Mn. Keene et al., HTS SQUID MAGNETOMETERS WITH INTERMEDIATE FLUX TRANSFORMERS, IEEE transactions on applied superconductivity, 7(2), 1997, pp. 3048-3051
We have modeled and fabricated a novel HTS magnetometer with a d.c. SQ
UID which couples to an on-chip flux transformer via an intermediate f
lux transformer. This configuration allows high efficiency coupling be
tween a 65 mm(2), 28 nH pickup loop and a low inductance (<15 pH) SQUI
D. The small SQUID inductance is required for low noise and high parti
al derivative V/partial derivative Phi so that direct readout schemes
may be used. The intermediate flux transformer was flip-chipped with a
sub-micron separation to the main substrate. The magnetometer makes u
se of CAM junction technology, double thickness (approximate to 0.7 mu
m) YBa2Cu3O7 layers, and a groundplaned SQUID slot. The measured resp
onsivity at 77 K was 0.57 mm(2) (3.6 nT/Phi(0)) with partial derivativ
e V/partial derivative Phi = 98 mu V/Phi(0) which is close to that pre
dicted by our model. No resonances, which degrade the performance of s
imilarly configured LTS devices, were observed. We attribute this to t
he use of PrBa2Cu3O7 as the isolation barrier. Excess white noise, pro
bably due to Johnson noise from the PrBa2Cu3O7, limited the sensitivit
y in our first device to 33 fT/root Hz at 77 K.