Tuning The Spin Dynamics Of Core-shell Magnetic Vortices

In recent years,the new physics discovered in various magnetic nanostructures proved its importance in fundmental reserch as well as in applied areas.Magnetic vortex state,commonly found in micro-/nano-scale magnetic materials,possesses elegant topological structure and symmetry.Our research foucuses on the interactions between spin wave and topological singularity in magntic nanostructures.In a magnetic vortex,there are three main spin-wave modes,namely gyrotropic,azimuthal,and radial modes.Utilizing giga-hertz frequency AC magnetic field and spin polarized current to stimulate spin wave modes in magnetic vortices,the polarity and chirality of the vortex can be switched within nanoseconds.This opens new routes for designing magnetic vortex based information storage and logic devices.Previous studies focus on the magnetic vortex in homogeneous nanodisks.However,the modulation on spin wave modes,as well as its interactions with the vortex polarity and chirality,using heterogeneous structures is neglected.We designed core-shell magnetic nanodisks,which have a permalloycore surrounded by Fe shell.We first studied the radial spin wave modes of this model.We know that only odd index radial modes can be effectively excited in homogeneous magnetic vortex.In the core-shell nanodisks,however,we found even index radial modes with signicant oscillation amplitude,and it can drive the vortex core reversal in subnanoseconds.The Fe shell only accommodate high-frequency spin wave.Therefore,the fundmental radial mode is effectively confined in the Py core,thus realizing much faster vortex switching.In high index radial modes,the high frequency radial spin wave is stimulated in the Py core which drives ultrafast core reversal.In comparison to homogeneous magnetic vortex,the core-shell structured one can realize core reversal in much wider frequency ranges with significantly faster speed.This research opens a new window for the studies on interaction of spin wave with vortex core.According to physics intuitions,the cubic magneto-crystalline anisotropy of Fe hinders the transmission of azimuthal spin wave.The immediate question to ask is:would it be difficult for exciting radial spin wave in Py/Fe core-shell nanodisks?Our results provide suprising answers to this question.Under the excitation of rotating magntic field,we observer neglegible azimuthal spin wave in Fe.However,the azimuthal mode excitation in the Py core is much stronger than in its homogeneous counterparts.As a result,the core reversal is much faster in the core-shell sample.Meanwhile,the azimuthal spin wave has four-fold symmetry.This suggests that the cubic anisotropy of Fe is "imprinted" into the Py through the exchange and dipolar interactions between the two materials.Consequently,the gyration route of the core is no longer round,but turns into square.Because of the compression and extension by the spin wave,the vortex core diameter oscillates periodically with a frequency 4 times that of the core gyration.We designed a thin Co nanodisk on top of Pt.The spin-orbital interaction in Pt induces interface Dzyaloshinskii-Moriya interaction,which causes the formation of magnetic Skyrmion in Co.Magnetic Skyrmions is more stable than the vortex state and has mobility comparable to the magnetic domain wall.Thanks to its small size and the merit of not easy been pinned by defects,Skyrmions have great importance in both fundamental and applied research areas.By triggering the gyrotropic motion of Skyrmion cores,its polarity can be switched unidirectionally within nanoseconds.The flipping event comprises core annihilation and nucleation process.In order to reduce the oscillation amplitude of in-plane AC field,we apply a perpendicular bias field.Our study demonstrates that the bias field can effectively change the skyrmion size and modulate the gyrotropical motion of the core.We therefore realized fast and unidirectional polarity reversal of the magnetic Skyrmion using small amplitude AC field.