D. T. Nemeth

dc SQUID magnetometers from single layers of YBa2Cu3O7−x

We have fabricated magnetometers patterned in a single layer of YBa2Cu3O7−x involving dc superconducting quantum interference devices (SQUIDs) with bicrystalline grain boundary junctions. The magnetometers consist of either a SQUID with a large area square washer or a single turn pickup loop coupled directly to the body of a small area SQUID. We found that the transfer function falls off with increasing SQUID inductance much more rapidly than predicted; implications for magnetometer performance are discussed. When operated at 77 K with a bias reversal technique a directly coupled magnetometer had a noise of (105±10) fT Hz−1/2 at 1 kHz, increasing to (145±10) fT Hz−1/2 at 1 Hz.

High performance dc SQUID magnetometers with single layer YBa2Cu3O7−x flux transformers

We have fabricated high‐transition temperature superconducting magnetometers involving a flux transformer patterned in a single film of YBa2Cu3O7−x (YBCO) on a 50‐mm‐diam substrate. This transformer is inductively coupled to a loop that in turn is directly coupled to a dc superconducting quantum interference device, patterned in a single layer of YBCO deposited on a SrTiO3 bicrystal. At 77 K, the lowest magnetic field noise achieved is 31 fT Hz−1/2 at frequencies above 5 Hz, increasing to 39 fT Hz−1/2 at 1 Hz.

Low noise YBa2Cu3O7−x–SrTiO3–YBa2Cu3O7−x multilayers for improved superconducting magnetometers

We have fabricated YBa2Cu3O7−x–SrTiO3–YBa2Cu3O7−x (YBCO–STO–YBCO) trilayers, in which each layer is patterned photolithographically, capping the first YBCO film with an in situ STO film. Atomic force microscopy demonstrates that the capping process dramatically improves the quality of the surface of the second layer, allowing the growth of an upper YBCO film with a substantially reduced level of low‐frequency flux noise. A magnetometer with a multiturn flux transformer coupled to a dc superconducting quantum interference device achieved a magnetic field noise of 74 fT Hz−1/2 at 1 Hz, improving to 31 fT Hz−1/2 at 1 kHz.

Bicrystal YBCO DC SQUIDs with low noise

The authors have fabricated 12 DC superconducting quantum interference devices (SQUIDs) by laser-depositing YBa/sub 2/Cu/sub 3/O/sub 7-x/ on a SrTiO/sub 3/ bicrystal substrate with a misorientation angle of 24 degrees . At 77 K all 12 devices had acceptable values of critical current, resistance, and voltage modulation produced by an external magnetic field. The white noise energy of one device with an estimated inductance of 41 pH was 1.8*10/sup -30/ JHz/sup -1/. The noise power scaled as 1/f at frequencies below about 1 kHz; by using a bias current reversal scheme it was possible to reduce this noise by two orders of magnitude at 1 Hz, to a value of about 1.5*10/sup -29/ JHz/sup -1/. A magnetometer was made by coupling the SQUID to a flux transformer with a 5-turn input coil. The measured magnetic field gain was 60, and the white noise was 36 fT/ square root Hz. However, the transformer produced relatively large levels of 1/f flux noise, not reduced by the bias reversal scheme, that limited the noise at 1 Hz to 1.7 fT/ square root Hz. A single-layer magnetometer with a single-turn pick-up loop is described.<<ETX>>

Low‐frequency excess noise in YBa2Cu3O7−x dc superconducting quantum interference devices cooled in static magnetic fields

We have investigated the performance of YBa2Cu3O7−x dc superconducting quantum interference devices (SQUIDs) cooled in static magnetic fields, B0, of 0.01–1 mT. For fields less than the earth’s ambient field, about 0.05 mT, the white noise of the devices at 77 K is not materially affected. However, at a frequency f of 1 Hz the spectral density of the 1/f noise, SΦ (1 Hz), at 0.05 mT increases by an order of magnitude over that for zero field. Furthermore, SΦ (1 Hz) scales approximately linearly with B0, suggesting strongly that the noise originates in the motion of vortices in the YBCO film. This increase in noise is likely to be an issue for SQUIDs operated in the earth’s field.

