Topological Defects In Low-dimensional Magnetic Materials

Spin structures in low-dimensional magnetic materials,such as nanodots,nanowires,atomic chains,and thin films,are the frontier and hot topic of condense matter physics.Recently,the realization of topological defects(e.g.,solitons,bubble domain,monopoles,vortices,and skyrmions)in these systems has attracted much attention both as a playground of unique physical phenomena,but also for the potential applications in electronic devices such as reconfigurable memories or logic based devices.Generally speaking,the spin structure in a magnetic system is governed by the competition among various interactions,including the exchange interaction,dipole-dipole interaction,Dzyaloshinsky-Moriya(DM)interaction,magnetic anisotropy,and so on.Typically,skyrmion spin crystal lattices were recently observed in the thin film of transition-metal silicides and germanides(for example MnSi,Fe1-xCoxSi and FeGe)with non-centrosymmetric cubic B20 crystal structure.In these materials,the competitions between strong ferromagnetic exchanges,weak DM interactions,and additional magnetic anisotropies always generate a long-wavelength helical spin order,which may be transformed into a skyrmion crystal structure under an external magnetic field.Therefore,the main physics responsible for the various topological defects,and the manipulation of these spin orders by different means become great interests.In this thesis,the very rich physics related to the topological defects in the low-dimensional frustrated spin systems is investigated.These studies aim at covering the following topics:(?)the exploration of new magnetic phases and new mechanisms in these systems,(?)the investigation of the magnetoelectric properties associated to chiral spin textures,and(?)evolution of the spin structures driven by external magnetic field and magnetic anisotropies.It is expected that the studies may provide new opportunities to tailor and control the magnetic topological defects,and potential applications in designing memory or logic devices.The whole thesis schedule is as follows:Chapter one starts from the background of low-dimensional magnetic materials.Subsequently,several basic concepts on the common magnetic interactions are introduced.Followed closely by a brief research history of topology,a detailed review on the physics of the vortex and skyrmion spin structures in several low-dimensional magnetic materials is presented.In the last section of this chapter,a short introduction to Monte Carlo(MC)simulation method is given.In chapter two,attention is paid to the transitions of spin configurations in ultrathin nanostructures by tuning the perpendicular anisotropy(Kz)and out-of-plane magnetic field(H),using MC simulation.It is revealed that enhancing the anisotropy Kz can drive the evolution of in-plane vortex state into intriguing saturated magnetization states under various H,such as the bubble domain state and quadruple-block-domain state etc.The spin configurations of these states exhibit remarkable H-dependence.In addition,the strong effects of geometry and size on the spin configurations of nanostructures are observed.It is suggested that the magnetic states can be manipulated by varying the perpendicular anisotropy,magnetic field,and geometry/size of the nanostructures.Furthermore,the stability(retention capacity)of the saturated magnetization states upon varying magnetic field is predicted,suggesting the potential applications of these saturated magnetization states in magnetic field-controlled data storages.In chapter three,motivated by recent experimental observations on helimagnet MnGe,the spin structures in chiral magnets with compass anisotropy are investigated using MC simulations.The studies are based on a classical Heisenberg spin model with DM interaction and compass anisotropy,which originates from the spin-orbit coupling(SOC).As a result,a phase diagram with emergent spin orders in the space of compass anisotropy(A)and out-of-plane magnetic field(H)is presented.It is revealed that the propagation directions of helical states and the symmetry of skyrmion structures depend strongly on the compass anisotropy(A).Meanwhile,an extended continuum spin-wave model with the lattice discretization anisotropy is proposed to interpret this phase diagram.It is demonstrated that specific helical propagating directions are favored by the high-order lattice anisotropies arising from the spin interactions in discretized lattice,leading to a gradual helical reorientation as a function of A.In addition,it is proposed that a hybrid super-crystal structure consisting of alternating half-skyrmion and half-anti-skyrmion is the possible zero-field ground state of MnGe.Especially,the simulated evolution of the spin structure driven by magnetic field is in good accordance with experimental observations on MnGe.Therefore,the Heisenberg spin model in this study captures the main physics responsible for the spin orders in MnGe,and the work also provides a theoretical guide to understand the spin structures in helimagnets with strong SOC.In chapter four,MC simulation is employed to explore the variations of spin structures and magnetic helicity ?m,in B20 compound Mn1-xFexGe as a function of concentration x,which is recently observed on experimental Mn1-xFexGe.MC simulation is carried out based on a simple spin model including hybrid DM interactions with different signs in FeGe and MnGe.The results reveal a series of spin structures with helicity ?m varying continuously from negative values to positive values upon decreasing x(0.0?<x?1.0).The simulated x-dependence of the spin structures is consistent with experimental observations,suggesting the validity of the hybrid model.This study sheds light on control of magnetic helicity in helimagnets,by mixing crystals with different DM interactions.In chapter five,a thin nanodisk of chiral magnets with perpendicular anisotropy and DM interaction is studied.Using a special method,a metastable skyrmion structure is created in this nanodisk at zero magnetic filed and zero temperature.Then the dynamics of the skyrmion driven by alternating current or direct current is investigated using Landau-Lifshitz-Gilbert equation.As a result,the mass center of the skyrmion exhibits different spiral motions for these two current modes.Also,it is interesting that some emergent phenomena appearing in the dynamic process of the skyrmion.In addition,the rotation sense(clockwise or counterclockwise)of the moving skyrmion trajectory driven by current depends on the polarization direction of the skyrmion mass center(up or down).For this reason,we propose an experimental scheme to control the polarization direction of the skyrmion mass center using the combined action of laser heating and magnetic field.Therefore,these studies provide us some theoretical foundations and experimental schemes to creat and manipulate the metastable skyrmion in the nanodisk,which has potential applications in designing the skyrmion-based memory storage devices.At last,the sixth chapter is devoted to the conclusion and perspectives to the future work.