Magnetometer based on transfer and modulation of magnetic flux using high-critical-temperature superconductor coils

We developed a new type of magnetometer to measure low-frequency magnetic fields with an operational principle based on magnetic flux transfer and modulation. This magnetometer consists of three coils: pickup, input and readout coils. The pickup and input coils were made from high-critical-temperature superconductor (HTS) tape and were connected with very low connection resistance to form a closed loop. The magnetic flux that is collected by the pickup coil can be transferred to the input coil even at low frequencies below 1 Hz. The magnetic flux at the input coil is then detected by the readout coil using a readout scheme based on modulation of the mutual inductance M between the input and readout coils. To modulate M over time, a magnetic wire was inserted into the readout coil and a time-varying current was supplied to the wire. Using this time-varying M, the magnetic flux is converted into an amplitude-modulated voltage across the readout coil for measurements. A prototype magnetometer was fabricated for demonstration. This magnetometer can operate at low frequencies down to f = 0.5 Hz without responsivity degradation. The magnetic field noise levels were 8 and 60 pT/Hz^1/2 at 50 and 1 Hz, respectively.