Tailoring Of Skyrmions By Material Modification And External Fields In Situ Lorentz TEM

Magnetic skyrmion is one of topologically protected spin textures.Owing to its small size and unique current-induced dynamics,magnetic skyrmion is expected to be a promising candidate for next-generation spintronic devices.The increase of skyrmion density with a controllable manner at designated locations is an optimal approach to further improve the recording density of magnetic memory devices and other relevant spintronic devices.It is well known that magnetic properties are inherent for the materials,which heavily depend on the microstructures and chemical composition.Therefore,the understanding of the relationship between structures and magnetism is essential for designing and controlling the excellent properties of skyrmion host materials.In this dissertation,we have used magnetic multilayered films and ferromagnetic alloys as example specimens to in situ transmission electron microscopy(TEM)investigate the tuning of skyrmion density under the stimuli of ion irradiation and external fields(including mechanical bending,low-temperature cooling and magnetic field).The microstructural changes of both modeled samples were characterized at the atomic scale.Theoretical and micromagnetic simulations were performed to unveil the relationship between the microstructures and density of skyrmions.Furthermore,the intrinsic physical mechanism of skyrmion nucleation and density stimulated by the structural modification and external fields were explored on the basis of the experimental and simulated data.This work should contribute the possibilities which to weaken skyrmions Hall effect and realize current-driven skyrmion motion.The main research contents are summarized as follows:1.The effect of mechanical bending stress on the density of skyrmions in magnetic multilayered films investigated by Lorentz mode TEM.Pt/Co/Ru multilayered films were prepared by magnetron sputtering technique.Slices for in situ TEM experiments were fabricated by the focused ion beam(FIB)and extracted by a nano-manipulator.It is found that the bending stress can cause an increase of skyrmion density in the area with small curvature,where surficial grain size was observed to enlarge and alloy was formed by spherical aberration-corrected scanning transmission electron microscopy(Cs-STEM).It is therefore deduced that the localized alloy and the increased grain size may provide the nucleation location for skyrmions and further result in the increase of skyrmion density.Whilst,micromagnetic simulations also confirmed that the bending stress changed the magnetic properties of the films and increased the density of the skyrmions.2.Precise tuning of skyrmions density by ion irradiation with controlled manner.Magnetic Pt/Co/Ta multilayered films were prepared and irradiated by Ga⁺ion beam.A sharp increase of skyrmions density in the exposed magnetic multilayered films was found to be purposefully realized at the site-specific position,which has industrial applicability.The Cs-STEM images and micromagnetic simulation results indicated that the skyrmions density can be sharply increased 5 times after applying an exposure with an irradiation dose of 1.5×10¹⁴ Ga⁺/cm~2,and the magnetic field required to create skyrmions was also reduced.The intrinsic physical mechanism of increasing skyrmion density is found to mainly originate from the formation of disorders through Ga⁺irradiation,which can induce a decrease of the nucleation energy barrier of skyrmions.This work should contribute a significant step towards the eventual realization of practical recording applications of magnetic skyrmions.3.Observation of the domain wall skyrmions in Fe₇₀Gd₃₀ films and their magnetization reversal process under low temperature cooling in situ Lorentz TEM.Single layered Fe₇₀Gd₃₀ films were grown by magnetron sputtering.At the spontaneous magnetization state,the Fe₇₀Gd₃₀ films were observed to have domain wall(DW)skyrmions,which were distributed along the chiral Néel walls.A series of DW meron chains were formed in the Fe₇₀Gd₃₀ film when an external magnetic field was applied.When the magnetic field was decreased,they were transformed into DW skyrmions.The two kinds of topological spin textures were further characterized.It was shown that the DW skyrmions moved along the domain wall when external magnetic field was applied,which can eliminate the influence of skyrmions Hall effect.The low temperature experiments which were performed by in-situ TEM cooling holder under Lorentz TEM revealed that DW meron chains had a high linear density with the external magnetized field.The observation and manipulation of these two types of topological domain walls should be significant for the understanding of topological magnetic structures.