high-Tc dc-SQUID Research Articles

Flip-Chip High-Tc DC SQUID Magnetometer With a Ferromagnetic Flux Antenna

We have combined a flip-chip high-Tc dc SQUID magnetometer with a ferromagnetic flux antenna made from 250 insulated 25-μm-thick stripes of amorphous soft magnetic Vitrovac 6025 foil inserted through the pick-up loop of the superconducting flux transformer. The ferromagnetic flux antenna provides improved magnetic coupling of the high-Tc SQUID to magnetic fields of ~0.4 nT/Φ0 to ~0.18 nT/Φ0. The orientation of the stripes was chosen to be normal to the foil tape rolling direction in order to minimize contributions from Barkhausen noise. A magnetic field resolution of ~2 fT/√Hz at 1 kHz and 77 K was achieved. Details of the technology and noise spectra of the novel composite high-Tc SQUID sensor are presented and their prospective applications for biomagnetic research and nondestructive evaluation are discussed.

Integration Issues of Graphoepitaxial High- <inline-formula> <tex-math notation="TeX">${\rm T}_{\rm c}$</tex-math></inline-formula> SQUIDs Into Multichannel MEG Systems

We have analyzed the possibility to construct multichannel magnetoencephalography (MEG) systems based on high-Tc direct current superconducting quantum interference devices (DC SQUIDs) with graphoepitaxial step edge Josephson junctions. A new layout of multilayer high-Tc superconducting flux transformers was tested and a new type of high-Tc DC SQUID magnetometer intended for MEG systems was realized. ameter 24 mm and a magnetic field resolution of ~4 fT/√Hz These magnetometers have a vacuum-tight capsule of outer diat 77 K. Crosstalk between adjacent sensors was estimated and measured for in-plane and axial configurations. The vibration-free cooling of sensors, minimization of the sensor-to-object distance and optimization of the sensor positions as well as the gantry design are discussed. Our findings may have implications for the next generation of noninvasive imaging techniques that will be used to understand human brain function.

Graphoepitaxial high-Tc SQUIDs

The fabrication process and physical properties of graphoepitaxially engineered high-Tc direct current superconducting quantum interferometer devices (DC SQUIDs) are studied. Double buffer layers, each comprising a graphoepitaxial seed layer of YBa2Cu3O7−x and an epitaxial blocking layer of SrTiO3, were deposited over textured step edges on (001) surfaces of MgO substrates. Scanning electron microscopy and high-resolution transmission electron microscopy were used to investigate the microstructural properties of DC SQUIDs with graphoepitaxial Josephson junctions. Both direct coupled and inductively coupled high-Tc DC SQUIDs with graphoepitaxial step edge junctions and flux transformers were studied.

Ultra-Low-Field MRI and Spin-Lattice Relaxation Time of $^{1}\hbox{H}$ in the Presence of $\hbox{Fe}_{3}\hbox{O}_{4}$ Magnetic Nano-Particles Detected With a High-$T_{\rm C}$ DC-SQUID

We have carried out ultra-low-field nuclear magnetic resonance (NMR) and magnetic resonance imaging experiments in microtesla magnetic fields using a high-Tc dc-SQUID sensor. The measurements were carried out in a home-made magnetically shielded room. NMR spectra of 1H from tap water and other substance samples were obtained in the field range from 10-200 μT. By adapting a transformer coupled detection system, the signal-to-noise ratio of NMR peak in a single-shot measurement for 10 ml tap water was increased significantly as compared with our previously used direct detection method. The improvement was attributed to the increased coupling efficiency and reduced SQUID noise. By applying magnetic field gradient in three directions, one- and two-dimensional imaging results were obtained from water phantom samples. Furthermore, the effect of Fe3O4 magnetic nanoparticles on the spin-lattice relaxation time T1 was studied.

