Low-frequency Flux Noise Research Articles

Low-frequency flux noise and visualization of vortices in a YBa2Cu3O7 dc superconducting quantum interference device washer with an integrated input coil

We used low-temperature scanning electron microscopy (LTSEM) for imaging quantized magnetic flux (vortices) in dc superconducting quantum interference devices (SQUIDs) with approximately 1 μm spatial resolution at temperature T=77 K in a controllable magnetic field up to 20 μT. We demonstrate that LTSEM allows to image the spatial distribution of vortices in a YBa2Cu3O7/SrTiO3/YBa2Cu3O7 multilayer thin-film structure consisting of a dc SQUID washer with an integrated input coil on top. Simultaneously, we are able to measure the low-frequency noise of the sample under test, which allows to correlate the spatial distribution of vortices with low-frequency noise in the SQUID.

Low-frequency flux noise in YBCO dc SQUIDs cooled in static magnetic fields

The low-frequency flux noise in bicrystal and step-edge YBa2Cu3Ox dc SQUIDs has been investigated. The width, w, of the superconducting strips forming the SQUID frame was varied from 4 to 42 µm. The SQUIDs were cooled in static magnetic fields up to 150 µT. Two types of low-frequency noise dominated, namely 1/f-like noise and random telegraph noise giving a Lorentzian frequency spectrum. The 1/f noise performance of the w = 4, 6 and 7 µm SQUIDs was almost identical, while the SQUIDs with w = 22 and 42 µm showed an order of magnitude higher noise level. Our analysis of the data suggests an exponential increase of the 1/f noise versus the cooling field, exhibiting a characteristic magnetic field around 40 µT.

Planar SQUID gradiometers fabricated on 24° and 30° SrTiO/sub 3/ bicrystals

HTS dc SQUID gradiometers have been fabricated on 24/spl deg/ and 30/spl deg/ SrTiO/sub 3/ bicrystal substrates with slots and flux dams in the SQUID washers. The spatial response of such devices has been measured experimentally and compared with modelled results. Single layer devices, in terms of gradient sensitivity, have characteristics that deviate only slightly from idealized first order gradiometers. The low frequency flux noise of these devices is discussed with particular emphasis on both the unshielded properties and the effect of various cooldown procedures on the noise.

Characterization of the low-frequency noise in high-temperature radio frequency superconducting quantum interference devices cooled in static magnetic fields

We have performed a study of the low-frequency noise in high-temperature radio frequency (rf) superconducting quantum interference devices (SQUIDs) cooled in static magnetic fields. At 77 K, the noise of different thin-film YB2C3O7−x washer rf SQUIDs was measured in static magnetic fields up to 55 μT. The SQUID inductances ranged from 150 to 300 pH. The measured low-frequency flux noise of the SQUIDs SΦ1/2 at 1 Hz was between 70 and 350 μΦ0/√Hz, and in terms of magnetic field noise SB1/2 between 120 and 1100 fT/√Hz. The low-frequency noise of all devices at 1 Hz showed no significant increase for all applied static fields.

Low noise operation of integrated YBa2Cu3O7 magnetometers in static magnetic fields

The noise of two integrated YBa2Cu3O7-SrTiO3-YBa2Cu3O7 multilayer magnetometers in static magnetic fields up to 110 μT is investigated: An inductively coupled magnetometer with integrated flux transformer and a multiloop magnetometer. In both samples, only a moderate increase of the low frequency flux noise is found in high fields, due to the high epitaxial quality of the involved multilayer films. So for moderately shielded or unshielded applications in the earth’s magnetic field, high-quality integrated YBa2Cu3O7 magnetometers can be operated with low excess noise.

