E. Dantsker

High-Tc super conducting quantum interference devices with slots or holes: Low 1/f noise in ambient magnetic fields

High-transition temperature dc superconducting quantum interference devices (SQUIDs) have been fabricated with arrays of slots or holes patterned in the 186×204 μm2 washers. In two designs, with seven slots or 248 holes, the linewidth of the remaining YBa2Cu3O7−x film was 4 μm. These devices exhibited virtually no increase in their low-frequency noise when they were cooled in static magnetic fields up to a certain threshold, above 100 μT in the best case. Furthermore, the mutual inductance between these devices and a seven-turn, spiral coil was at least 85% of that for a solid SQUID with the same outer dimensions.

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.

Reduction of 1/f noise in high‐Tc dc superconducting quantum interference devices cooled in an ambient magnetic field

The spectral density SΦ(f ) of the low‐frequency 1/f noise of high transition temperature dc superconducting quantum interference devices (SQUIDs) with narrow linewidths was independent of B0, the magnetic field in which they were cooled, up to a threshold value, about 33 μT in the best case. Above this threshold, which is associated with the entry of flux vortices into the film, the noise increased rapidly. By contrast, for large square washer SQUIDs, SΦ(f ) scaled linearly with B0. Estimates indicate that the 1/f flux noise produced by the pickup loop of a directly coupled magnetometer is negligible.

High-T/sub c/ SQUID microscope for room temperature samples

A high-T/sub c/ SQUID microscope has been constructed for imaging samples at room temperature and atmospheric pressure. The SQUID is mounted on the upper end of a sapphire rod, cooled to liquid nitrogen temperature, below a 3-/spl mu/m thick silicon nitride vacuum window. This arrangement yields a SQUID-to-sample separation as low as 15 /spl mu/m. A sample can be placed on the window or scanned over the SQUID to produce magnetic images.

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.

Integrated high‐Tc multiloop magnetometer

Two integrated, thin‐film multiloop magnetometers (fractional turn SQUIDs) have been fabricated using a YBa2Cu3O7−x‐SrTiO3‐YBa2Cu3O7−x multilayer process. The magnetometers have 16 parallel loops, an outer diameter of 7 mm and effective areas of 1.84 and 1.89 mm2, respectively. The magnetic field noise of the better device, measured in a YBCO tube at 77 K with a bias reversal scheme, was 37 fT Hz−1/2 at 1 Hz and 18 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.

High-Tc superconducting gradiometer with a long baseline asymmetric flux transformer

The planar gradiometer consists of a directly coupled magnetometer inductively coupled to the smaller loop of an asymmetric, two-loop flux transformer patterned in a single film of YBa2Cu3O7−x. The coupling is adjusted mechanically to balance the gradiometer to about 1 part in 3000 with respect to fields perpendicular to its plane. The baseline is 48 mm and the intrinsic balance with respect to in-plane fields about 1 part in 1400. When a magnetic field is applied solely to the magnetometer, its sensitivity is reduced by only 5% by the presence of the flux transformer.