Design and Fabrication of Directly-Coupled HTS-SQUID Magnetometer With a Multi-Turn Input Coil

To improve the field resolution of a magnetometer for use in the Earth´s field, we have developed a new magnetometer using a hybrid superconducting quantum interference device (SQUID). A multi-turn input coil (N_i turns) is inserted between a directly-coupled SQUID and a pickup coil in series in the hybrid SQUID. Screening current couples with the SQUID by both direct current injection and magnetic coupling. To avoid flux trapping, which occurs during cooling in the Earth´s field, the width of the SQUID is restricted to be 5 μm. Therefore, the input coil is not stacked on the SQUID inductor like a conventional integrated SQUID but formed around the SQUID. There is an optimal N_i as a result of the trade-off relationship between the increase of the coupling strength and the decrease of the screening current with increasing N_i. The magnetometer using the hybrid SQUID with a slit hole inductor showed an effective area of 0.45 mm>;² at the optimal N_i of 3. The magnetic coupling was enhanced by using a square hole inductor that had a larger flux capturing area. The obtained A_eff (~ 0.6 mm²) was almost doubled from that of the conventional directly coupled magnetometer, resulting in a field noise as low as 18 fT/Hz^1/2 (white).