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ARIADNE (Axion Resonant InterAction Detection Experiment) is a table-top experiment that intends to search for QCD axions from exotic spin-dependent interactions mediated by axion between nuclei at sub-mm range. This experiment includes a non-magnetic mass to source the axion field, and a dense ensemble of hyper-polarized 3He nuclei to detect the axion field with nuclear-magnetic-resonance (NMR)-based method. The expected NMR signal from the interaction could be easily buried in the noise spectrum of the magnetometer, especially in a frequency range (~ 100 Hz) where the interaction signal is supposed to exist. In this work, we report optimization of SQUID gradiometer for ARIADNE including noise spectrum measurement. [ABSTRACT FROM AUTHOR]

We have developed a thin-film dc SQUID susceptometer for precision measurements of the magnetization of microscopic samples. The miniature SQUID susceptometer, fabricated using a Nb/Al - A1Ox/Nb Josephson junction process on a 2-mm x 3-mm chip, consists of two 40-μm ID pickup ioops in a series, ‘figure-of-eight’ gradiometric configuration that forms the SQUID inductance and two magnetizing field coils, one centered on each pickup loop. The magnetizing field coils, which apply equal and parallel magnetic fields to each pickup loop, may be operated in persistent current mode using an on-chip thin-film persistence switch. The sample to be measured is placed inside one of the pickup loops, and the SQUID gradiometer is used to sense the change in magnetization of the sample as the temperature is varied. The SQUID susceptometers can be operated using conventional locked-loop feedback electronics over the temperature range from 0 to around 6 K with a typical white flux noise of 3 μΦ0 /Hz1/2 (rms) at 4 K. These susceptometers are currently being used to measure the magnetization of Mn- and Fe-doped Pd samples. [ABSTRACT FROM AUTHOR]

We have developed several types of low-Tc first-order planar gradiometers with long baselines ranging from 3.6 cm to 4 cm. Thin-film planar gradiometers are very attractive for measurements of the off-diagonal components of the magnetic field gradient tensor in noisy environments because of the high intrinsic balance that can be achieved with the precision photolithographic techniques used to fabricate these devices. The gradiometer pickup loops are series configured and connected to the input circuit of a dual-washer gradiometric dc SQUID. Series and parallel SQUID washer configurations have been investigated. For a given chip size, model calculations using a magnetic dipole point source and a distributed current dipole source show that gradiometer performance can be improved by increasing the size of the pickup loops at th expense of a small reduction of the baseline length. Based on these results, an improved planar gradiometer has been developed with 1.175 cm x 1.175 cm square pickup loops integrated with a series-configured dual washer SQUID on a 1.2cm x 4.8cm chip. The white flux noise of the improved gradiometer with 3.6 cm baseline is as low as 2.4 μΦ0/Hz1/2 (rms), and the magnetic field sensitivity referred to one pickup loop is 0.63 nT/Φ0. This results in a magnetic field gradient noise of 0.42 fT/cm - Hz1/2. The gradiometers are operable unshielded in typical laboratory environments without losing lock. [ABSTRACT FROM AUTHOR]

A six-layer process is described for the fabrication of Josephson junction devices and Superconducting QUantum Interference Devices (SQUIDs) on silicon wafers up to 150 mm in diameter. The Nb/Al - AlOx/Nb trilayers are sputter-deposited in a load-locked vacuum chamber with base pressure in the low 10-7 Pa (10-9 Torr) range and a very low leak up rate of 5 × 10-6 Pa/min (3.5 × 10-5 mTorr/min). The barrier process is optimized for a critical current density of 100 A/cm², and all resistors are fabricated from palladium with a sheet resistivity of 1Ω /sq. A charge dissipative dielectric layer is used in order to reduce the probability of damage caused by electrostatic charge build-up and discharge. The trilayer films are wet etched, while the subsequent metal and dielectric films are patterned using fluorine-based reactive ion etch, Ar ion milling, or lift-off techniques. This process is currently being used to fabricate dc SQUIDs for various instrumentation applications, integrated SQUID magnetometers and planar gradiometers, and single tunnel junction detectors. [ABSTRACT FROM AUTHOR]