Detection of nuclear magnetic resonance in the microtesla range using a high Tc dc-SQUID

We have detected the ultra-low field nuclear magnetic resonance signal from water samples using a high-Tc dc-SQUID sensor. The measurements were carried out in a homemade magnetically shielded room. Resonance spectra of 1H from tap water and other substance samples were obtained in the field range from 7-110μT corresponding to resonance frequency 300-4.68kHz. Two kind of experimental systems were built, the first one is a directly coupled system, its signal to noise ratio in a single-shot measurement is around 4 for about 15 ml water. The second one used a Cu coil to transfer the flux to the SQUID sensor. Signal to noise ratio was improved to about 20 in a single-shot measurement for 5ml water, which benefits from the improvement of coupling efficiency. The effect of residual gradient in the magnetically shielded room was also investigated. J-coupling of 2,2,2-Trifluoroethyl alcohol was measured, the peaks are consistent with high field results.

Magnetoencephalography using a Multilayer hightc DC SQUID Magnetometer

We describe tests of the use of a multilayer highTc DC SQUID magnetometer for magnetoencephalography (MEG) and compare our measurements with results obtained using a lowTc SQUID sensor. The integration of bias reversal readout electronics for highTc DC SQUID magnetometry into a commercial MEG data acquisition system is demonstrated. Results of measurements performed on a salinefilled head phantom are shown and the detection of an auditory evoked magnetic response of the human cortex elicited by a stimulus is illustrated. Future modifications of highTc DC SQUID sensors for applications in MEG, in order to reach a resolution of 1 fT/√Hz at 77.5 K over a wide frequency band, are outlined.

Ultra-low field magnetic resonance imaging based on high Tc dc-SQUID

A systematic research on ultra-low field magnetic resonance imaging (MRI) is conducted based on high-Tc dc-SQUID senor. The coil system is first updated to reach the requirements of MRI experiment. After that one-dimensional and two-dimensional imaging are performed and images consistent well with original phantoms are obtained successfully. Two different methods are used to rebuild the image: direct back projection and Fourier transform reconstruction. Both of them can obtain the profile of water phantom. A comparative discussion between these methods is proposed: the Fourier transform method has a better profile, while the direct back projection has a better signal-to-noise ratio. Imaging of biological sample such as green pepper and celery is also performed, and it is consistent well with the physical object.

The Effect of Low Frequency External Field Disturbance on the SQUID Based Ultra-Low Field NMR Measurements

We have carried out NMR experiments in the microtesla range using a high-Tc DC-SQUID sensor. The measurements were carried out in a simple magnetically shielded room. Resonance spectra of 1H nuclei from tap water and other substance samples were obtained in the field range 7-70 μT. The effect of frequency, strength, direction and phase of a low frequency external disturbance field on the NMR spectra was investigated. A sinusoidal field was applied along or perpendicular to the measurement field to simulate the external disturbance. The results were compared with the numerical calculations based on the Bloch equation and good agreement was obtained. In all cases, the field disturbance less than a few Hz are proved to influence the NMR spectra more severely, suppressing the resonance peak and driving it into split bands. On the other hand, the influence of the disturbance field at the frequency range around 50 Hz on the resonance peak position and profile is small, even though some additional small peaks appear in the spectra due to frequency mixing. The effect is more significant when the disturbance field is along the direction of the measurement field. The phase difference between the disturbance field and the free-induction-decay signal could change the NMR peak position and profile drastically.

Circulating Current Ratio for DC SQUIDs

The circulating current in one Josephson junction of a DC SQUID is in the direction of translation current while in the other Josephson junction is in the opposite direction. Therefore, one interesting function is the circulating current ratio (CCR) which we define as: CCR = (dissipative translation current1 − circulating current)/(dissipative translation current2 + circulating current). We discuss the circulating current ratio in the loop of DC SQUIDs by considering the 20 on-chip high TC DC SQUIDs that Novikov et al. measured their transfer functions and flux shifts and reported in I.L. Novikov et al. (Physica C 469:30, 2009). A comparison between the CCR characteristics of those SQUIDs may be helpful in understanding the voltage-current and voltage-flux characteristics of asymmetric high TC DC SQUIDs.

