Understanding dynamic frequency-matching resonance driven magnetic switching

Aiding magnetization reversal of a single domain particle usingan ac magnetic field at resonance condition can lower the switching field threshold. This is the underlying mechanism for microwave assisted magnetic recording. However, the resonance frequency changes during the reversal of magnetic particles with uniaxial anisotropy. For an ac field with constant frequency, resonance conditions cannot be maintained during a reversal process, thereby limiting the effectiveness of the ac field assistance. In this paper, we provide an in-depth analysis on the effect of time varying frequency ac field for the magnetization reversal. It is found that matching the ac field frequency with the particle’s resonance frequency would yield a 90° phase angle between the field and the magnetization, the optimal condition for maximizing driving torque by the ac field. Matching this optimal condition, even approximately, magnetization reversal can be solely accomplished by the ac field alone.