The role of kinetic inductance on the performance of YBCO SQUID magnetometers

Abstract

Inductance is a key parameter when optimizing the performance of superconducting quantum interference device (SQUID) magnetometers made from the high temperature superconductor YBa 2 Cu 3 O 7−x (YBCO) because lower SQUID inductance L leads to lower flux noise, but also weaker coupling to the pickup loop. In order to optimize the SQUID design, we combine inductance simulations and measurements to extract the different inductance contributions, and measure the dependence of the transfer function V Φ and flux noise SA comparison between two samples shows that the kinetic inductance contribution varies strongly with film quality, hence making inductance measurements a crucial part of the SQUID characterisation. Thanks to the improved estimation of the kinetic inductance contribution, previously found discrepancies between theoretical estimates and measured values of V Φ and S 1/2 Φ could to a large extent be avoided. We then use the measurements and improved theoretical estimations to optimize the SQUID geometry and reach a noise level of S 1/2 B = 44 fT/ √ Hz for the best SQUID magnetometer with a 8.6 mm × 9.2 mm directly coupled pickup loop. Lastly, we demonstrate a method for reliable one-time sensor calibration that is constant in a temperature range of several kelvin despite the presence of temperature dependent coupling contributions, such as the kinetic inductance. The found variability of the kinetic inductance contribution has implications not only for the design of YBCO SQUID magnetometers, but for all narrow linewidth SQUID-based devices operated close to their critical temperature.

Properties of grooved Dayem bridge based YBa2Cu3O7−δ superconducting quantum interference devices and magnetometers

“…This result is almost a factor 2 lower than that which was previously obtained with GDB weak links 18 and represents the lowest field noise achieved in SQUID magnetometers implementing Dayem bridge based weak links. Moreover, the measured magnetic field noise is comparable with the lowest reported field noise values achieved with single-layer grain boundary-based SQUIDs galvanically coupled to a pick-up loop made on 10 mm×10 mm substrates 12,14,16,38,41 , S In conclusion, we have studied the temperature dependence of GDB based SQUIDs. At low temperatures, the GDBs behave like short and thin Dayem bridges, whereas at temperatures above…”

“…Here the effective area was separately measured via responsivity measurements using a calibrated Helmholtz coil. 38 In Table I we summarize the magnetic noise properties of three different SQUID magnetometers. Devices SQ2 and SQ3 were fabricated on the same chip sharing the same pick-up loop, whereas SQ1 was realized on a different chip.…”

“…Hz, achieved in devices based on grain boundary junctions [12][13][14][38][39][40] ). The lowest magnetic noise spectral density was achieved for SQ2 and is shown in Fig.…”

Studying Quantum Materials with Scanning SQUID Microscopy

“…The pickup loop of these chip-based planar SQUIDs is then scanned over the sample to produce magnetic field images. Fabrication procedures have been improved over the years, and scanning SQUIDs are now available with loops from 10 mm (15,16) to 40 nm (17) in diameter. Note that for small loops, the effective pickup area is larger than the loop's inner diameter owing to magnetic field line distortions from the finite loop thickness (18).…”

Determining the temperature-dependent London penetration depth in HTS thin films and its effect on SQUID performance

“…A technique like this was used for HTS SQUIDs by Forrester et al 17 and subsequently by others. [18][19][20][21][22][23][24] Measurements similar to ours have been made by several others on SQUIDs to find λ . Li et al 21 measured L sq for five nano-slit SQUIDs fabricated by a focused helium ion beam on a 25 nm thick YBCO film and found λ = 180 nm gave the best match to experiment at 9 K. Ruffieux et al 22,23 20 made earlier inductance measurements on seven different CPS SQUIDs made from 300 nm YBCO films and found the inductance per unit length L 0 = 0.97 pH/µm for w 1 = 2 µm and s = 4 µm.…”