  A common understanding of magnetization switching in microwave-assisted
magnetization reversal is that the rotation direction of the microwaves should
be the same with the precession direction of the magnetization. In this letter
however, we show that microwaves initially rotating opposite to the
magnetization precession destabilize the magnetization at an equilibrium and
induces the switching more efficiently, when the microwave frequency depends on
time. This argument is analytically deduced from energy balance equation. We
also establish a model satisfying this condition, and confirm magnetization
switching solely by microwaves by using numerical simulation.
