In-situ TEM Study Of Polar Vortex In Oxide Superlattice

Ferroelectric vortices are topological structures with the electric dipole continuously rotating around a stable core.These structures can be characterized by toroidal moment order and exhibit exotic emergent properties,such as negative capacitance and chirality.Thus,they are promising for lowering the energy-consuming and increase the capacity of the applications in high density data storage.To realize the real applications,it is crucial to control and manipulate the vortices under external stimuli.However,the size of the vortex is small?typically 2?4 nm?,making the characterization a challenge for the technology such as X-ray or atomic force microscopy?AFM?.Thus,the dynamics of the vortex remain largely elusive.In-situ transmission electron microscopy?TEM?allows the real-time observation of the specimen under the external stimuli,such as force,electric fields and light,enabling the measured properties to be correlated with the applied excitations.Recently,the largely improved the stability of in-situ double-tilt TEM holder,combined with newly-installed double-Cs corrected TEM,enables us to reach stable atomic resolution in the scanning transmission electron microscopy?STEM?mode even during application of mechanical loads and electric fields.Thus,we study the transition of the vortex under external excitations at atomic level.The main results are as follows:1. The polar vortex arrayin the Pb Ti O₃/Sr Ti O₃ superlattice films was characterized and the polarization was quantitively analyzed.The long range order of the vortex can be reflected by TEM dark field,selected aperture electron diffraction?SAED?and high resolution TEM images.By calculating the offset between Pb and Ti sublattice in the high angle annular dark field?HAADF?image,the vortex can be directly shown.However,the quantitive measurement of the polarization inside vortex is based on an empirical method,which may be inaccurate for such an inhomogeneous system like vortex,due to the absence of oxygen atoms of HAADF images.For accurate calculation,it is necessary to know the positions of oxygen atoms,which is feasible for annular bright field?ABF?images.Through ABF images,the distribution of out-of-plane and in-plane polarization is calculated.Furthermore,in subunit cell level,the contribution of Pb O plane and Ti O₂ plane is precisely measured,which exhibit a highly inhomogeneously behavior.Finally,the polarization of vortex calculated by HAADF and ABF images are compared.These results are crucial for figuring out the screening mechanism inside vortex and provide insights into the understanding of lattice-charge interaction.2. The vortex transition under external mechanical stress is investigated.Themobile tungsten tip of the in-situ TEM act as the indenter and exert the mechanical stress to the vortex.A time series of dark field images indicates vortex transition started from the surface and the nucleation growth in an inhomogeneous way.With the removal of the applied stress,the vortex recovered,indicating the reversity of the vortex transition.The mechanical loads required to drive vortex transition is estimated to be0.6 GPa,which is indeed lower than the stress used to switch ferroelectric domains.The evolution of in-plane lattice a,out-of-plane lattice c and the c/a ratio is demonstrated,implying vortex transitioned into a domain.Furthermore,a series of HAADF images acquired under the applying mechanical stress unveil the transition process at atomic level and confirmed the newly formed structure is a domain.Both high resolution TEM images and HAADF images shed light on the single vortex evolution and demonstrated the vortex core is the most stable part under the stress.Phase-field simulation reproduce the transition event.Besides,the contribution of flexoelectric effects is also discussed,which prove the driving force of the transition is the mechanical loads while flexoelectric effect align the direction of the formed a domains.These results offer a realistic way to control and manipulate the vortex,and thus pave the way for the vortex based applications.3. The vortex transition andchirality switching under electric fields was investigated.Only when the electric fields exceed the threshold value,the vortex transition began.In contrast to the mechanical case where the vortex directly transitioned into a domains,vortex first evolved into intermediated a/c domain due to the‘melting'of vortex cores and then became uniform c domain with the applying electric fields.Inversing the direction of the applying electric fields lead to a similar transition event but with a more rapid transition speed due to the Schottky barrier build-in fields.After the removal of the electric fields,the vortex chirality was found to be switched.The ability to control and manipulate vortex and toroidal moment by electric fields bring to the forefront of the applications.