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Magnetization reversal condition for a nanomagnet within a rotating magnetic field

The reversal condition of magnetization in a nanomagnet under the effect of rotating magnetic field generated by a microwave is theoretically studied based on the Landau-Lifshitz-Gilbert equation. In a rotating frame, the microwave produces a dc magnetic field pointing in the reversed direction, which energetically stabilizes the reversed state. We find that the microwave simultaneously produces a torque preventing the reversal. It is pointed out that this torque leads to a jump in the reversal field with respect to the frequency. We derive the equations determining the reversal fields in both the low- and high-frequency regions from the energy balance equation. The validities of the formulas are confirmed by a comparison with the numerical simulation of the Landau-Lifshitz-Gilbert equation.