Integrated multilayer high-Tc SQUID magnetometers with slotted washer

We have fabricated and characterized multilayer high-T_c SQUID magnetometers designed for a magnetically unshielded environment. The magnetometers consist of YBa₂Cu₃O_7-x(YBCO)/SrTiO₃ (STO)/YBCO trilayers deposited by pulsed laser deposition (PLD) on an STO bi-crystal substrate. We fabricated SQUIDs with slotted washers at the bottom or top YBCO layer. The slots consist of 4 μm wide superconducting lines separated by 8 μm. A flux transformer with a multi-turn input coil was defined on the top or bottom YBCO layer, and the SQUID washer was used as a crossover pattern to simplify the fabrication process. The pickup loop size was 4 mm×7 mm, and the magnetometer effective area was found to be 0.67 mm². The SQUID inductance was 85 pH. For the operation of our SQUID in unshielded environments, compact 3 MHz modulation flux locked loop (FLL) electronics with bias-reversal were developed. The system bandwidth was greater than 500 kHz, and maximum slew rate was about 10⁶ Φ₀/sec. The flux noise spectra of the magnetometers were measured and compared with our single layer devices. We also investigated the effect of slots on the coupling strength between the input coil and SQUID