  We present a highly sensitive miniaturized cavity-enhanced room-temperature
magnetic-field sensor based on nitrogen-vacancy (NV) centers in diamond. The
magnetic resonance signal is detected by probing absorption on the 1042\,nm
spin-singlet transition. To improve the absorptive signal the diamond is placed
in an optical resonator. The device has a magnetic-field sensitivity of 28
pT/$\sqrt{\rm{Hz}}$, a projected photon shot-noise-limited sensitivity of 22
pT/$\sqrt{\rm{Hz}}$ and an estimated quantum projection-noise-limited
sensitivity of 0.43 pT/$\sqrt{\rm{Hz}}$ with the sensing volume of $\sim$ 390
$\mu$m $\times$ 4500 $\mu$m$^{2}$. The presented miniaturized device is the
basis for an endoscopic magnetic field sensor for biomedical applications.
