We developed a magnetometer for measuring low-frequency magnetic fields. The magnetometer consists of two coils, namely, pickup and modulation coils made of high-critical-temperature superconducting material. The inductance of the modulation coil is modulated with time, and the magnetic field signal is converted to an amplitude-modulated voltage across the modulation coil. The prototype magnetometer can operate at low frequencies down to f = 0.01 Hz without diminishing the response. The noise in the magnetic field signal is 40 pT/Hz1/2 at 10 Hz. An order of magnitude improvement will be possible with a modulation coil of suitable geometry.
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