nonlinear effects of magnetization dynamics in nanosize ferromagnetic disks
Feng Guo1,2, Han-Jong Chia1,2, Lyuba M. Belova4 and Robert D. McMichael1
We have developed a ferromagnetic resonance force microscope (FMRFM) to probe the magnetization dynamics in magnetic nanostructures and devices. Ferromagnetic resonance (FMR) is a standard tool for probing magnetization dynamics in ferromagnets where the magnetic samples are excited with the microwaves to induce magnetization precession. In the ferromagnetic resonance microscope, we use a sensitive, magnetized cantilever to detect precession in individual structures. To detect extremely small structures, it is desirable to drive precession to higher amplitudes, so that the precession signal gets stronger. However, nonlinear phenomena emerge and alter the linear resonance behavior of the low power regime. The nonlinear behavior of magnetic nanostructures limits the signals in the FMRFM, but it also has important implications for rapid switching of memory devices and for frequency stability of spin torque oscillators.
We use ferromagnetic resonance force microscopy (FMRFM) to probe nonlinear magnetization dynamics in individual Permalloy disks with diameters ranging from 100 nm to 500 nm. In these nanosize ferromagnets, the nonlinear effects that include asymmetry in resonant line shape and the appearance of additional peaks are observed. We report that the nonlinear effects are size dependent. In the larger disks, the power dependence shows a complex response while much simpler behavior is observed in smaller disks. Furthermore, the onset power for the nonlinear effects are lower in the larger disks, while higher power needs to be applied for the smaller disks to show nonlinear effects. We have modeled our results using micromagnetic simulations, and they display good correspondence with our experimental data.