Noise analysis of DC SQUIDs with damped superconducting flux transformers

An analysis was performed of intrinsic noise for high-Tc DC SQUID with superconducting flux transformer (FT) containing resistive elements. For a SQUID with a loop inductance of about 40 pH we observed voltage swings of ~55 μV and a flux noise of ~4 μΦ0/√Hz at 77 K. Inductive coupling of an 8-mm multilayer superconducting FT to the SQUID increased voltage swings to ~70 μV due to effective reduction of the SQUID loop inductance. This also increased the flux noise to ~6μΦ0/√Hz, corresponding to a field resolution of ~18 fT/√Hz at 77 K with a white noise spectrum down to frequency ~10 Hz. The main sources of white flux noise were the Nyquist noise in the Josephson junctions and the FT, as well as the suppression of the DC SQUID voltage swings caused by parasitic capacitance between the FT and the SQUID. An ultra-low-ohmic resistor with resistance value between the flux-creep-induced resistances of superconductors (below ~0.1 nΩ) and resistances of conventional resistors (above ~0.1 mΩ) was developed. An RL-circuit based high-pass filter (HPF) with time constant ~7 sec was realized and integrated in the superconducting FT. The contribution of the HPF to the noise of the sensors was measured and compared with calculated values.

Tunable high transition temperature superconducting quantum interference device magnetometer with gate-voltage-controlled bicrystal junctions

A gate-voltage-controlled high transition temperature direct current superconducting quantum interference device (high-Tc dc SQUID) magnetometer was fabricated to study how the electric field effect affects the device’s voltage-flux and voltage-current characteristics. The magnetometer consists of a monolithic YBa2Cu3O7−δ superconducting film and bicrystal junctions covered by a polymethylmethacrylate film as the gate insulator. It was found that the magnetic flux modulated voltage swing of the SQUID magnetometer varies by 48% when the gate voltage is changed from −1.9 to 9.5 V. The variation in voltage swing is attributed to the change of effective capacitance in grain-boundary junctions.

Voltage–current and voltage–flux characteristics of asymmetric high TC DC SQUIDs

We report measurements of transfer functions and flux shifts of 20 on-chip high T C DC SQUIDs half of which were made purposely geometrically asymmetric. All of these SQUIDs were fabricated using standard high T C thin-film technology and they were single layer ones, having 140 nm thickness of YBa 2Cu 3O 7− x film deposited by laser ablation onto MgO bicrystal substrates with 24° misorientation angle. For every SQUID the parameters of its intrinsic asymmetry, i.e., the density of critical current and resistivity of every junction, were measured directly and independently. We showed that the main reason for the on-chip spreading of SQUIDs’ voltage–current and voltage–flux characteristics was the intrinsic asymmetry. We found that for SQUIDs with a relative large inductance ( L > 120 pH) both the voltage modulation and the transfer function were not very sensitive to the junctions asymmetry, whereas SQUIDs with smaller inductance ( L ≃ 65–75 pH) were more sensitive. The results obtained in the paper are important for the implementation in the sensitive instruments based on high T C SQUID arrays and gratings.

Epitaxial thick film high-Tc SQUIDs

Low-noise operation of superconducting quantum interference devices (SQUIDs) in magnetic fields requires high critical current and strong pinning of vortices in the superconducting electrodes and in the flux transformer. Crack-free epitaxial high-Tc dc-SQUID structures with a total thickness ?5 μm and a surface roughness determined by 30 nm high growth spirals were prepared with YBa2Cu3O7-x (YBCO) films on MgO substrates buffered by a SrTiO3/BaZrO3-bilayer. HRTEM demonstrated a high quality epitaxial growth of the films. The YBCO films and SQUID structures deposited on the buffered MgO substrates had a superconducting transition temperature Tc exceeding 91 K and critical current densities Jc > 3 MA/cm2 at 77 K up to a thickness ~5 μm. The application of thicker superconducting and insulator films helped us to increase the critical current and dynamic range of the multilayer high-Tc flux transformer and improve the insulation between the superconducting layers. An optimization of SQUID inductance allowed us to fabricate 8 mm SQUID magnetometers with SQUID voltage swings of ~60 μV and a field resolution of ~30 fT/√Hz at 77 K.

Operation of a high-Tc DC-SQUID-gradiometer on a non-metallic pulse tube refrigerator

A planar high-Tc DC-SQUID gradiometer has been operated with a specially developed low noise pulse tube refrigerator. The cold finger of the refrigerator consists only of non-metallic and non-magnetic materials. The high sensitive SQUID-gradiometer with a base length of 4 mm has a field gradient resolution of about 3 pT/(cm Hz1/2) at 1 Hz and 0.7 pT/(cm Hz1/2) in the white noise region in magnetically shielded environment. During the operation of both, the sensor and the refrigerator, the noise generated by the cryocooler is below the noise level of the SQUID-gradiometer. We demonstrate the potential of this non-metallic pulse tube refrigerator by measuring the magnetic field originating from a human heart (magnetocardiogram), without additional suppression of the intrinsic refrigerator noise.

