Magnetic Dynamics In Ferromagnetic Nanostructures

With the breakthrough of new physics and new spintronic effects in the past decades,the study of magnetic reversal featured by domain wall(DW)motion has been increasingly focused in various complex multilayers with comprehensive spintronic effects,like Spin transfer torque(STT),spin hall effect(SHE),Dzyaloshinskii-Moriya interaction(DMI)etc.This growing tendency caters to the development of modern memory technology towards the industrial application on the one side.On the other side,it requires a comprehensive establishment of DW dynamics theory in different magnetic reversal systems.To this end,we carry out the study of characteristic domain structure and magnetic-field/current driven DW dynamics in several simple film stack systems with different dimension and magnetic anisotropy,which fills the gap of the following researches:1)the characteristic domain structure and DW mobility in the ferromagnetic thin films with the absolute uniaxial magnetic anisotropy;2)asynchronous DW depinning and mobility in the ferromagnetic zigzag nanowires with shape induced uniaxial anisotropy along the wire axis.Besides,we successfully fabricated perpendicular magnetic anisotropy(PMA)two-dimensional MoSe2/CoFeB(2D MoSe2/CoFeB)bilayer with contamination-free interface which will enable us to investigate the DW dynamics in such a new PMA bilayer system with strong hybrid interface in the future.To start with,we study the effect of DC power and annealing on the crystalline structure,damping and magnetic properties of sputtered CoFeB films with two different compositions aiming for a better film growth.In addition,the enhanced magnetization and magnetic anisotropy of the CoFeB films with different thickness deposited with an in-plane magnetic field have also been investigated which shows an absolute uniaxial magnetic anisotropy(UMA)for 15-30 nm thick CoFeB films.Based on the above knowledge,we successfully deposit CoFeB layer on the surface of MBE-grown 2D MoSe2 and prove the perpendicular magnetic anisotropy(PMA)at 2D MoSe2(2 ML)/CoFeB(3 nm)interface by performing the element-distinguishable XMCD hysteresis loop along film normal(NI)and grazing direction(GI).By applying sum rule to the Co(Fe)L2,3-edge XMCD measured along NI and GI,we demonstrate that the observed PMA at 2D MoSe2/CoFeB interface is originated from the perpendicular orbital moment mL of 3d transition metal Co and Fe rather than the spin moment ms which has been found isotropic.It has been further pointed out that the interfacial d-d hybridization between transition metal atoms plays an essential role in the observed anisotropic orbital moment at the 2D MoSe2/CoFeB interface.We also demonstrate favorable magnetic softness and considerable magnetic moment preserved at the interface and theoretically predict the interfacial band matching for spin filtering.Our work reveals the 2D MoSe2/CoFeB interface allowing for the PMA development in a broader CoFeB thickness-range than common systems such as MgO/CoFeB and highlights the 2D MoSe2/CoFeB interface as a promising platform for examination of TMD-based spintronic applications and might stimulate further development with other combinations of 2D TMD/FM interfaces.By Kerr microscope and magnetic field pulse generation system,we investigate the structure and spin configuration of revised magnetic domain driven by pulsed field in the CoFeB films with absolute UMA.It has been demonstrated that The rectangular magnetic domain with two different DW types,zigzag DW parallel to the hard axis(HA)and straight DW parallel to the easy axis(EA),has been demonstrated the characteristic domain shape in CoFeB films with absolute UMA when the external field is applied parallel to the film EA.Our experiments show that the field-driven motion of zigzag DW and straight DW follows completely different dynamic rules,namely linear v-H relation predicted by the 1D model for the former and creep motion with-1/4 law for the latter.And the mobility of zigzag DW from the linear fitting is ?=390 m s-1 mT-1,higher than that measured in FM single layer nanowires(?300 m s-1 mT-1).Our analysis shows that the divergence comes from the special effect of magnetic pining sites on zigzag DW which in detail is that the DW with zigzag shape maintains intact on velocity and only changes the zigzag amplitude when passing pinning sites.This makes the whole CoFeB film a pinning free system for the moving zigzag DW which therefore can be predicted by the 1D model,a theory established in an ideal UMA spin system without magnetic pinning.In addition,the effect of magnetic pinning to the shape of zigzag DW is apparent in the low field region but negligible after a critical field,e.g.0.6 mT for 30 nm thick CoFeB film in our study.Our theoretical analysis shows that the zigzag DW is the equilibrium result of several competing magnetostatic energies in the film system and the zigzag angle can be determined by the material parameters through our estimation formula which has a good agreement with that obtained by statistical analysis of the observed zigzag DW in CoFeB films.Besides,our study of slant DWs moving in the wide CoFeB strips with different width shows a linear dependence of DW mobility ? on 1/Sin? in which ? is the slant angle of slant DWs with respect to the strip axis.This suggests that the mobility ? is proportional to the effective DW width Aeff,the length of Neel transition region parallel to the DW propagation direction.Our results show that the asymmetric DW motion can be also induced by anisotropy field which may provoke the thinking on asymmetric DW dynamics which was currently thought to be contributed by Dzyaloshinsky-Moriya Interaction.Our work highlights the significance of 2D magnetic domain in the fundamental research of domain wall motion which is benefit for the future research on 2D regular DW motion in magnetic films.Not like the 2D DW motion in the full film,depinning and dynamics of the DW confined in the nanowire is governed by the shape anisotropy.We investigate the current induced depinning of magnetic domain walls(DWs)trapped at the kinks in a patterned Py nanowire with two-kink zigzag shape on Si/SiO2 substrate.By injecting pulse current with varying amplitude,two threshold current densities with a slight difference of around 6×1010 A m-2 have been determined according to the different depinning process.The larger threshold corresponds to the simultaneous depinning of two DWs which has been commonly observed in a variety of shaped nano wires.The smaller threshold current density corresponds to the depinning of a single DW which is always the first DW facing to the electron flow.This fact indicates that the different pinning potential caused by the structural asymmetry is not the key to determine which DW is to depin.Micromagnetic simulations provide evidence that the specific current-driven behavior of the vortex DW might account for the single depinning process.We demonstrate that current-induced Magnus force of the polarized vortex core which introduces an energy split to the chirality-dependent asymmetric pinning distribution of vortex DW is the key for the single depinning process.This asynchronous depinning phenomenon provides the possibility of the selective DW depinning in the future logic devices.Besides,vortex DW has been demonstrated by our simulation to move in the breathing mode which is usually considered to result in energy consumption by emitting spin wave.Our study shows that the breathing frequency increases with the decreasing damping suggesting that the energy consumption of spin wave might be nonnegligible compared with damping consumption which is the main contribution of a driving DW.