  We describe our approach to atomic magnetometry based on the push-pull
optical pumping technique. Cesium vapor is pumped and probed by a resonant
laser beam whose circular polarization is modulated synchronously with the spin
evolution dynamics induced by a static magnetic field. The magnetometer is
operated in a phase-locked loop, and it has an intrinsic sensitivity below
20fT/\sqrt(Hz) using a room temperature paraffin-coated cell. We use the
magnetometer to monitor magnetic field fluctuations with a sensitivity of
300fT/\sqrt(Hz).
