  Quantum sensors offer unparalleled precision, accuracy, and sensitivity for a
variety of measurement applications. We report a compact magnetometer based on
a ferrimagnetic sensing element in an oscillator architecture that circumvents
challenges common to other quantum sensing approaches such as limited dynamic
range, limited bandwidth, and dependence on vacuum, cryogenic, or laser
components. The device exhibits a fixed, calibration-free response governed by
the electron gyromagnetic ratio. Exchange narrowing in the ferrimagnetic
material produces sub-MHz transition linewidths despite the high unpaired spin
density ($\sim 10^{22}$ cm$^{-3}$). The magnetometer achieves a minimum
sensitivity of 100 fT/$\sqrt{\text{Hz}}$ to AC magnetic fields of unknown phase
and a sensitivity below 200 fT/$\sqrt{\text{Hz}}$ over a bandwidth $\gtrsim \!
1$ MHz. By encoding magnetic field in frequency rather than amplitude, the
device provides a dynamic range in excess of 1 mT. The passive, thermal
initialization of the sensor's quantum state requires only a magnetic bias
field, greatly reducing power requirements compared to laser-initialized
quantum sensors. With additional development, this device promises to be a
leading candidate for high-performance magnetometry outside the laboratory, and
the oscillator architecture is expected to provide advantages across a wide
range of sensing platforms.
