Nb fabrication process. To evaluate current resolution, flux noise and flux-voltage (Φ-V) characteristics were measured at 4.2 K. The Φ-V characteristics showed a large voltage swing of more than 4 mV. The SQUID amplifier was operated at a smooth slope in the Φ-V characteristics, while asymmetric behavior appeared in the same Φ-V characteristics.

We have observed local shielding current flows in oxide superconducting thin films under low magnetic field by scanning probe microscopy using a superconducting quantum interference device (SQUID). The strength and direction of current flow were characterized simultaneously and directly by observing magnetic signals around artificial holes. This method allows us to map the current flow without reverse Fourier transform. In this paper, experimental data and analyses of the current flows in oxide superconducting thin films up to 50 µT are shown. Furthermore, results of calculation based on a very simple model are also presented. The possibility of using scanning SQUID microscopy to characterize current flow is presented.

We developed a compact moving-sample magnetometer that uses a high-temperature superconductor–superconducting quantum interference device (high-T c SQUID) to directly measure the flux coupled to a normal detection coil from a sample's magnetic moment in the presence of an external DC magnetic field. The moving-sample method is employed by inserting the sample between the poles of a DC electromagnet and vibrating the sample along the axis perpendicular to the external field axis using an actuator at a frequency of 2.693 Hz. First, the magnetic field of the sample is transferred by a first-order differential normal Cu coil to a SQUID for detection. Then, the SQUID output is fed to a lock-in amplifier for detection. The critical feature of the system design is the use of high-T c SQUID, which enables the realization of a compact system. The basic characteristics of the developed system are presented, and the current system exhibited a detection limit of 1×10-7 emu.

A superconducting-qantum-interference-device (SQUID) nondestructive evaluation (NDE) system using a small room-temperature probe is developed for active scanning rather than for a massive movement occurring in a traditional SQUID NDE system. The small room-temperature probe is composed of a quadruple excitation coil and a double D-shaped pickup coil. It is connected to the input coil surrounding a high-Tc rf SQUID, immersed in liquid nitrogen, and shielded by a shielding can. Beyond the NDE function, the SQUID NDE scheme has spatial recognition functions, including the detection of the orientation and depth of a narrow line crack using different parameters, and the scanning of images of large objects with arbitrary shapes. Furthermore, the spatial sensitivity, limited by the size of the probe, reaches up to only 7 µm in the aspect of crack widths and 1 mm in the aspect of spatial intervals for precision NDE on a printed circuit board.

Properties of magnetic nanoparticles are studied for application to magnetic immunoassays utilizing a superconducting quantum interference device (SQUID). In this application, a magnetic marker is made of magnetic nanoparticles, and the binding reaction between an antigen and its antibody is detected by measuring the magnetic field from the marker. Magnetization of an assembly of the particles is simulated when the effects of the thermal noise and the distribution of the particle size are taken into account. Magnetic signals from the assembly of the particles are clarified for three detection methods, i.e., susceptibility, relaxation and remanence. From the comparison with experimental results, it was shown that the demagnetization effect also significantly influences the magnetic signal from the particles. When these effects are taken into account, the simulation results agree semi-quantitatively with the experimental ones.

Accepted: 2004-05-26, 資料番号: SA1001085000

A superconducting quantum interference device (SQUID) for application to biological immunoassays has been designed. In this application, the magnetic field from a magnetic marker made of magnetic nanoparticles is detected with the SQUID. It is shown that the conventional design of the SQUID that was developed for the case of uniform magnetic field cannot be used in this application, since the spatial variation of the magnetic field from the marker is large. Therefore, a numerical simulation method is developed for obtaining a signal flux detected with the SQUID when the magnetic field changes spatially. It is shown that the signal flux strongly depends on the geometrical parameters of the measurement system, such as the size of the pickup coil, size of the sample and the distance between the sample and the SQUID. The dependence of the signal flux on these geometrical parameters is quantitatively clarified, and the optimum design of the SQUID for the application to magnetic immunoassays is discussed.

