Electron Microscopic Investigations Of Perovskite Functional Oxide Film Interfaces

Due to the interplay between charge,spin,orbital,and lattice degrees of freedom,solid interface between two distinct materials display pronounced phenomena such as semi-conductor-metal transitions,magnetoresistance,the quantum hall effect and superconductivity,which show great potential in low dimension device application.Similarly,technologically enticing phenomena were also expected in compositionally homogeneous interfaces including ferroelectric domain walls(DWs).Generally,many factors coupled in determining these exotic properties,which means we need a comprehensive and detailed analysis.In the nanoscience era,the properties of many exciting new materials and devices will depend on the details of their composition down to the level of single atoms.Thus,the combination of atomic-resolution Z-contrast scanning transmission electron microscopy(STEM)and electron energy loss spectroscopy(EELS) represents a powerful method to link the atomic and electronic structure to macroscopic properties,allowing materials,nanoscale systems,and interfaces to be probed in unprecedented detail.Hence,the characterization of the structure and electronic properties of matter at the atomic scale is becoming ever more vital for economic and technological as well as for scientific reasons.In this paper,we use spherical aberration correction TEM and correlation spectroscopy to investigate the perovskite functional oxide films interface.The study mainly includes the following three parts.In the first part,using advanced STEM characterization techniques,we directly studied the valence states of both Ti and Fe with atomic spatial resolution at the interface of an LaTiO3/LaFeO3 heterostructure grown with atomic layer precision by reactive molecular beam epitaxy(MBE).Based on that,we demonstrate that the charge transfer from Ti to Fe indeed occurs near the interface,and the charge transfer depth is two-unit cells.In the second part,by atomically resolved EELS,we show a direct demonstration that the emergent magnetism results from the multivalence Mn rather than an interfacial effect in a LaMnO3 epitaxial thin film system.Furthermore,the electronic reconstruction and oxygen excess both responsible for the occur of multivalence Mn.At the same time,the observed thickness-dependent ferromagnetism is controlled by the magnetic dead-layer effect in manganite thin films,which is accompanied by the accumulation of Mn2+ induced by the electronic reconstruction.In the last part,utilizing quantitative TEM analysis and correlation characterization methods,we have found the unusual buffer domain formed on the head-to-head DWs in n-type free-standing BiFeO3 thin films,due to local strain relaxation and charge screening.The size of these buffer domain changes with the polarization charges compensation and the reduction of trap concentration.