An integrated planar gradiometer based on a double relaxation oscillation SQUID

Abstract

The design and performance of an integrated planar gradiometer based on a double relaxation oscillation SQUID (DROS) are presented. The DROS was made from hysteretic Nb/AlO x /Nb junctions and the devices were fabricated by a simple four-level process. The signal SQUID loop is a gradiometric type with two square holes connected in parallel and a reference junction is used instead of the reference SQUID. The high flux-to-voltage transfer coefficient of typically 3 mV −1 0 enabled direct readout by a simple electronics with a modest voltage noise. The pickup coil integrated on the same wafer as the SQUID consists of two planar 10 × 10 mm 2 coils connected in series and has a baseline of 30 mm. The overall size of the device is 10 × 40 mm 2 . The field gradient noise is 2.6 fT cm −1 Hz −1/2 in the white region and 4.4 fT cm −1 Hz −1/2 at 1 Hz.

First-Order Planar Superconducting Quantum Interference Device Gradiometers With Long Baseline

“…Connections between the two die are made using superconducting wire bonds. This long baseline multi-chip gradiometer had a field gradient noise of 1.2 , better than a factor of two improvement over their earlier design [3]. Such long baseline gradiometers are very desirable for applications requiring sensitivity to deep-lying sources such as magnetocardiography (MCG) [5].…”

“…In 1981, the development of a transformer coupling scheme [2] led to improved thin-film planar gradiometers with a baseline of 4 cm, a projected factor of 100 improvement of the magnetic field gradient noise, and an intrinsic balance of better than 1 part in 10,000. In 1996, Lee et al [3] relaxation oscillation (DROS) SQUID integrated on a 1 cm 4 cm chip. This gradiometer had a baseline of 3 cm and exhibited a field gradient noise of 2.6 .…”

Double relaxation oscillation superconducting quantum interference devices with gradiometric layout

“…In order to make a sensitive magnetometer, the effective area of the DROSs has to be enhanced considerably, e.g., by connecting a relatively large superconducting pickup circuit to the input coils. This pickup circuit can either be integrated on the same chip as the DROS, 15 or it can be fabricated separately and connected to the input coil afterwards. In the latter case, the pickup circuit can be a wire wound coil 11,16 or a thin film device, 17 as in the present case.…”