  Sensing small magnetic fields is relevant for many applications ranging from
geology to medical diagnosis. We present a fiber-coupled diamond magnetometer
with a sensitivity of (310 $\pm$ 20) pT$/\sqrt{\text{Hz}}$ in the frequency
range of 10-150 Hz. This is based on optically detected magnetic resonance of
an ensemble of nitrogen vacancy centers in diamond at room temperature. Fiber
coupling means the sensor can be conveniently brought within 2 mm of the object
under study.