Arrays of dc-superconducting quantum interference devices (dc-SQUIDs) were investigated, in which each dc-SQUID element has a multiturn input coil. The dependence of flux noise on the number of turns of the input coil was studied for current amplifiers. SQUID arrays were fabricated by a Nb/AlO x /Nb Josephson tunnel junction fabrication process. Results show that the flux noise of the SQUID array amplifier does not depend on the turn number of the input coil. A SQUID array amplifier consisting of 128 dc-SQUID elements was fabricated, in which each element had a 7-turn input coil and a mutual inductance between the input coil and the SQUID loop of 460 pH. This amplifier showed an ultrahigh current resolution of 1 pA/Hz0.5 and a maximum input current of 11 µA for signals of 50 kHz, which is sufficient bandwidth for X-ray detection applications. These values correspond to a large dynamic range of 87 dB. The flux-voltage characteristics of this SQUID array, however, showed asymmetric behavior, which originated from electronic resonance in stray capacitance and the mutual inductance between the SQUID loop and the multiturn input coil.

In order to investigate the local magnetic properties of high-T c superconductors, we have performed scanning superconducting quantum interference device (SQUID) microscopy (SSM) on the ab-surfaces of an underdoped La1.9Sr0.1CuO4 (LSCO) single crystal (T c=26 K) at various temperatures. The obtained magnetic images below T c clearly demonstrated diamagnetic behavior together with trapped vortices in units of Φ0 (= hc/2e). Above T c up to 97 K, on the other hand, we did not observe any magnetic structures within an experimental error of ≤0.2 µT, contrary to the recent SSM results of LSCO thin films exhibiting diamagnetic regions of over-micron scales. From the present SSM observations, it can be safely said that local diamagnetism of such scales is unique to thin film specimens.

A new type of superconducting interference device (SQUID) microscope was designed and fabricated. A direct-coupled SQUID magnetometer with a high µ-metal needle was used and the substrate was machined to create a dimple for the needle at the center of the pick-up loop. One end of the needle penetrated through the superconducting pick-up loop in a vacuum; the needle was fixed in the vacuum window with the other end at room temperature in the outside atmosphere. As a demonstration, a laser printed output was scanned by the microscope. Line bars with a line width of 100 µm and a spacing between lines of 200 µm were clearly imaged.

The nondestructive detection of small cracks and deep-lying flaws in structures is important for safety inspection. Carbon-fiber-reinforced plastics (CFRP) are complex materials for use in aircrafts and structures. We investigated the possibility of detecting flaws in CFRP using a nondestructive evaluation system using high-T c superconducting quantum interference device (SQUID). A SQUID system for the nondestructive evaluation was developed using a SQUID magnetometer and a SQUID gradiometer fabricated based on a YBa2Cu3O7-δ thin film. Holes, the diameters of which were equal to or larger than 0.4 mm, were made in CFRP plates and all holes except the smallest one were detected by a current-injection method using this system. The results show that the location and the diameter of the holes can be estimated from the distribution of the magnetic flux density, B z , measured by the magnetometer or the distribution of the first-order gradient of B z , dB z /dx, measured by the gradiometer. The effect of texture of the carbon fiber in CFRP on the magnetic signal strength was discussed.

A system is developed to magnetically measure biological antigen-antibody reactions with a superconducting quantum interference device (SQUID) magnetometer. In this system, antibodies are labeled with magnetic nanoparticles of γ-Fe2O3, and the antigen-antibody reactions are measured by detecting the magnetic field from the magnetic nanoparticles. A setup of the system is described, and the sensitivity of the system is studied in terms of detectable weight of nanoparticles. Magnetic particles as small as 600 pg can be detected at present. An experiment is also conducted to measure antigen-antibody reaction with the present system. It is shown that the sensitivity of the present system is better than that of the conventional optical method. A one order of magnitude improvement of sensitivity will be realized by the sophistication of the present system.

