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

We have developed several types of low-T_c 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 the expense of a small reduction of the baseline length. Based on these results, an improved planar gradiometer has been developed with 1.175 cm times 1.175 cm square pickup loops integrated with a series-configured dual washer SQUID on a 1.2 cm x 4.8 cm chip. The white flux noise of the improved gradiometer with 3.6 cm baseline is as low as 2.4 muPhi₀/Hz^1/2 (rms), and the magnetic field sensitivity referred to one pickup loop is 0.63 nT/Phi₀. This results in a magnetic field gradient noise of 0.42 fT/cm - Hz^1/2. The gradiometers are operable unshielded in typical laboratory environments without losing lock.