  We study inelastic cotunneling through a strong Coulomb-blockaded quantum dot
subject to a static magnetic field and a perpendicular circularly-polarized
magnetic field using a quantum Langevin equation approach. Our calculation
predicts an interesting controllable cotunneling current characteristic,
splitting--zero-anomaly--splitting transition of the differential conductance
with increasing the driving frequency, ascribing to the role of {\em
photon-assisted spin-flip} cotunneling processes.
