In Situ Transmission Electron Microscope Studies On Magnetic Domains

Big data makes big difference.It is always of a great challenge for us to achieve the storage and retrieval of the exploding massive data efficiently,by means of the existing principles and techniques,in a manner combined with extremely low energy consumption,superfast speed,and ultrahigh density.Storage based on magnetism is one of the most important approaches to the information storages,and will be continuously and widely employed for the foreseeable future.For the exploration of novel magnetic storage media,materials of the topological magnetic domain structure resulting from the so-called non-collinear and/or non-coplanar magnetic moment arrangement having the characteristics of a nanoscale special spin configuration,which is protected by topological symmetry and can be manipulated via the spin-polarized current with low current density,are expected to break through the physical limitation of superparamagnetism in traditional magnetic storage media,and to reduce the energy consumption meanwhile.Therefore,this kind of matters is believed to be one candidate for new generation information storage unit with high-density,high-speed and low-energy consumption.In the viewpoints of basic research and practical application,it is of great significance to study the generation and manipulation mechanism of topological magnetic domains.In this dissertation,magnetic domains with different topological properties,such as magnetic vortices in amorphous CeFeB,magnetic skyrmions in Ni₅₀?Mn,In?₅₀ and magnetic bubbles in MnNiGa?Y,C,In?alloys had been systematically studied and especially manipulated via external fields?temperature,electric current,and magnetic field?.Researches greatly help ones to deepen the understanding of the topological properties of magnetic domains.The main contents of the dissertation are concluded as follows:1.Generation and manipulation of magnetic vortices in amorphous CeFeB alloy.Spontaneous magnetic vortices were observed in amorphous CeFeB ribbons for the first time.The dynamic behaviors of magnetic vortices under external fields were systematically studied via Lorentz transmission electron microscopy.Via temperature manipulation,it was found that the magnetic vortex and antivortex nucleated or annihilated pairwise when the temperature changed and the vortex density increased with the decrease of temperature in a certain range.Via electric current manipulation,it was found that vortices could be driven by a small current and the vortices and antivortices periodically nucleated while the current exceeded the critical value,which significantly increased the density of magnetic vortices.In addition,a process similar to relaxation behavior appeared while applying current,with vortices and antivortices spontaneously nucleated or annihilated even at the same electric current.Via in-plane magnetic field manipulation,it was found that the magnetic vortex prefered to nucleate in the defective region and vortices/antivortices pairwise nucleated under magnetic fields.The high-density magnetic vortices appeared near the external magnetic field close to its coercivity but only within a very limited magnetic field range.The discovery of magnetic vortices in the amorphous CeFeB has not only broadened the application of rare earth materials,but also provided a new class of candidate materials for the study of magnetic vortices.Moreover,the direct observation of the dynamic behavior of magnetic vortices is of guiding significance to the design of the connate materials and devices.2.Exploration of the magnetic skyrmions and martensitic transition in Ni₅₀?Mn,In?₅₀ Heusler alloys.The magnetic domain structure and martensite phase transition behavior in Ni₅₀?Mn,In?₅₀0 Heusler alloy were studied via in-situ Lorentz transmission electron microscopy.The effects of valence electron concentration,grain size and atomic order on phase transition,magnetic domain structure and martensite twin structure were discussed.It was found that the magnetic domain and structure evolution of the Ni₅₀?Mn,In?₅₀ Heusler alloy were closely related to the composition and the atomic order of the material.The magnetic domains of martensite varied with the microstructure.In the Ni₅₀Mn₃₅In₁₅ bulk alloy,the martensite phase had a paramagnetic-ferromagnetic transition.The ferromagnetic phase of the martensite had stripe magnetic domains,which evolved into a biskyrmion-like nanodomain structures under a vertical magnetic field.In the Ni₅₀Mn₃₅In₁₅ ribbons,the martensite showed rich magnetic domain structures and microstructures,which demonstrated the complex structures and magnetism in martensitic phase.In Ni₅₀Mn_35.2In_14.8 bulk alloy with the slightly different composition,pre-martensitic transition was directly observed during martensite phase transition and zero-field skyrmions with three kinds of spin textures were spontaneously generated in the nanotwinning of martensite.The generation of skyrmions was originated from the emergence of the intermediate phase and martensite twinning confinement.The discovery of spontaneous magnetic skyrmions in Ni₅₀Mn_35.2In_14.8 alloy with premartensite phase provides an alternative way to manipulate skyrmions by external strain/stress.3.Nanoscale magnetic bubbles in MnNi?Ga,M??M=Y,In?and MnNiGaC_?alloys.Magnetic bubbles with various internal structures instead of biskyrmions in MnNiGa alloy are generated in MnNiGa-based alloys,with modified anisotropy and other parameters which results in the change of the quality factor Q in MnNiGa alloy,due to the substitution/addition of Y and In of large atomic radius and C of small atomic radius.The magnetic domain structures of MnNiGaY alloy were manipulated by electric current and magnetic field simultaneously,and high-density magnetic bubble lattices obtained at zero magnetic field.The high-density magnetic bubbles with uniform internal structure had been achieved due to the spin transfer torque.The discovery of controllability of topological properties of magnetic domain structures provides a guidance for the design of materials with nanoscale magnetic domain structure.That the hexagonal magnetic bubble lattice at zero field generated by electromagnetic dual-manipulation will not only promote the potential application in the future non-volatile magnetic memory,but also benefit the understanding of the interaction between magnetic moments and conduction electrons.