Lorentz Electron Microscopy Studies On Magnetic Skyrmions

Nanometric magnetic skyrmions are expected to overcome the technical and physical challenges such as Moore’s Law and super-paramagnetic limit in traditional magnetic materials due to their special topological spin configurations and ultralow-current driving properties.They are considered as ideal candidates for high-density,high-speed,low-energy magnetic information storage bits in the future and thus become one of the emerging research hot topics in the field of magnetism.This dissertation mainly studies the generation and manipulation of magnetic skyrmions under external fields（magnetic field,current,temperature,etc.）in centrosymmetric Mn NiGa,La_1-xSr_xMnO₃,and non-centrosymmetric FeGe.And new skyrmion material in rare-earth compound is further explored.The main results are as follows:1)Magnetic skyrmions in centrosymmetric MnNiGa material:i)Magnetic skyrmions are firstly discovered in MnNiGa by using Lorentz TEM.The spin configuration is analyzed to be composed of two single skyrmions with opposite chiralities,and thus the topological number is 2,named biskyrmion.ii)High-density and zero-field biskyrmions are achieved in untra-wide temperature range（16-338 K）by means of field-cooling manipulation.The energy barrier is extracted from thermal equibrilium skyrmion distribution and decreases rapidly with increasing temperature based on phenomenological free-energy model.We propose that the manipulation of short-range ordered ferromagnetic clusters slightly above T_C,the topological protection and the significantly increased energy barrier while FC are the physical mechanisms for the zero-field skyrmion lattice.iii)The spin transfer torque effect induced by electric current can manipulate the skyrmion density.The pre-applied current suppresses the transition from ferromagnetic state to stripes while decreasing magnetic fields,prefering to generate residual skyrmions.iiii)The microscopic origin of magnetic biskyrmion is explored and the influence of various factors such as constratint of grain boundary,in-plane magnetic fields,and sample thickness etc.,on the generation and stability of skyrmion are also investigated.2)Magnetic skyrmions in centrosymmetric La_1-xSr_xMnO₃ magnet:With decreasing the temperature,the anisotropy transition from in-plane 180°magnetic domain to spiral magnetic structure is observed,spontaneously forming zero-field magnetic bubbles with a size about 100 nm.The dynamic behavior of the Bloch lines is studied under perpendicular magnetic fields,leading to the topological transition from magnetic bubbles with topologic number 0 into magnetic skyrmions with topologic number 1.3)Magnetic skyrmions in FeGe non-centrosymmetric material:Through FC manipulation,the temperature range for skyrmion lattice is broadened with zero-field skrymions realized at lower temperatures.The relaxation behavior of metastable zero-field skyrmion is quantitatively analyzed based on the relationship between relaxation time and temperature,which approximately obeys Arrhenius’s law.Therefore,the energy barrier and lifetime of skyrmion can be extracted.4)Exploration of magnetic skyrmion in rare-earth compound:The temperature dependence on magnetic domain transition is studied in TbGe,GdGe and GdSi,and the skyrmion-like nanodomain is spontaneously generated at lower temperatures.We propose that the decreased magnetic anisotropy due to the amorphous layer at the edge of GdGe and GdSi might explain the generation of nanodomains.