The ac bias method is used in the readout system of dc-superconducting quantum interference devices (SQUIDs) to suppress the low-frequency noise that originates in the fluctuation of parameters of two junctions in the SQUID. From the analysis of various schemes of the ac bias method, we propose a simple form which expresses the alternation of the operation variables, i.e., the bias current, modulation flux, and bias flux. We considered the modification of the previous schemes in the context of this form and have found a possible method to directly detect the noise component due to the parameter fluctuation. In the modified ac bias scheme, the low-frequency noise of the parameter fluctuation can either be detected or suppressed by manipulating a single variable. We fabricated the electronics to verify this function, and confirmed the versatility of this method from the measurements of the high-T C SQUID.

A high-T c superconducting quantum interference device (SQUID) microscope with a sample chamber isolated by a shutter gate was designed and constructed for imaging samples at both room temperature and 77 K. According to our scheme, the separation of the sample from the SQUID can be less than several micrometers, in principle. We have successfully imaged a trapped flux in a YBa2Cu3O7-y thin-film ring at 77 K.

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.

A comprehensive quantitative comparison between the measured performance of direct-coupled high T c superconducting quantum interference device (SQUID) magnetometers with 30° bicrystal junctions and a numerical simulation is presented. It is shown that the characteristics of the SQUID magnetometer are considerably affected by resonances due to the large dielectric constant of the SrTiO3 substrate. In the realized magnetometer layout, the strip line resonance occurring in the SQUID inductance increases the voltage modulation depth and the output voltage noise of the SQUID, while the flux noise of the SQUID is nearly unchanged. It is also shown that the distortion of the voltage versus flux characteristic is caused by the LC resonance in the pickup loop of the magnetometer in combination with a capacitive feedback. Good agreement between experiment and simulation has been obtained.

We propose a new configuration for direct-coupling-type high-T c superconducting quantum interference device (SQUID) magnetometers consisting of two pickup coils per SQUID on a bicrystal substrate. The SQUID and pickup coils can be arranged symmetrically without covering the bicrystal line on the substrate. This configuration should produce a large coupling coefficient from the pickup coil to the SQUID, and thus a large effective pickup area. First, we optimized the structure and dimensions of the pickup coils using an analogous model made of normal metal. Second, we fabricated SQUIDs for this configuration, in which the slitlike holes were located parallel to the bicrystal line. We also fabricated magnetometers consisting of pickup coils and SQUIDs of this configuration from YBCO film on bicrystal 10 mm × 10 mm SrTiO3 substrates. An effective area of 0.42 mm2 has been achieved, which was measured by applying an external magnetic field.

We have fabricated high-T c dc superconducting quantum interference devices (SQUIDs), which have different geometries of the hole and configurations of current injection leads, in the direct-coupling magnetometer, and measured the coupling efficiency of the flux generated by the injected current. It has been found that the coupling is efficient in the case where the current flows uniformly across the width of the path around the SQUID hole. High coupling of more than 80% was obtained in a geometry of narrow parallel strips with wide slit of the SQUID hole between them.

We developed a four-Josephson-junction superconducting quantum interference device (4JJ-SQUID) with a large modulated voltage to realize a highly sensitive magnetic sensor. This letter describes the design and fabrication, and discusses the potential and the performance of the 4JJ-SQUID. The 4JJ-SQUID was fabricated using Nb/Al–AlO x /Nb tunnel junctions. The SQUID consists of two pairs of Josephson junctions that form two superconducting loops via a shared washer coil. We confirmed using both calculation and experiment that the SQUID had a higher magnetic sensitivity than that of a conventional DC-SQUID.