Excess low-frequency flux noise in dc SQUIDs

We have fabricated dc superconducting quantum interference devices (SQUIDs) incorporating Nb/Al-oxide/Nb Josephson junctions in both stripline and washer geometries. Low-frequency noise in excess of that predicted by the resistively-shunted junction model is present in both geometries and is demonstrated to be flux noise. This flux noise is not environmental. Improvements in fabrication processing over the past four years have reduced the level of this flux noise. SQUIDs are now fabricated with PdAu resistors, Nb wiring layers, and SiO/sub 2/ interlayer dielectric. In our best well-coupled SQUIDs, the white-noise energy sensitivity is 5/spl times/10/sup -31/ J/spl middot/s, with a 1/f knee below 0.1 Hz. We believe further reduction in the flux noise might be obtained with the use of on-chip flux shielding and/or trapping structures.

Flux dam, a method to reduce extra low frequency noise when a superconducting magnetometer is exposed to a magnetic field

This letter discusses the effects of a moderate magnetic field applied to a SQUID magnetometer and introduces the invention of the ‘‘flux dam.’’ A flux dam is a weak link or Josephson junction in series with the pickup loop which allows magnetic flux to enter and leave the inside of the superconducting loop easily for moderate changes in magnetic field. This largely prevents the flux lines from entering and being pinned in the superconducting parts of the magnetometer which would create extra low frequency noise. SQUID magnetometers with flux dams in the pickup loop can be operated in the Earth’s magnetic field without a significant increase in low frequency flux noise.

Effects of radio frequency radiation on the dc SQUID

The effects of radio frequency radiation on the dc SQUID are examined. Simulations show how the shape of the SQUID transfer characteristic is distorted by radio frequency interference (RFI). How this affects three commonly used SQUID modulation methods is discussed, and the results explain why we experimentally observe the bias current reversing readout method to be the least susceptible to RFI. The commonly seen increase in the low frequency flux noise power spectrum of dc SQUIDs in unshielded environments is also explained.

Thin-film flux transformers of YBa/sub 2/Cu/sub 3/O/sub 7-x/

A three-layer in situ laser deposition process was used to construct superconducting thin-film flux transformers of YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO). The transformers are designed for efficient coupling to a planar thin-film DC SQUID, have 0.7-cm/sup 2/ magnetic field pickup areas, and have ten-turn input coils. When coupled to low transition temperature (T/sub c/) SQUID, the resulting hybrid magnetometer exhibits excess low-frequency flux noise which arises in the transformer. This noise depends on the geometry of the flux transformer, and the observed behavior agrees with model calculations of the expected contribution from flux motion in the YBCO. The hybrid magnetometer attains a magnetic field sensitivity of about 0.9 pTHz/sup -1/2/ at 1 Hz with the transformer at 60 K; the RMS noise decreases as 1/f/sup 1/2/ at 1 Hz up to a frequency f approximately=1 kHz. It is believed that the sensitivity is high enough for use, for example, in magnetocardiography.

Excess low-frequency flux noise in dc SQUIDS incorporating Nb/AIOXIDE/Nb Josephson junctions

We have fabricated thin-film do SQUIDs (Superconducting QUantum Interference Devices) incorporating Nb/AlOxide/Nb tunnel junctions. The spectral density of the voltage noise, Sv, of stripline SQUIDs is characterized between 1 Hz and 2000 Hz. In this frequency range, S v is proportional to the square of the responsivity (ϖV/ϖΦ) over a significant range of bias conditions with an unusual frequency dependence. In a 7 pH SQZIK the spectral density of the flux noise, S Φ, at 1 Hz is less than 10 −11 Φ 0 2/Hz, where Φ 0 ≡ h/2 e. The observed noise does not appear to be environmental; also, it is independent of the value of the junction shunt resistance and whether the stripline material is a Pbln alloy or Nb. Subject to the constraint of a constant product of the junction area, critical current density, and SQUID self-inductance in the SQUIDs studied, S Φ is inversely proportional to the junction area.

