  We propose a theoretical framework of a magnetization switching induced
solely by a microwave. The microwave frequency is always close to but slightly
different from the oscillation frequency of the magnetization. By efficiently
absorbing energy from the microwave, the magnetization climbs up the energy
landscape to synchronize the precession with the microwave. We introduced a
dimensionless parameter $\epsilon$ determining the difference between the
microwave frequency and the instant oscillation frequency of the magnetization.
We analytically derived the condition of $\epsilon$ to switch the
magnetization, and confirmed its validity by the comparison with numerical
simulations.
