Influence of shape anisotropy on magnetization reversal induced by nonlinear down-chirp microwave pulse

It has been demonstrated that a circularly polarized linear down-chirp microwave field pulse (LDCMWP) reverses a single-domain of high anisotropy magnetic nanoparticle but not efficient as expected. Therefore, based on the Landau-Lifshitz-Gilbert equation, we study the nonlinear down-chirp microwave field pulse (NLDCMWP) driven magnetization reversal, which can induce fast and energy-efficient reversal since the reversal time is close to the theoretical limit and the required field amplitude and initial frequency band are smaller than that of the LDCMWP. The fast and energy-efficient magnetization reversal is obtained because the frequency change of the NLDCMWP closely matches the frequency change of the magnetization precession which leads to efficient stimulated microwave energy absorption (emission) by (from) the magnetic particle before (after) it crosses over the energy barrier. Moreover, we found that the enhancement of hard anisotropy reduces the initial frequency band and the microwave amplitude of NLDCMWP and the material with the larger damping is better for fast magnetization reversal. These investigations may give a pathway to realize the fast and energy-efficient magnetization reversal.