  We report a new spin manipulation protocol for periodically reversing the
sample magnetization for Magnetic Resonance Force Microscopy. The protocol
modulates the microwave excitation frequency synchronously with the position of
the oscillating detection cantilever, thus allowing manipulation of the spin
magnetization independent of both magnetic field gradient strength and
cantilever response time. This allows continuous variation of the detected
sample volume and is effective regardless of spin relaxation rate. This
enhanced flexibility improves the utility of MRFM as a generally applicable
imaging and characterization tool.
