Dual bias current operation of 2-SQUID directly coupled magnetometers

High-T_c directly coupled magnetometers made from thin films of single layer YBa₂Cu₃O_y deposited on bicrystal substrates were investigated. A series 2-SQUID structure with a common pickup coil was designed to increase voltage modulation depth (ΔV). A second bias current was introduced to compensate for the disagreement between the two optimal bias currents and the voltage peak positions. The distribution of the second bias current was controlled by selecting the proper electrodes formed on the pickup coil, thus eliminating the difference in the voltage peak positions. The value of ΔV was doubled. The white noise in the magnetometer decreased by using this 2-SQUID design, but the low-frequency noise increased slightly. In addition, another type of the 2-SQUID magnetometer was proposed. This magnetometer has SQUIDs with different inductances. The overlapped V-Φ curve showed the modulation beat caused by the difference in V-Φ periodicity, and a high ΔV was observed at several Φ₀ intervals