Research On Hexagonal Manganites And Ferrites Single Phase Multiferroics By Electron Microscopy

Multiferroic materials have become an attractive class of strongly correlated systems with rich emergent physical properties and appealing potential for future applications.As an important branch of multiferroic materials,single-phase multiferroic materials have multiple ferroic orders in a single system,which make it a great platform for exploring the multiferroic coupling mechanism.Among the known single-phase multiferroic systems,hexagonal manganites and ferrites are two representative ones that have been the focus of research efforts due to their various exotic properties.Plenty of interesting scientific problems are embedded,which deserves continual in-depth investigations both experimentally and theoretically.Transmission electron microscopy is a systematic methodology that can provide key information on both microstructure and electronic structure of materials in real-（sub-angstrom scale）,reciprocal-and energy（less than 1e V）spaces,making it an irreplaceable research method in material science.In the research of this dissertation,giving full play to state-of-the-art electron microscopy and combining it with physical property characterizations,theoretical calculations and simulations,hexagonal yttrium manganite（h-YMnO₃）,hexagonal lutetium ferrite(h-(Lu_0.5,Sc_0.5)FeO₃)and a charge ordered system(Lu Fe₂O_4+d)are systematically studied for exploring the interplay between different degrees of freedom.Our study has certain systematicness and comprehensiveness covering from the mesoscopic scale to the atomic scale,from structural analysis to property improvement.For the h-YMnO₃ system,dynamic mesoscale study reveals the reversible evolution of vortex domains and charged domain walls under the control of electron beam illumination,providing a perspective on potential applications in ferroelectric storage;In terms of performance improvement,experimental and theoretical studies have demonstrated the key roles of the oxygen vacancies at different sites in tuning the geometric ferroelectricity and antiferromagnetic properties:the in-plane oxygen vacancies can alter Y 4d-O 2p hybridization and thus regulate geometric ferroelectric properties,while the out-of-plane oxygen vacancies can influence the antiferromagnetic configurations,which makes it possible to tailor the magnetic properties and create rich strain-accommodated magnetic states in h-YMnO₃film via strain engineering.On this basis,the oxygen vacancy is proposed to be treated as an atomic multiferroic element.For the h-(Lu_0.5,Sc_0.5)FeO₃system,by taking full advantages of the high-resolution electron microscopy,the vortex domain structure is systematically analyzed at the atomic level,revealing the contribution of Sc ion to stabilize the hexagonal phase.For the study of the hole-doped Lu Fe₂O_4+dsystem,we demonstrate atomic-scale observation and analysis of a new modulation wave that requires significant modifications to the conventional modeling of ordered structures.On the basis of the systematic investigations using advanced electron microscopy,density-functional-theory calculations and simulations,the interesting physics discovered here is that through introducing oxygen-hole into the system we are able to manipulate the degree of freedom of the charge-lattice order and interplay,which alter the primary and secondary wave vectors of the modulation and modify the order parameter.Furthermore,a new lattice-charge second-order modulation structure formulism is developed,which adds additional degrees of freedom in both modulation phase and amplitude parameter spaces and can be widely applicable to numerous ordered systems.This study illustrates a new approach to manipulate singularity in modulation waves via targeted hole doping,insight from which may shed light on deciphering how the doped holes entangle with charge and lattice that determines many emergent quantum states in materials.This dissertation focuses on the close couplings between lattice,charge,and spin in single-phase multiferroic materials.The research results to some extent provide the impetus for understanding the interplay between multiferroic orders and improving the performance of multiferroic materials.