Superconducting Kinetic Inductance Field-Frequency Sensors: High-Sensitivity Magnetic Field Sensing in Moderate Background Fields

We present a lumped-element superconducting resonator magnetometer fabricated from a thin film of NbTiN. By taking advantage of the kinetic inductance non-linearity of the superconductor, we achieve frequency shifts of order 1 MHz for 1 mG field changes in a perpendicular magnetic field of 600 G. We show that field modulation up to 10 kHz can be sensed at a temperature of 2K. Our device operates in background fields as large as 2000 G, and can be designed to operate at even higher fields. This feature makes it attractive for applications requiring high sensitivity magnetometry at moderate to high magnetic fields, where superconducting quantum interference devices are typically unable to operate. One such application is x-band pulsed electron spin resonance, which typically requires large magnetic fields and becomes particularly susceptible to magnetic field fluctuations of the order of 10 ppb when measuring spin systems with long coherence times. While traditional field-frequency locking techniques based on NMR have been used to compensate for slow magnetic field drifts, our resonator devices can provide the required field sensitivity for higher noise frequencies.