  We experimentally demonstrate a simple and robust protocol for the detection
of weak radio-frequency magnetic fields using a single electron spin in
diamond. Our method relies on spin locking, where the Rabi frequency of the
spin is adjusted to match the MHz signal frequency. In a proof-of-principle
experiment we detect a 7.5 MHz magnetic probe field of 40 nT amplitude with <10
kHz spectral resolution over a T_1-limited noise floor of 0.3 nT/rtHz.
Rotating-frame magnetometry may provide a direct and sensitive route to
high-resolution spectroscopy of nanoscale nuclear spin signals.
