Low-Noise Serial High- $T_{c}$ $\hbox{YBa}_{2}\hbox{Cu}_{3}\hbox{O}_{y}$ Superconducting Quantum Interference Devices Based on Bicrystal Junctions

The fabrication of high-T
_c
YBa
₂
Cu
₃
O
_y
(YBCO) superconducting quantum interference devices (SQUIDs) that produce low flux noise while maintaining a high fabrication yield continues to challenge scientists. In this paper, we report the characterization of low-noise serial high-T
_c
SQUIDs for biomagnetic applications. Epitaxial high-T
_c
YBCO thin films with a root-mean-square surface roughness less than 5 nm are optimized for the low-noise serial SQUID coupled to a common pickup coil to form magnetometers and gradiometers. Current-voltage curves, voltage-flux curves, and flux noise of SQUID devices are characterized. A smooth surface morphology across the bicrystal line is essential for SQUIDs to show good characteristics. The field sensitivity is improved by a factor of (2)
^1/2
with the designed serial SQUID array, which is coupled to a common pickup loop. This is demonstrated, and its fabrication yield is higher than 80% for devices with peak-to-peak voltage V
_PP
≥ 15 μV. The challenges and perspectives of the high-T
_c
serial SQUID array for biomagnetic applications are addressed and discussed.