Manipulation On Statics And Dynamics Of Domain Walls By Transverse Magnetic Fields In Magnetic Nanostructures

In the past decade,thin magnetic nanowires bearing transverse domain walls(TDWs)have attracted intense attention as basic units of future magnetic memories,logics and shift registers.When driven by axial magnetic fields,current or temperature gradient,etc.,TDWs propagate along wire axis thus realize high-speed information storage and processing.Further increase of the response speed of these nanodevices relies on further boosting of TDW propagation.Compared with other strategies,using transverse magnetic fields(TMFs)is the simplest and most basic one.Therefore,a thorough understanding of TDW statics and dynamcis under TMFs is crucial for the improvement and optimization of furture magnetic nanodevices.In this thesis,with the help of asymptotic expansion method(AEM)on LLG equation,we systematically investigate the manipulation of TMFs on TDW dynamics driven by magnetic fields and/or currents.The main results concerning the manipulation of TMFs onfield-driven TDW statics and dynamics are concluded as follows:As the beginning point of most works in this theis,we first introduce the manipulation of uniform TMFs(UTMFs)on the static profile of TDWs,and then the boosting effect of UTMFs to TDWs' axial propagation.For TDW dynamics,the AEM is a powerful tool for investigations on travelling-wave mode.Although UTMFs greatly enhance the TDW velocity in travelling-wave mode,they inevitably lead to a twisting in the azimuthal plane of TDWs.The twisting will induce stochastic magnetic fields thus be not good for some certain circumstances.To eliminate the twisting around TDW center,we fix the TMF strength and let its orientation become variable.For statics,given tilting attitude,the TMF strength must fall into a finite region.For any allowed TMF strength,we provide its corresponding orientation profile.When driven by axial fields,this planar TDW is also boosted.At last,the possible realization strategies of this TMF profile in real experiments are discussed.The related results are published in:Scientific Reports,2017,7:43065.The works with respect to the manipulation of UTMFs on current-driven TDW dynamics in magnetic nanostructures are concluded as follows:1.The s-d exchange coupling between the spin of itinerant electrons and the local magnetization in various magnetic nanostructures results in the transfer of spin angular momentum between them,thus leading to the so-called spin-transfer torque(STT).We first briefly introduce the main existing results of this field:the detailed formulations of STT in spin valves(discrete magnetization case)and in magnetic nanowires(continuous magnetization case),as well as the travelling-wave solutions of LLG equation including the related STT in each case.However,these results are short of a simple and clear picture.Inspired by the roadmap of field-driven TDW dynamics and based on the s-d exchange interaction,we propose the energy-dissipation picture of current-driven TDW propagation.Combined with the traditional torque picture,all existing results can be well explained.2.Next,with the help of AEM,we systematically investigate the manipulation of UTMFs on current-driven dynamics of TDWs in various magnetic nanostructures.Three common stack configurations are selected:(?)narrow and long spin valve with perpendicular current injection(CPP);(?)monolayer nanowire with axial current injection(CIP);(?)ferromagnetic nanowire on heavy-metal substrate with axial current injection(bilayer).In all three configurations,analytical investigations via AEM provide the manipulation mechanism as well as the related boosting factors.Within the framework of energy dissipation we already proposed,all these results can be well understood.Our results help to explain a series of confusing experimental measurements.Also,they contribute to future optimization of magnetic nanodevices based on current-driven TDW propagation.The related results have been submitted to Phys.Rev.B and are now under review.