Magnetometer Using Inductance Modulation of High-Critical-Temperature Superconducting Coil for Low-Frequency Field Measurement in Presence of Excitation Fields

We have developed a new type of magnetometer using a high-critical-temperature superconductor (HTS) coil. This magnetometer consists of pickup and input coils made from HTS tape. The two coils are connected with very low joint resistance, and the signal flux that is collected by the pickup coil is transferred to the input coil. The signal at the input coil is then read out using a newly developed inductance-modulation scheme. The inductance of the input coil is modulated over time using a magnetic wire that is inserted into the input coil, and the magnetic flux is converted into a signal voltage using this time-dependent inductance. It is demonstrated that this magnetometer can measure low-frequency signal fields down to 0.03 Hz without any reduction in responsivity. The noise level was measured to be 0.85 pT/Hz(1/2) at frequencies above 25 Hz and reached 11 pT/Hz(1/2) at 1 Hz when using a pickup coil with 50 turns and an average diameter of 25 mm.