Long Baseline 2nd-Order High- $T_{\rm c}$ SQUID Gradiometer Made From a Single-Layer ${\rm YBa}_{2}{\rm Cu}_{3}{\rm O}_{7}$ Film

We have fabricated a 30 mm long baseline 2nd-order SQUID gradiometer from a single-layer YBa
₂
Cu
₃
O
₇
film. The gradiometer was patterned from a YBa
₂
Cu
₃
O
₇
film on a sapphire substrate by argon ion milling with a photoresist mask. The device had an overall size of 70.2 mm times 10.6 mm and a baseline of 30 mm with three parallel-aligned 10.2 mm times 10.2 mm pickup loops coupled directly to a dc SQUID to measure part
²
B
_z
/partx
²
. Josephson elements of the device were YBa
₂
Cu
₃
O
₇
nanobridges which were made by a focused ion beam technique. Design of the gradiometer was based on the same rules as those of 2.9 mm short baseline gradiometers. Gradient sensitivity of the gradiometer was estimated to be ~ 1.8 times 10
^-10
T/m
²
/Hz
^1/2
, which was equivalent to a field noise of 160 fT/Hz
^1/2
, for an intrinsic SQUID flux noise of 10
^-5
Phi
₀
/Hz
^1/2
. We also studied the transition properties of the YBa
₂
Cu
³
O
₇
nanobridge. The bridge showed an superconductor- normal metal-superconductor (SNS) behavior with the top Au layer intact and an superconductor-insulator-superconductor (SIS) behavior with Au removed. We believe that the Josephson nature of the bridge is ascribed to naturally formed grain boundaries in the bridge.