Lorentzian Transmission Electron Microscopy Studies Of Topological Magnetic Domains In TbFeCo Films

Magnetic Skyrmion is a spin structure with spherical spin configuration topology and nanoscale dimensions,which can be driven by low-density current at high speed and exhibits rich dynamic characteristics.Using Skyrmion as the information bits is expected to solve the energy consumption problem,quantum limit problem and Moore’s law problem faced by current memory application.The skyrmions spintronic devices are therefore expected to be next generation of low-power memory devices.The zerofield and room temperature stability and the high speed write/erase capability of the skyrmions are the main areas of concern in the application of spintronics devices.On the other hand,the transformation between skyrmion and other topological structures is also crucial for memory application.In this thesis,the generation and stabilization of skyrmions in amorphous ferrimagnetic Tb Fe Co films and the transformation between zero-field skyrmions and other topological magnetic structures is studied by using Lorentz transmission electron microscopy combined with femtosecond laser pulse excitation and in-situ manipulation techniques.The main research results are summarized as follows:1.Femtosecond laser-induced generation and manipulation of skyrmions: in an amorphous ferrimagnetic Tb Fe Co film,the creation of high density zero field skyrmions from the saturated ferromagnetic state upon femtosecond laser pulse excitation is observed via in-situ Lorentz transmission electron microscopy.Through further comparative experiments and micromagnetic simulations,the topological skyrmions are demonstrated to emerge under fs laser pulse excitation through a unique ultrafast nucleation mechanism.Notably,large intrinsic uniaxial anisotropy could substitute the external magnetic field for the creation and stabilization of skyrmions.The ultrafast switching between topological trivial magnetic state(saturated ferromagnetic state or stripe domain state)and nontrivial skyrmions state are realized at an optimum magnitude of magnetic field and laser fluence.Our results would broaden the alternative options to generate zero-field skyrmions from versatile magnetic states especially from the saturated state of magnets and provide a new perspective for advanced applications with magnetic state ultrafast switching in skyrmionic devices.2.Magnetic domain evolution and topological state analysis of Tb Fe Co films: in the amorphous ferrimagnetic Tb Fe Co film,zero-field skyrmions state is generated by femtosecond laser pulse excitation,and the in-situ temperature dependent magnetic domain evolution is revealed by Lorentz electron microscopy.The experimental results show that zero-field skyrmions have a high thermal stability,most of skyrmions remain unchanged even at 470 K.Above 470 K,with the increase of temperature,zero-field skyrmions suddenly expand into topologically equivalent magnetic bubbles and almost all skyrmions expand into magnetic bubbles at 510 K.In addition,the femtosecond laser pulse enables nucleation not only topological non-trivial magnetization textures but also some topologically trivial structures.The transformation of topologically trivial structures into topologically non-trivial is observed during the heating process.The magnetic parameters measurement of the ferrimagnetic Tb Fe Co amorphous film reveals that the saturation magnetization shows no significant change and uniaxial anisotropy decreases significantly with the increase of temperature.From the perspective of the reduction of uniaxial anisotropy,a series of micromagnetic simulations are set up,and successfully explain the transformation from zero field skyrmions to topologically equivalent bubbles and the transformation from topologically trivial structures into topologically non-trivial.