An atomic magnetometer system is disclosed that includes a variable magnetic field source (14) configured to provide an oscillating primary magnetic field to cause a sample (16) to produce a secondary magnetic field. The atomic magnetometer system also includes an atomic magnetometer for detecting the secondary magnetic field. The atomic magnetometer includes an atomic specimen, a pump and probe subsystem configured to pump the atomic specimen to create a polarisation and to probe atomic coherence precession within the atomic specimen with a probe beam, a detector configured to detect the probe beam to produce a detection signal. The system is configured to drive the variable magnetic field source (14) in dependence on the detection signal with a frequency tuned to rf resonance. A method of operating an atomic magnetometer is also disclosed.