We have constructed a multiplexed 2-channel, 2-stage superconducting quantum interference device (SQUID) amplifier using double-washer SQUIDs and a 35-SQUID series array. The biasing current of the first-stage SQUIDs is switched by a complimentary metal oxide semiconductor (CMOS) analog multiplexer at liquid helium temperature, and an on-stage SQUID transfers the signal to the second-stage SQUID. Each SQUID has an additional negative feedback circuit to enlarge the linear input region and decrease distortion. Maximum input flux at the first-stage was 0.9Φ opp and the total harmonic distortion of the overall two-stage SQUID was 1% for a sinusoidal input flux of 0.1Φ opp. The voltage swings of the two channels were 1.1 and 1.3 mV.

Reducing the number of loops in a Drung-type superconducting quantum interference device (SQUID) from eight to four minimally reduces the voltage swing. A four-loop SQUID, called a clover-leaf SQUID (CLS), has been designed in order to reduce the area of the SQUID chip. The CLSs are fabricated by the Nb/Al/AlO x /Nb tunnel junction process. They occupy a 4.5-mm-square area on the substrate and have high field-to-flux sensitivity (S=0.63 nT/Φ0). Typical magnetic field resolution is 4.0 fT/Hz0.5 at the white-noise level. The measured biomagnetic fields of this SQUID confirm that its performance level is acceptable.

It has been shown that the voltage versus flux (V-Φ) relation of dc superconducting quantum interference devices (SQUIDs) operating at T=77 K is degraded considerably by thermal noise. Since degradation of modulation voltage in the V-Φ relation becomes more significant when inductance increases, a very restricted range of inductance exists which can be used at T=77 K. In order to reduce the degradation due to thermal noise and to enable the use of large inductances, a new SQUID using three Josephson junctions (3J-SQUID) is proposed. Numerical simulation shows that inductance of 3J-SQUID can be increased to twice as large as that of the conventional case without degrading modulation voltage. It is also shown that modulation voltage is highly improved by damping resistance parallel to inductance. As a result, we can expect to use inductances as large as L

A cryostat for simultaneous measurements of spontaneous magnetization and AC susceptibility by using a SQUID with a temperature control system was constructed. The temperature of samples can be controlled to an accuracy of 1.2×10-5 at temperatures from 1.6 K to 6.5 K during measurements. This accuracy is obtained by good thermal isolation from their surroundings and use of an isolation amplifier which breaks ground loop currents interfering with the detection of temperature. The SQUID magnetometer consists of a flux locked loop followed by another phase-sensitive detector referenced to the frequency of AC susceptibility. The frequency range of AC susceptibility is from 1 Hz to about 200 Hz and the sensitivity is 2×10-10 emu at 10 Hz with a time constant of 3 seconds, which corresponds to 10-4 of the flux quantum at a SQUID.

DC SQUID operation utilizing all ceramic YBa2Cu3O7-δ and GdBa2Cu3O7-δ Josephson junctions is reported. It is confirmed that the macroscopic quantum interference effect is observable up to 90 K, almost the transition temperature of bulk cermaics. The periodic patterns of GdBa2Cu3O7-δ SQUID are rather deformed; probably due to the presence of magnetic element Gd. The temperature dependence of Josephson critical current shows a nearly linear behavior down to low temperatures, in disagreement with Ambegaokar-Baratoff's calculation.

We experimentally demonstrate that energy sensitivity of a dc superconducting quantum interference device (SQUID) with large β=2LsIo/φo is lowered by connecting a damping resistor across the loop inductance of the SQUID, where Ls is the loop inductance of the SQUID, Io is the critical current of each Josephson junction, and φo is the flux quantum. This is due to the considerable increase of the flux-to-voltage transfer function of the SQUID.

