Jen-Tzong Jeng

Detection of deep flaws by using a HTS-SQUID in unshielded environment

We reported the nondestructive evaluation of flaws in conductive samples with the high-T/sub c/ SQUID operated at 77 K. By nulling the SQUID with an additional compensation, we can sensitively detect the excited magnetic field due to the eddy current around a deep flaw. To investigate the effects of the size and the depth of flaws on the induced magnetic field, the excited magnetic fields as a function of the depth were measured for buried slots with various widths and thickness. Besides, the amplitude and the phase with different excitation frequencies for a fixed slot size are also studied. It was found that for a fixed excitation frequency, the phase of the measured signal linearly responds to the depth of the slot, while the amplitude correlates with the height of the slot. The results are valuable for quantitative evaluation of flaws.

High-sensitivity low-noise miniature fluxgate magnetometers using a flip chip conceptual design.

This paper presents a novel class of miniature fluxgate magnetometers fabricated on a print circuit board (PCB) substrate and electrically connected to each other similar to the current “flip chip” concept in semiconductor package. This sensor is soldered together by reversely flipping a 5 cm × 3 cm PCB substrate to the other identical one which includes dual magnetic cores, planar pick-up coils, and 3-D excitation coils constructed by planar Cu interconnections patterned on PCB substrates. Principles and analysis of the fluxgate sensor are introduced first, and followed by FEA electromagnetic modeling and simulation for the proposed sensor. Comprehensive characteristic experiments of the miniature fluxgate device exhibit favorable results in terms of sensitivity (or “responsivity” for magnetometers) and field noise spectrum. The sensor is driven and characterized by employing the improved second-harmonic detection technique that enables linear V-B correlation and responsivity verification. In addition, the double magnitude of responsivity measured under very low frequency (1 Hz) magnetic fields is experimentally demonstrated. As a result, the maximum responsivity of 593 V/T occurs at 50 kHz of excitation frequency with the second harmonic wave of excitation; however, the minimum magnetic field noise is found to be 0.05 nT/Hz1/2 at 1 Hz under the same excitation. In comparison with other miniature planar fluxgates published to date, the fluxgate magnetic sensor with flip chip configuration offers advances in both device functionality and fabrication simplicity. More importantly, the novel design can be further extended to a silicon-based micro-fluxgate chip manufactured by emerging CMOS-MEMS technologies, thus enriching its potential range of applications in modern engineering and the consumer electronics market.

Vector Magnetometer with Dual-Bridge GMR Sensors

A three-axis vector magnetometer based on two sets of dual-bridge GMR sensors and a fluxguide was investigated in this work. The sensors were mounted on a printed circuit board (PCB) with their sensing directions along the x- and y-axes respectively. A cylindrical ferrite fluxguide was put at the center of four sensors with the symmetric axis normal to the PCB plane. The fluxguide deflects the magnetic flux lines of an out-of-plane field to generate the in-plane components, making it possible to detect the z-axis field component with the x - y plane sensor pairs. When the device was driven by a peak-to-peak excitation voltage of 0.4 V, the demodulated dV/dB were found to be 230, 240, and 220 V/T, respectively, for the x, y, and z sensing axes. The field noise spectral densities at 1 Hz were found to be between 3 and 9 nT/√(Hz) for all three sensing axes. The proposed magnetometer design can be adapted to reduce the size and to lower the power consumption by integrating all the components on a chip.

Tri-axis magnetometer with in-plane giant magnetoresistance sensors for compass application

A tri-axis magnetometer comprising three giant-magnetoresistance sensors and a cylindrical fluxguide are implemented for compass application. The sensors are mounted on a single printed circuit board (PCB) board with their sensing axes in a plane. A calibration process involving matrix manipulation was employed to make the device function as a tri-axis magnetometer with orthogonal sensing directions. The capability of the device for compass application was demonstrated by measuring its azimuth response to the geomagnetic field about different rotation axes. The proposed calibration technique can be used for the magnetometer system with either orthogonal or non-orthogonal sensor arrangement.

