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Applied Physics Letters

A design of planar multi-turn flux transformers for radio frequency SQUID magnetometers

Y. Zhang, H. R. Yi, Jürgen Schubert, W. Zander, M. Banzet, Alex I. Braginski

Published Apr 20, 1998• 11 citations
Publisher

Abstract

We propose a design for planar multi-turn flux transformer for radio frequency (rf) superconducting quantum interference device (SQUID) magnetometers. This transformer is integrated with a coplanar resonator, e.g., on a 1 cm2 LaAlO3 substrate. Its pickup loop is connected with two input coils, separate for dc (low-frequency) and rf currents. A double-hole washer SQUID is coupled to these coils in a flip-chip configuration to form a magnetometer. The separation of rf and dc current paths in the transformer made it possible to demonstrate the rf SQUID magnetometer operation.

DOI
10.1063/1.121254

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Journal of Applied Physics•Aug 15, 1993•16 citations
We propose a design for planar multi-turn flux transformer for radio frequency (rf) superconducting quantum interference device (SQUID) magnetometers. This transformer is integrated with a coplanar resonator, e.g., on a 1 cm2 LaAlO3 substrate. Its pickup loop is connected with two input coils, separate for dc (low-frequency) and rf currents. A double-hole washer SQUID is coupled to these coils in a flip-chip configuration to form a magnetometer. The separation of rf and dc current paths in the transformer made it possible to demonstrate the rf SQUID magnetometer operation.
This work showed experimentally how the planar multiturn input coil of a flux transformer could be integrated with a coplanar resonator while still permitting one to couple radio frequency (rf) signals to an rf SQUID. A new layout of a multiturn flux transformer with one pickup loop and two input coils, one single turn and one multiturn, has been fabricated on (001) substrates and evaluated. A planar double-hole washer SQUID was inductively coupled to the two separate input coils via a flip chip configuration. With this layout the best magnetometer showed magnetic field white noise of at 77 K in a magnetically shielded surrounding.
This paper demonstrates a design of a planar multi-turn flux transformer integrated with a superconducting labyrinth resonator serving as the planar tank circuit for a radio frequency (rf) superconducting quantum interference device (SQUID) magnetometer. All structures were patterned from 200 nm-thick epitaxial YBa/sub 2/Cu/sub 3/O/sub 7/ (YBCO) films grown on 10/spl times/10 mm/sup 2/ LaAlO/sub 3/ substrates. A double-hole washer SQUID had one hole coupled to the input coil of the labyrinth resonator and the other hole coupled to the input coil of the multi-turn flux transformer using a flip-chip configuration to form a magnetometer. This resonator has a good high-frequency coupling to the double-hole rf SQUID, thus securing its optimum operation. For the voltage-to-flux (transfer function) coefficient, a value of 300-500 /spl mu/V//spl Phi//sub 0/ was obtained. A SQUID magnetometer with an inductance of 210 pH exhibited white flux noise of 11.5 /spl mu//spl Phi//sub 0///spl radic/Hz at 77 K. This corresponded to a white magnetic field noise of 11.5 fT//spl radic/Hz.
This article describes three planar layouts of superconducting multiturn flux transformers integrated with a coplanar resonator for radio frequency (rf) superconducting quantum interference device (SQUID) magnetometers. The best magnetic field noise values of 22 and 11.5 fT/Hz1/2 in the white noise regime were obtained for the layout with two input coils and the layout with the labyrinth resonator, respectively. Excess low-frequency noise (about 200 fT/Hz1/2 at 10 Hz) was present. Computer simulation showed that the loss in this trilayer system was dominated by the high loss tangent of the dielectric film used for the separation of the upper and lower superconducting films. The rf coupling coefficient krf between the resonator and the flip-chip-coupled SQUID was also estimated. The values krf2≈14×10−3 obtained for the layout with two input coils, and krf2≈45×10−3 for the layout with the labyrinth resonator were considerably higher than the typical value of krf2≈7×10−3 for the single-layer coplanar resonator. These high coupling coefficients have compensated the somewhat degraded unloaded quality factor of the resonator, thus securing the optimum operation of the rf SQUID.
We have developed a novel coplanar resonator serving as a tank circuit for radio frequency washer superconducting quantum interference devices (SQUIDs). Two coplanar lines surrounded the circular flux concentrator washer, 13.4 mm in diameter. The SQUID, 3.5 mm in diameter, was coupled to the concentrator in the flip-chip configuration. All parts were fabricated from single-layer epitaxial YBa2Cu3O7 films on LaAlO3 substrates. In this layout, the resonant frequency could be set between 600 and 850 MHz. With SQUID loops of 100×100 μm2 (SQUID) inductance Ls=150 pH or 10×500 μm2 (Ls=260 pH), we determined at 77 K white flux noise levels Sφ1/2 of 1.0×10−5 and 1.5×10−5φ0/Hz, respectively. Both figures corresponded to the field resolution of about 30 fT/Hz.
The operation of YBa2Cu3O7 rf superconducting quantum interference devices (SQUIDs) using a dielectric SrTiO3 resonator as tank circuit was tested. At 77 K, we attained for such resonators unloaded quality factors Q0 up to 7000 at resonant frequencies near 1 GHz, when measuring without electromagnetic shielding. For a washer SQUID with an area of 4×4 mm2 and an inductance of ≂100 pH, which was properly coupled to the resonator, we measured an approximately 60 μV peak-to-peak SQUID signal and a white flux noise spectral density of SΦ1/2=1×10−5Φ0/√Hz, corresponding to a field sensitivity of BN=105 fT/√Hz. With an additional single-layer flux transformer with a diameter of the pickup loop of 22 mm in flip-chip arrangement, the field sensitivity was improved to 30 fT/√Hz.
Conventional rf SQUIDs (superconducting quantum interference devices) consist of a superconducting loop interrupted by one weak link, and when read out via a tank circuit they produce output signals in the order of a few tens of microvolts. When the single weak link is replaced by a dc SQUID (a superconducting loop with two junctions) a so-called double quantization loop SQUID is formed. In this paper it is shown theoretically, and supported experimentally with thin-film devices of three different designs, that the general behaviour can be explained by a simplified picture of the fixed critical current of the single junction of the rf SQUID being replaced by an effective critical current of a dc SQUID loop that is varied by the applied magnetic flux (i.e., we have a rf SQUID with a variable β parameter). A more detailed analysis of the experimental data and the theory of this novel device reveals its interesting nonlinear character and shows the deviations from this simple picture. It is demonstrated experimentally that output signals in the order of ≥1 mV can be obtained with a suitable read-out mode for the tank circuit, a clear advantage when compared to conventional rf SQUIDs. With respect to dc SQUIDs, the coupling to integrated planar input coils will be unproblematic since no microwave resonances should occur in the structures.