An experimental strategy to test for macroscopic quantum coherence in a rf SQUID is described. The back action of broadband noise sources in a conventional dc SQUID magnetometer measurement system destroy the coherent state. A novel measurement scheme is suggested in which the rf SQUID is weakly coupled to an intermediate bistable system. This system consists of a pair of dc SQUIDs biased below threshold. The combined rf and dc threshold SQUID system is described by a single potential. The form of this potential is varied by changes in the external bias to the dc SQUIDs at the moment in which the measurement is to occur. This causes a transition in the dc SQUID system which serves to freeze the coherent oscillations. Amplification by a conventional SQUID linear amplifier chain then follows.

The first successful dc SQUID operation at 77 K utilizing Y-Ba-Cu-O bulk ceramics Josephson junctions is reported. The observation of clear periodic patterns due to the macroscopic quantum interference effect assures promising applications of this device at liquid nitrogen temperature.

Using YBa2Cu3O7-x (YBCO) thin films (0.5∼1.0 µm in thickness) with c axis orientation, we fabricated dc superconducting quantum interference devices (SQUIDs). YBCO films were deposited on MgO(100) single-crystal substrates by MOCVD. These films exhibit zero resistance above 80 K and critical current density of 104 to 105 A/cm2 at 77 K. The SQUIDs with microbridge are fabricated by photolithography and chemical etching techniques. The maximum voltage modulation is about 80 µV for the YBCO dc SQUID with an inductance of 70 pH, and intrinsic energy sensitivity is 740 h (h: Planck's constant) at 4.2 K. Furthermore, the SQUID was operated in flux-locked loop. The minimum flux noise in the flux-locked SQUID is 1.5×10-5 \varPhi0/√Hz in the white noise region at 4.2 K. The experiments suggest that the current-phase relationship of a micrometer-long YBCO bridge may be sinusoidal.

Noise characteristics of dc SQUIDs in large β regimes have been studied experimentally in the case when a SQUID inductance is resistively shunted. Here, β=2L I 0/Φ0 is the modulation parameter, L is the SQUID inductance, I 0 is the critical current of the Josephson junction and Φ0 is the flux quantum. It is shown experimentally that the performance of the SQUID does not degrade significantly even for large β values; for example, the minimum energy resolution of the SQUID is as small as ε=25h and the flux region where εh is as wide as 0.1Φ0, even for the case β=12, where h is the Planck constant. The experimental results agree quantitatively with the theoretical ones.

Construction and properties of a thin film SQUID is described. The SQUID is based on resistively shunted niobium-niobium oxide-lead tunnel junctions deposited on a flat substrate. The origin and consequencies of flux trapping in these SQUID's is presented.

This paper describes the I-V characteristics (as a function of applied magnetic field) and magnetic field response of a dc SQUID which incorporates two weak-link Josephson junctions in a superconducting ring. These characteristics are derived by digital computer simulation, using the Stewart-McCumber model. The effects of asymmetry (dissimilarity between the junctions) on SQUID critical current I c, normal resistance R and capacitance C are also simulated. With microwave excitation applied, the Josephson oscillations of the dc SQUID are found to be “frequency-locked” to the applied microwave signal even when critical currents I c of the two junctions differed, Subharmonic voltage steps are obtained for a symmetric dc SQUID in a magnetic field even when the current-phase relationship is purely sinusoidal.

A new essential multichannel dc SQUID system has been devised. The system consists of two stages of dc SQUIDs; in the first stage, a number of dc SQUIDs equivalent to the input number detect the input signals, and the sum of the first-stage output modulates the carrier of the second-stage dc SQUID and is sent by the carrier to a room temperature amplifier, where the mixed input signals are separated into the original signals. To verify the system operation, a three channel dc SQUID system has been developed and good results have been obtained.

A new, simple and stable type of microwave SQUID has been developed successfully at 10 GHz by using a bridge type junction which can be fabricated very easily. In consequence of adopting a junction of this type, the microwave SQUID has the merit of being free from adjustment and endures several heating cycles between room and liquid He temperatures. This type of SQUID has a slightly poorer S/N ratio, to be improved in future, but has the same characteristics as previously reported SQUIDs with a point contact junction.