Design, Fabrication, and Characterization of a 3-D CMOS Fluxgate Magnetometer

A dual-core 3-D microfluxgate magnetometer fabricated by a simple and inexpensive fabrication process is described in this paper. The microfluxgate is able to operate along a nearly linear V-B relationship at the second harmonic frequency and features good characteristics of high sensitivity and low noise response. These characteristic results indicate a field-to-voltage transfer coefficient of 11 V/T measured at the second harmonic frequency, power consumption of 67.3 mW, and a field noise response less than 12 nT/√ Hz at 1 Hz. In brief, our proposed device not only enhances responsivity capability and linear V-B characteristics, but also is CMOS process compatible, which is considered both function-efficient and cost-effective.

Influence of bicrystal microstructural defects on high-transition-temperature direct-current superconducting quantum interference device

Using atomic force microscopy and scanning electron microscopy (SEM), we investigate the correlations between the microstructural defects and the electrical characteristics of the bicrystal grain-boundary Josephson junctions and dc superconducting quantum inference devices (SQUIDs). The structural defects are shown to correlate qualitatively with the characteristics of grain-boundary Josephson junctions patterned on the YBa2Cu3O7−x film. SEM images show that these defects grown on the grain boundary were a few submicron depth of the groove. The low flux noise characteristics were observed when the groove depth was smaller than 18nm in the junctions of the SQUID. The existence of these defects is expected to affect the supercurrent and the motion of the magnetic flux in the films, which dominate the excess noise in the SQUID with bicrystal junctions.

Transport properties in FeSe0.5Te0.5 nanobridges

FeSeTe nanobridges of different widths have been fabricated on MgO substrates using focused ion beams. These nanobridges exhibit the Josephson effects. The current-voltage curves of junctions with 248–564 nm wide follow the resistively and capacitatively shunted junction model. Shapiro steps under microwave radiation were clearly observed in these nanobridges. The products of the critical current and normal state resistance (IcRn) are remarkably high. The temperature dependence of IcRn product followed the Ambegaokar-Baratoff (A-B) relation. The value of energy gap of FeSeTe calculated from the A-B relation is 3.5kBTc. The nanobridge junctions have a strong potential for high frequency applications.

A highly sensitive YBCO serial SQUID magnetometer with a flux focuser

We have designed and fabricated a low noise, high Tc superconducting quantum interference device (SQUID) magnetometer with an improved yield. In order to reduce the field noise level of the magnetometer, we increased the voltage modulation depth with a series SQUID array and enhanced the effective area with a flux focuser. Ten washer-type SQUIDs were connected in series, thereby increasing the voltage flux transfer function by a factor of 10. An enhancement in effective area by a factor of 5 was achieved by coupling the 10-SQUID array to a single-layered flux focuser. For the directly coupled SQUID array magnetometer, the yield was improved by selecting the best SQUIDs to have in series in order to reduce the flux noise. The magnetic field sensitivity of 40 fT Hz−1/2 in the white regime and 100 fT Hz−1/2 at 1 Hz is demonstrated by using a single layer of high Tc film. The proposed high yield magnetometers would be suitable for building low noise multi-channel magnetocardiographs.

Evaluation of the flaw depth using high-Tc SQUID

Abstract Using high- T c SQUID, we investigated quantitative nondestructive evaluation for flaws in conducting samples. The spatial derivative of the defect field was found to provide valuable information about the position and the depth of the flaw. By analyzing the spatial derivative of the defect field, the quantitative flaw evaluation was demonstrated.

High quality step-edge substrates for high-Tc superconducting devices

Despite the significant progress in fabrication methods of step edge, the lack of reproducibility still hinders their use in more complicated systems. To pursue the high reproducibility and quality of step edge for high-Tc superconducting devices, we have developed the technique to fabricate high quality step-edge substrates with arbitrary step angles. We used two steps to improve the step ramp quality substantially. The surface microscopy of step substrates shows high uniformity with respect to any step angle. There are no needles, waves, trenches, cascades, or other flaws on these surfaces. Serial Josephson junctions and superconducting quantum interference device arrays were fabricated onto step-edge substrates. The step-edge devices exhibit excellent results.