High-Tc dc SQUID readout electronics with low noise and high bandwidth

Using AD797 low noise op amps and 2SA1048 low noise transistors, we have developed a composite preamplifier for use in dc SQUID readout electronics. This preamplifier has a small dc drift and super low noise at high frequencies. The equivalent input voltage noise of the preamplifier is about 0.35nV/√Hz from 100kHz to 10MHz. Using this preamplifier, we developed dc SQUID readout electronics having low noise and high bandwidth. Used with a 3mm2 high-Tc dc SQUID, the white flux noise was about 18μΦ0/√Hz above 100kHz and the FLL bandwidth was about 2MHz. This readout electronics can be used for the applications of SQUID-based NDE and SQUID-based NQR.

The influence of the second harmonic in the current-phase relation on the voltage-current characteristic of high TC�DC�SQUID

A theory for the voltage-current characteristic in high TC DC SQUIDs, which accounts for a second harmonic in the junction current-phase relation, is developed. The comparison with experiment is performed. It is shown that if the second harmonic is prevailed, the theory can explain the large deviations of the experimental voltage modulation from theoretical predictions and computer simulations based on conventional sinusoidal current-phase relation.

Performance of high-TC dc SQUID magnetometers for use in a magnetically disturbed environment

YBCO dc SQUID magnetometers based on bicrystal Josephson junctions on 10 mm × 10 mm STO substrates have been fabricated. We have designed three different types of pickup coils for the SQUID magnetometers, i.e., solid-type, 12 and 16 parallel loops with 50 μm line width to test performances of the SQUIDs for use in a magnetically disturbed environment. Magnetometer with 16-parallel-loop pickup coil exhibit most stable FLL operation under external dc magnetic field. Finally, we could obtain optimised direct coupled YBCO SQUID magnetometer design having flux transfer coefficient BΦ of 4.5 nT/Φ0 and magnetic field noise BN of 30 fT/Hz1/2 measured at 100 Hz. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Research on high- T c SQUID based non-destructive evaluation

A non-destructive evaluation system based on high-Tc dc-SQUID (superconducting quantum interference device) incorporating a gradient field excitation has been built. By using this system a 1mm-diameter hole at a depth of 2mm inside an aluminium plate at room temperature can be easily detected and imaged in an unshielded environment. The relation between the spatial resolution, or the smallest detectable flaw size and experimental parameters is briefly analysed in terms of a simple metal ring model. The result shows that the spatial resolution depends strongly on the sensor-sample separation as well as on some other parameters, such as signal-to-noise ratio of excitation, excitation frequency and material conductivity.

Theory of the voltage–current characteristic of high Tc DC SQUIDs

A new analytic theory for the voltage–current characteristic of symmetric high T c DC SQUIDs is developed. The theory is based on analytical solution of a two-dimensional Fokker–Planck equation, which describes the dynamical behavior of DC SQUID in presence of thermal noise. For high T c DC SQUIDs operating at 77 K with L>100 pH, where L is geometrical inductance of DC SQUID loop, our theory is valid for any values of critical currents of Josephson junctions and it allows a correct evaluation of transfer function of practical high T c DC SQUIDs. The theoretical voltage modulation is shown to be in a good agreement with the results of known computer simulations.

Novel excitation coils for non-destructive evaluation of non-magnetic metallic structures by high-Tc dc SQUID

A non-destructive evaluation (NDE) system was set up in our laboratory. A high-Tc dc superconducting quantum interference device (dc SQUID) made from YBa2Cu3O7−δ thin film on a 24° bicrystal SrTiO3 substrate was adopted as the magnetic flux sensor. Novel excitation coils and a balance circuitry were proposed and employed for evaluation of multilayer aluminium structure with artificial slits in an unshielded noisy environment. Our experimental data demonstrate that the NDE system with proper location of two thin excitation coils could make full use of the dynamic range of the SQUID and enhance the spatial resolution of detection.