  We demonstrate a cavity-enhanced room-temperature magnetic field sensor based
on nitrogen-vacancy centers in diamond. Magnetic resonance is detected using
absorption of light resonant with the 1042 nm spin-singlet transition. The
diamond is placed in an external optical cavity to enhance the absorption, and
significant absorption is observed even at room temperature. We demonstrate a
magnetic field sensitivity of 2.5 nT/sqrt(Hz), and project a photon
shot-noise-limited sensitivity of 70 pT/sqrt(Hz) for a few mW of infrared
light, and a quantum projection-noise-limited sensitivity of 250 fT/sqrt(Hz)
for the sensing volume of 90 um x 90 um 200 um.
