  Magnetization dynamics described by the Landau-Lifshitz-Gilbert-Slonczewski
(LLGS) equation can be formulated to have the form of the well-known
two-level-system (TLS) equations. Recently, we showed that a DC spin-transfer
torque (STT) term in the LLGS equation can be mapped to a modulation of the
carrier relaxation rates in the analogous TLS equations. Here, we extend the
analogy to the TLS dynamics by including the AC magnetic field, AC
demagnetization field, and AC STT excitation that we show constitute the
interaction term in the analogous TLS picture. Interestingly, we find that the
carrier injection rate in the TLS equations that is responsible for transitions
between the excited and ground states of the system naturally translates to an
intense short magnetic pulse that reverses the magnetization state.
Furthermore, we also show that the two helicities of circularly polarized
magnetic pulses correspond to the two carrier injection rates in the analogous
TLS picture. In the context of the highly debated all-optical helicity
dependent switching experiment, it offers a new explanation of the
magnetization reversal from first principles.