Excess low-frequency flux noise in de SQUIDS incorporating Nb/Al_oxide/Nb Josephson junctions

Excess low-frequency flux noise in de SQUIDS incorporating Nb/Al_oxide/Nb Josephson junctions

Performance of dc SQUIDs with resistively shunted inductance

We have fabricated and characterized the noise performance of well-coupled Nb-Pb alloy edge junction dc superconducting quantum interference devices (SQUIDs) with a resistively shunted inductance. The extra damping resistor removes the resonances in the current-voltage characteristics resulting in a very low value of the extrinsic energy sensitivity in flux-locked-loop operation. Numerical simulations, with the junction capacitance included, indicate that the damping resistance does not degrade noise performance until it is significantly less than the junction-shunting resistance. We also tested different SQUID designs and demonstrated that a large SQUID inductance, proposed earlier to be desirable when resistively shunted, can seriously affect the low-frequency flux noise.

Flux noise and flux creep in YBCO thin films

The authors used a DC SQUID (superconducting quantum interference device) to measure the low-frequency magnetic flux noise produced by thin-film rings of YBa/sub 2/Cu/sub 3/O/sub 7- delta / (YBCO) with various microstructures. Below the transition temperature T/sub c/ of the YBCO, the spectral density of the noise scales as 1/f (f is the frequency) from 1 Hz to 1 kHz. This noise generally increases with temperature and vanishes abruptly at T/sub c/. Improvements in crystalline microstructure greatly reduce the magnitude of the noise, which was lowest for a highly orientated sample with its c-axis perpendicular to the substrate. Making a radial cut to interrupt current paths around the sample ring does not significantly affect the magnitude of the noise, demonstrating that the noise arises from a local mechanism such as the thermally activated hopping of flux bundles. Flux creep was observed in one sample cooled in a magnetic field of 1 mT, and the creep rate exhibited a sharp maximum near 80 K. It is concluded that SQUIDs and flux transformers of YBCO must be fabricated from highly orientated films to obtain low noise at low frequencies.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Magnetic flux noise in thin-film rings of YBa2Cu3O7−δ

The low-frequency magnetic flux noise in thin-film rings of YBa2Cu3O7−δ (YBCO) is measured over the temperature range 1.3–125 K by means of a dc superconducting quantum interference device (SQUID) maintained at liquid-helium temperatures. Below the transition temperature Tc of the YBCO, the spectral density of the noise scales as 1/f, where f is the frequency, and generally increases with increasing temperature. The magnitude of the noise depends strongly on the microstructure of the film, and is lowest for a sample which is predominantly oriented with its c axis perpendicular to the substrate. These results imply that SQUID’s and flux transformers of YBCO must be fabricated from highly oriented films to produce good resolution at low frequencies.

Excess noise in dc SQUIDs from 4.2K to 0.022K

Four types of excess noise have been identified in dc SQUIDs operated in the temperature range 0.022 to 4.2 K. At temperatures between about 2 and 4 K, the spectral density of the low frequency flux noise of a wide variety of thin-film dc SQUIDs scales as 1/fmwhere m = 1.0 ± 0.1. In SQUIDs with Nb loops the noise originates as an flux noise, whereas in those with Pb or PbIn loops the noise is substantially lower and originates in critical current fluctuations. When any of these devices is cooled to temperatures below about 0.5 K, the spectral density of the excess flux noise scales as 1/fm, with m = 0.66 ± 0.08 in most cases, and the noise always originates as an apparent flux noise. At the lowest temperatures, the white noise saturates at an effective temperature of about 150 mK; this excess noise probably arises from self-heating in the resistance shunting each tunnel junction.

A novel modulation technique for 1/f noise reduction in dc SQUIDs

We describe a novel modulation scheme for dc SQUIDs in flux locked loop applications that cancels the effects of Josephson junction critical current fluctuations. The scheme involves switching the SQUID between three states and detecting the output at the second harmonic of the modulation frequency. With this scheme we have achieved a ten fold reduction of the low frequency flux noise spectral density over that obtained with a conventional flux locked loop readout. A dc SQUID with a 20 pH inductance had a resultant noise spectral density of 10-12 \Phi\min{0}\max{2} /Hz at 0.1 Hz, three times lower than the lowest flux noise previously reported for any SQUID system at this frequency.