  Quantum sensors based on spin defect ensembles have seen rapid development in
recent years, with a wide array of target applications. Historically, these
sensors have used optical methods to prepare or read out quantum states.
However, these methods are limited to optically-polarizable spin defects, and
the spin ensemble size is typically limited by the available optical power or
acceptable optical heat load. We demonstrate a solid-state sensor employing a
non-optical state preparation technique, which harnesses thermal population
imbalances induced by the defect's zero-field splitting. Readout is performed
using the recently-demonstrated microwave cavity readout technique, resulting
in a sensor architecture that is entirely non-optical and broadly applicable to
all solid-state paramagnetic defects with a zero-field splitting. The
implementation in this work uses Cr$^{3+}$ defects in a sapphire (Al$_2$O$_3$)
crystal and a simple microwave architecture where the host crystal also serves
as the high quality-factor resonator. This approach yields a near-unity filling
factor and high single-spin-photon coupling, producing a magnetometer with a
broadband sensitivity of 9.7 pT/$\sqrt{\text{Hz}}$.
