Fabrication of Low-Noise HTS-SQUID Gradiometers and Magnetometers With Ramp-Edge Josephson Junctions

We have studied the fabrication process of HTS-SQUIDs with ramp-edge Josephson junctions (JJs). SmBa
₂
Cu
₃
O
_y
(SmBCO) and Er
_0.95
La
_0.1
Ba
_1.95
Cu
₃
O
_y
(L1ErBCO) having
T
_c
's around 90 K were used for base- and counter-electrodes, respectively. SrSnO
₃
(SSO) was used for insulating layers. A black-colored Pr
_1.4
Ba
_1.6
Cu
_2.6
Ga
_0.4
O
_y
(P4G4) layer was first grown on MgO substrate via a thin BaZrO
₃
buffer layer. It was expected to work as a temperature homogenizer over the whole substrate area during deposition of the upper layers. We fabricated planar gradiometers having pickup coils with sizes of 0.5 × 1.0, 1.0 × 1.0 and 3.0 × 3.0 mm
²
. Their baselines are 1.0, 1.0 and 3.0 mm, respectively. The gradiometers exhibited flux noise at 77 K as low as 3.3 ~ 4.5 μΦ
₀
/Hz
^1/2
at 1 kHz. Directly and inductively coupled magnetometers having a 13.5 mm square pickup coil were also fabricated. The latter has an integrated 20-turn input coil. They showed field noise levels of 55 and 10 fT/Hz
^1/2
at 1 kHz, respectively.