The intrinsic imbalance of a Superconducting QUantum Interference Device (SQUID) gradiometer is a critical physical parameter that determines its common-mode rejection ratio(CMRR) and ultimate performance in unshielded environments. Traditional fabrication methods for high-temperature superconducting (HTS) Josephson junctions, such as bicrystal and step-edge techniques, suffer from strict substrate grain boundary constraints or poor etching reproducibility, making it difficult to precisely control the geometric symmetry and the electrical uniformity of the two pickup coils. To address this issue, this study explores a novel approach to optimize the imbalance of HTS planar gradiometers utilizing helium focused ion beam (He-FIB) technology.
A highly symmetric HTS planar gradiometer was designed and fabricated on a single-layer (YBCO) thin film. The He-FIB direct-write technology, featuring maskless and nanometer-level processing precision, was employed to precisely define the Josephson junctions via local irradiation-induced superconductor-normal metal-superconductor (SNS) transitions or superconductor-insulator-superconductor(SIS) transitions. The total footprint of the device was strictly limited to 5×10 mm² with a baseline length of 4.2 mm.
A systematic calibration experiment, utilizing homogeneous Helmholtz coils and gradient Maxwell coils, was established to evaluate the effective parasitic area and the effective gradient area. Extensive electrical and magnetic characterizations were performed at liquid nitrogen temperature 77 K. The fabricated devices exhibited high-quality resistively shunted junction (RSJ) model like current-voltage characteristics, with critical currents (I_c) ranging from 22.4 to 144 μA and voltage modulation depths of approximately 10 μV. To further suppress the low-frequency 1/f noise originating from the intrinsic defect states in the He-FIB irradiated barriers, an alternating current (AC) bias reversal scheme was applied, successfully pushing the 1/f noise corner frequency down to approximately 20 Hz. In the white noise region (>1 kHz), the gradiometer achieved an excellent magnetic field gradient resolution as low as 1.48 pT·cm^-1·Hz^-1/2. More importantly, the measured imbalance levels of the five prepared samples consistently fell within the narrow range of 2.7×10^-3 to 9.8×10^-3.
The experimental results demonstrate the superior reliability and high yield of the He-FIB technology in fabricating highly symmetric HTS planar gradiometers. Although the two-dimensional topology of the single-layer YBCO film limits the absolute imbalance to the 10^-3 level due to the unavoidable parasitic area of the SQUID loop itself, the overall performance is completely comparable to, and in terms of yield surpasses, state-of-the-art devices fabricated by complex multilayer or bicrystal techniques.
Combined with software-based reference compensation, the achieved balance level and gradient resolution provide a solid technological foundation for next-generation unshielded magnetic detection applications, including deep-earth geophysical exploration and mobile magnetoencephalography.