Phase Transition In Functional Materials And Structural Dynamics As Studied By Ultrafast Transmission Electron Microscopy

Ultrafast Transmission Electron Microscopy?UTEM?with high spatiotemporal resolution plays an increasingly critical role in directly observing the microstructure and transient states of materials.This dissertation describes the development process of the independently developed UTEM,and the effort in equipment improvement and performance optimization of photoelectron.Via high spatiotemporal resolution UTEM,we reveal the structural dynamic features of martensitic?MT?transition in shape memory alloy MnNiSn initiated by ultrafast laser pulses,and demonstrated a real-space imaging of ultrafast demagnetization in MnNiGa.In addition,the interplay between the topological magnetic structure Skyrmion and the crystallography twin structure has been investigated by transmission electron microscopy?TEM?and in-situ techniques,which prepared for further exploration of the ultrafast magnetic dynamics in UTEM.The main results of this dissertation are listed as following:1.Participated in the development and equipment performance optimization process of the UTEM at IOP in China,and evaluated the photoemission performance of UTEM with different filaments?LaB₆,W,Ta?.We demonstrate its ability for ultrafast electron diffraction and real space imaging,with the high spatial?0.35nm?and temporal?1ps?resolutions.2.The photoinduced martensitic?MT?transition and reverse transition in a shape memory alloy Mn₅₀Ni₄₀Sn₁₀0 have been examined by UTEM,and imaging and diffraction observations clearly show a variety of structural dynamic features at picosecond time scales.We have estimated the temperature rise for both lattice and electron subsystems as a function of optical fluence and specimen thickness by a phenomenological two temperature model?2TM?.And the strong coupling between the MT transition and different strain wave modes,the modulation by the defect during phase transition are observed.3.We explore the high spatiotemporal capability of Lorentz UTEM for direct imaging photoinduced ultrafast magnetization dynamics.The Lorentz UTEM images with a picosecond temporal and sub-50-nm spatial resolution reveal remarkable features of magnetic transient states after a femtosecond pulsed laser excitation,and three successive dynamical processes involving four distinct magnetic states are evidently observed in MnNiGa crystals.Moreover,a full dynamic cycle for the thermally-driven ferro-paramagnetic transition in a time span up to tens of microseconds has been recorded by using nanosecond-laser-generated photoelectrons.The temporal changes of electron,spin and lattice in the present system have been briefly discussed based on a three temperature model?3TM?.4.In order to develop the ultrafast magnetic dynamics,we explore the topological magnetic structure Skyrmion by TEM-based techniques.The in situ capability with varying field and temperature allow direct address the relationship between the Skyrmion and the twin structure in the magnetic material MnPtSn.We reveal such twinning that host the Skyrmion,and study the interplay between crystallography and spin degrees of freedom.