Haruhisa Toyoda

A multi-channel high- SQUID system and its application

A multi-channel high temperature superconducting interference device (high Tc SQUID) system has been developed. Step edge junctions were employed as weakly coupled Josephson junctions for the SQUID. The magnetic field resolution was 70 to 250fT/Hz1/2 at 1kHz. The SQUID was mounted on a special chip carrier and was sealed with epoxy resin for protection from humidity. We have developed a 32-channel high Tc SQUID system. We used it to measure magnetic signals of the human heart in a magnetically shielded box. We obtained clear multi-channel magnetocardiac signals, which showed clear so called Q, R, S and T wave peaks. Its clear isofield contour map was also obtained. We could observe activities of a stomach using a tiny steel ball as a tracer. These data indicated that the high Tc SQUID is feasible for these biomagnetic applications.

Four-channel YBa2Cu3O7-y dc-SQUID magnetometer for biomagnetic measurements

A 4-channel YBa2Cu3O7-y (YBCO) thin film dc-SQUID magnetometer was fabricated. Biomagnetic measurements were performed by using the 4 channel SQUID array at 77

Iron-Based P/M Soft Magnetic Material for Electric Devices Operating at High Frequencies

KAPPA in a magnetically shielded room. We have successfully measured 4 channel magnetocardiac signals clearly in real time mode. The best magnetic field resolution of the four SQUIDs was 370fT/Hz1/2 at 10Hz and 200fT/Hz1/2 in the white noise region.

MULTI-CHANNEL HIGH Tc SQUID SYSTEM FOR MCG

Recently, there has been a growing demand for soft magnetic materials with high conversion characteristics in the high-frequency range due to the trend where electric devices are being operated at higher frequencies. P/M soft magnetic materials, which were developed for such devices, utilizing iron-based powder and alloy powder. Through this work, we were able to reduce core loss and achieve remarkable magnetic characteristics in the high frequency range. Core loss, which includes hysteresis loss and eddy-current loss, is predominant in the high-frequency range. Two methods can be used to reduce eddy-current loss: using fine-grained powder, and using high-resistivity powder. However, both these present compressibility problems, with high-resistivity iron-based alloy powders such as Fe-Si being particularly poor in deformability. By studying composition of the material, we successfully developed a material that offers both high resistivity and high compressibility, reduction of core loss by more than 20% over the conventional material was achieved.

A high- superconducting quantum interference device with an alternating current bias DOIT and additional positive feedback

We have developed a multi-channel high Tc SQUID system for magnetocardiography. The system consists of a 16ch SQUID in a liquid nitrogen dewar, a cylindrical permalloy magnetic shield including a movable bed, an adjustable gantry and electronics with a personal computer. The SQUIDs whose diameter is 17mm were arranged in 4 by 4 matrix with spacing of 20mm. The field resolution of the SQUID is less than 1pT/√Hz @10Hz. The liquid nitrogen dewar is compact with the diameter of 190mm. The magnetic shield is compact with the diameter of 0.85m and the length of 1.6m. It has open ends for a patient to feel relaxed. The development of a compact and low cost system is aimed for use in hospitals and bioengineering laboratories. We obtained a 16ch magnetocardiogram of the 6cm by 6cm area above the human heart and constructed time-varying magnetocardiac isofield contour maps. System noise level was about 25pT in real time mode and less than 5pT by averaging of 11 times.

Large Washer SQUID

This study shows that the alternating current (AC) bias direct offset integrated technique (DOIT) reduces low-frequency noise without increasing the level of white noise in a high- DC superconducting quantum interference device (SQUID). Additional positive feedback (APF), which can improve the effective flux-to-voltage transfer function, it used to reduce the equivalent flux noise due to the voltage noise of the preamplifier of the magnetometer. The use of APF combined with the AC bias DOIT reduced the noise of the magnetometer by factors of 1.5 at 100 Hz, 3.5 at 10 Hz, and 5.3 at 1 Hz when compared with noise levels obtained with the static current bias DOIT.

Improvement of super low core-loss soft magnetic materials

Focussing effect of YBa2Cu3O7−y large washer SQUID and flux shielding plates were studied. Optimizing the hole size of the shielding plates, the flux capture area was improved up to 60% of the theoretical value. The field resolution was 370fT/Hz1/2 at 10Hz. We have developed a 16-channel high-Tc SQUID magnetometer system, and demonstrated that the 16 channel magnetocardiogram can be obtained. The devices used in this system were sealed by epoxy resin; there were no significant degradation of the SQUID performances after the heat cycles (room temperature to 77K) of 50 times.

Noise Evaluation of High Tc SQUID

In the previous work, we reported a P/M soft magnetic material with super low core loss value of W10/1k = 68 W/kg which is lower than that of 0.35mm-thick flat rolled soft magnetic laminated steel sheets. But this material lack of strength characteristics due to spherical particles produced by a gas-atomizing method. That is, the value of transverse rupture strength (TRS) was only 20MPa when a non-hygroscopicity resin was used as binder. In order to achieve both low core loss and high strength, the iron powder (shape, surface morphology) and binder strength was improved, and we were able to obtain a material with TRS of 80 MPa and core loss (W10/1k) of 108 W/kg of. Furthermore, by using this binder system, we were able to obtain a TRS of over 50MPa for the material with spherical particles (W10/1k = 81 W/kg).

YBa2Cu3O7−y SQUID with Step-Edge Junctions

A high Tc DC Superconducting Quantum Interference Device (SQUID) with step edge junctions on SrTiO3(100) substrate have been fabricated. Noise components of four high Tc DC-SQUIDs have been evaluated. Intrinsic thermal noise levels of four SQUIDs have been estimated 7.9∼11.6μФ0/Hz1/2. 1/f noise comes from three noise sources, in-phase critical current fluctuation at two junctions, out-of-phase critical current fluctuation at two junctions and residual 1/f noise. In-phase and out-of-phase 1/f noise levels at 1Hz have been estimated 131∼781μФ0/Hz1/2 and 55.6~164μФ0/Hz1/2, respectively. In-phase 1/ f noise level was much larger than out-of-phase 1/f noise level, though two noise sources were deduced from the critical current fluctuation at two junctions. Residual 1/f noise levels have been estimated 19.6 ∼43.2μФ0/Hz1/2 and relatively smaller than the in-phase and out-of-phase 1/f noise levels.

Soft Magnetic Material and Dust Core

YBa2Cu3O7− junctions were fabricated. We have studied characteristics of the SQUIDs with different inductances. The flux transfer function was affected significantly by the SQUID inductance. The minimum energy sensitivity at 10Hz of 3.3 × 13.10−29J/Hz was obtained at the SQUID with inductance of 40pH. The superconducting focussing plate was applied to the SQUID to enhance the magnetic field sensitivity. The flux capture area increased by a factor of 3.6 and was 1.8 × 10-8m2. The flux noise Sϕ 1/2 at 10 Hz was 3.4 x× 10−5ϕ0/Hz1/2, corresponding to 3.7pT/Hz1/2.