Eddy current microscopy using a 77-K superconducting sensor

We have used a scanning magnetic flux microscope based on a high transition temperature YBa2Cu3O7 superconducting quantum interference device (SQUID) to produce magnetic images of eddy currents in patterned Cu thin films and 11–30‐μm‐thick Cu on printed circuit boards. The fields produced by the eddy currents are imaged with a spatial resolution of about 80 μm over a 100‐mm2 sample area. With the sample and SQUID at 77 K, the microscope uses typical probing fields of 80 nT and can obtain simultaneously eddy current and static magnetic field images. At probing frequencies of 26–100 kHz, the system achieves a field sensitivity of about 7 pT Hz−1/2.

Picovoltmeter based on a high transition temperature SQUID

We have fabricated and tested picovoltmeters in which the voltage source is connected in series with a calibrated resistor and a coil inductively coupled to a high‐transition temperature superconducting quantum interference device operating at 77 K. The coil consists of either seven or ten turns of copper wire or of two turns patterned in a film of YBa2Cu3O7−x(YBCO). The lowest voltage noise achieved at 1 Hz was 30 pV Hz−1/2 and 2.3 pV Hz−1/2 for input coils made of copper and YBCO, respectively.

High-T/sub c/ multilayer magnetometers with improved 1/f noise

We have fabricated three types of high T/sub c/ magnetometers using a YBa/sub 2/Cu/sub 3/O/sub 7-x/-SrTiO/sub 3/-YBa/sub 2/Cu/sub 3/O/sub 7-x/, multilayer process: multiturn flux transformers coupled to single layer dc SQUIDs in a flip-chip arrangement, multiturn flux transformers integrated with dc SQUIDs, and multiloop magnetometers (fractional turn SQUIDs). The magnetic field noise of the best flip-chip magnetometer was 74 fTHz/sup -1/2/ at 1 Hz and 31 fTHz/sup -1/2/ at 1 kHz. The magnetic field noise of the multiloop magnetometer, measured in a YBCO tube, was 37 fTHz/sup -1/2/ at 1 Hz and 18 fTHz/sup -1/2/ at 1 kHz.<<ETX>>

High‐Tc three‐axis dc SQUID magnetometer for geophysical applications

A prototype three‐axis magnetometer has been developed, intended for geophysical applications, involving high‐transition temperature dc superconducting quantum interference devices (SQUIDs). Each SQUID was fabricated from a thin film of YBa2Cu3O7−x deposited on a SrTiO3 bicrystal to form two Josephson junctions, and was directly coupled to a single‐turn pickup loop patterned in the same film. The three orthogonally mounted sensors were immersed in liquid nitrogen in a fiberglass Dewar and operated in separate flux‐locked loops. In nominally zero ambient magnetic field, the best magnetic field resolution was 170 fT Hz−1/2 in the white noise, increasing to 225 fT Hz−1/2 at 1 Hz. The dynamic range was ±1.6×106 Hz1/2 in the white noise, and the highest slew rate achieved was 1.6 mT s−1, at 900 Hz. The 3 dB point in the frequency response was about 90 kHz. The three channels were mutually orthogonal to about 1%. The three‐axis magnetometer was also operated outside the laboratory in the presence of 60 Hz and r...

Fabrication issues in optimising YBa2Cu3O7-x flux transformers for low 1/f noise

We describe an improved interconnect technology for the fabrication of multiturn flux transformers from YBa2Cu3O7-x-SrTiO3-YBa2Cu3O7-x multilayers. The essential improvements are reductions in the thicknesses of the trilayer films, typically to 100 nm, 250 nm and 250 nm respectively, and in the deposition rate, to 0.07 nm/laser pulse. This process yields crossovers in which the critical current density in the upper YBa2Cu3O7-x film at 77 K is (2-3)*106 cm-2 In situ trilayers exhibited 1/f flux noise levels at 1 Hz below the measurement sensitivity of 15 mu Phi 0 Hz-1/2, Where Phi 0 is the flux quantum. However the flux noise of trilayers in which each layer had been patterned was significantly higher. The best flip-chip magnetometer had a white noise of 40 fT Hz-1/2, increasing to 340 fT Hz-1/2 at 1 Hz; the corresponding flux noise levels were 9 mu Phi 0 Hz-1/2 and 75 mu Phi 0 Hz-1/2.