A. H. Miklich

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.

Multilayer YBa sub 2 Cu sub 3 O sub x -SrTiO sub 3 -YBa sub 2 Cu sub 3 O sub x films for insulating crossovers

We describe our procedure for fabricating YBa2Cu3Ox‐SrTiO3‐YBa2Cu3Ox thin‐film trilayer structures. Each film is grown in situ by excimer laser deposition onto a heated (100)MgO substrate. The geometrical configuration of each layer is defined by a metal mask; the vacuum chamber is opened between depositions to allow the targets and masks to be changed. The lower and upper YBa2Cu3Ox films in the best trilayer structure had transition widths of 1 and 3 K (10–90%), respectively, and transition temperatures (zero resistance) of 87 K. The resistance between the YBa2Cu3Ox films at 77 K was 108 Ω for an overlapping area of 0.2 mm2, corresponding to a SrTiO3 resistivity of 4×109 Ω cm.We describe our procedure for fabricating YBa{sub 2}Cu{sub 3}O{sub {ital x}}-SrTiO{sub 3}-YBa{sub 2}Cu{sub 3}O{sub {ital x}} thin-film trilayer structures. Each film is grown {ital in} {ital situ} by excimer laser deposition onto a heated (100)MgO substrate. The geometrical configuration of each layer is defined by a metal mask; the vacuum chamber is opened between depositions to allow the targets and masks to be changed. The lower and upper YBa{sub 2}Cu{sub 3}O{sub {ital x}} films in the best trilayer structure had transition widths of 1 and 3 K (10--90%), respectively, and transition temperatures (zero resistance) of 87 K. The resistance between the YBa{sub 2}Cu{sub 3}O{sub {ital x}} films at 77 K was 10{sup 8} {Omega} for an overlapping area of 0.2 mm{sup 2}, corresponding to a SrTiO{sub 3} resistivity of 4{times}10{sup 9} {Omega} cm.

Sensitive YBa2Cu3O7−x thin‐film magnetometer

Our YBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital x}} thin-film magnetometer consists of a dc superconducting quantum interference device with bi-epitaxial Josephson junctions, fabricated on one chip, and a flux transformer with a multiturn input coil, fabricated on a second chip. Photolithographic processing is used to pattern all layers. The magnetometer operates in a flux-locked loop at temperatures up to 81 K, with the flux transformer improving the magnetic field sensitivity by a factor of 83{plus minus}3. The low-frequency rms magnetic field noise scales approximately as 1/{ital f}{sup 1/2}, where {ital f} is the frequency, with a magnitude of 0.6 pT Hz{sup {minus}1/2} at 10 Hz and 0.09 pT Hz{sup {minus}1/2} at 1 kHz with the magnetometer immersed in liquid N{sub 2}.Our YBa2Cu3O7−x thin‐film magnetometer consists of a dc superconducting quantum interference device with bi‐epitaxial Josephson junctions, fabricated on one chip, and a flux transformer with a multiturn input coil, fabricated on a second chip. Photolithographic processing is used to pattern all layers. The magnetometer operates in a flux‐locked loop at temperatures up to 81 K, with the flux transformer improving the magnetic field sensitivity by a factor of 83±3. The low‐frequency rms magnetic field noise scales approximately as 1/f1/2, where f is the frequency, with a magnitude of 0.6 pT Hz−1/2 at 10 Hz and 0.09 pT Hz−1/2 at 1 kHz with the magnetometer immersed in liquid N2.

Flicker (1/f) noise in biepitaxial grain boundary junctions of YBa2Cu3O7−x

At low frequencies f the 1/f noise power in single biepitaxial junctions of YBa2Cu3 O7−x peaks sharply for bias currents just above the noise reduced critical current and increases as I2 for high bias currents I. This behavior is explained by a model in which both critical current fluctuations δI0 and resistance fluctuations δR contribute to the measured voltage noise. The magnitude of the normalized critical‐current fluctuations ‖δI0/I0‖ is always much greater than that of the normalized resistance fluctuations ‖δR/R‖. Switching the bias current between positive and negative values at 2 kHz greatly reduces the magnitude of the 1/f noise from both sources, implying that the coherence of the noise generating process is not affected by the current reversal.

Sensitive YBa sub 2 Cu sub 3 O sub 7 minus x thin-film magnetometer

Our YBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital x}} thin-film magnetometer consists of a dc superconducting quantum interference device with bi-epitaxial Josephson junctions, fabricated on one chip, and a flux transformer with a multiturn input coil, fabricated on a second chip. Photolithographic processing is used to pattern all layers. The magnetometer operates in a flux-locked loop at temperatures up to 81 K, with the flux transformer improving the magnetic field sensitivity by a factor of 83{plus minus}3. The low-frequency rms magnetic field noise scales approximately as 1/{ital f}{sup 1/2}, where {ital f} is the frequency, with a magnitude of 0.6 pT Hz{sup {minus}1/2} at 10 Hz and 0.09 pT Hz{sup {minus}1/2} at 1 kHz with the magnetometer immersed in liquid N{sub 2}.Our YBa2Cu3O7−x thin‐film magnetometer consists of a dc superconducting quantum interference device with bi‐epitaxial Josephson junctions, fabricated on one chip, and a flux transformer with a multiturn input coil, fabricated on a second chip. Photolithographic processing is used to pattern all layers. The magnetometer operates in a flux‐locked loop at temperatures up to 81 K, with the flux transformer improving the magnetic field sensitivity by a factor of 83±3. The low‐frequency rms magnetic field noise scales approximately as 1/f1/2, where f is the frequency, with a magnitude of 0.6 pT Hz−1/2 at 10 Hz and 0.09 pT Hz−1/2 at 1 kHz with the magnetometer immersed in liquid N2.

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.