Lattice Strain Modulation And Magnetic/ferroelectric Properties Of Perovskite Multiferroic Thin Films

As an important functional basic material,multiferroic thin films have great application prospects in emerging spintronic devices such as multi-state magnetoelectric storage and ultra low power information devices.Thus,the regulation of physical properties of multiferroic thin films and the identification of underlying physical mechanism have become the foucs of attention in the field of material science and condensed matter physics in recent years.The orthorhombic rare-earth ferrite(o-REFeO3,RE=rare-earth)family is a class of prototype perovskite materials with rich magnetic characteristics and multiferroic properties,whose crystal structure and physical properties can be effectively tuned via chemical doping,epitaxial strain and other means.Starting from the magnetic and electrical properties of multiferroics,this paper has obtained excellent room temperature ferroelectric polarization and ferromagnetism in representative compound SmFeO3 and EuFeO3 multiferroic thin films through strain and defect engineering,and systematically investigates the microscopic origins of their local structural distortions,electronic orbital hybridization,and spin interactions,revealing the underlying physical mechanisms of performance enhancement.Based on the coupling characteristics exhibited in the above single-phase multiferroics,the SrTiO3-NiO composite films with ordered self-assembled nanopillar and tightly coupled lattice were designed to further improve the magnetoelectric effects.By finely controlling the oxygen partial pressure during the film growth,the multiferroic SmFeO3-δ epitaxial films with a unique triaxial lattice expansion state was prepared.Accompanied by room temperature ferromagnetism,a colossal ferroelectric polarization of 4.14 μC/cm2 was achieved,which is two orders of magnitude higher than its bulk and is also the largest one among the o-REMO3(M=Mn,Fe)family.Combining the integrated differential phase-contrast imaging and theory calculations,we reveal the origin of this excellent ferroelectricity in which the purposely introduced oxygen vacancies(Vo)in the Fe-O layer drives the displacement of Fe ions in FeO6 octahedron under special strain state.This study not only provides a promising candidate for exploring the magnetoelectric coupling properties of single-phase multiferroic materials above room temperature,but also proposes a feasible strategy for improving the ferroelectricity of the classical singlephase multiferroic o-REMO3 family—strain defect engineering.According to the findings in SmFeO3-δ thin films,the valence electron structures of different Re3+ were further investigated,and Vo was introduced into EuFeO3-δ epitaxial thin films with special 4f6 electron arrangements.The polarized neutron reflection and magnetometry revealed the magnetization of thin films at RT was significantly increased by about 5 times compared to the bulk phase.Combined with annular bright-field images,X-ray absorption spectroscopy,and theory calculations,we revealed the underlying mechanism for this magnetic enhancement is that the reduction of the Fe-O-Fe bond angles induced by Vo strengthens magnetic interactions and tilts Fe spins.Further,the internal relationship between magnetism and Vo was built by illustrating how the magnetic magnitude and structure change with Vo configuration and concentration.Such results can deepen our understanding of the room temperature ferromagnetic nature of o-REFeO3 family,and the developed strategies may also be applicable to the improvement of ferromagnetism in other multiferrous material.Using two-phase composite to further improve the magnetic and electrical properties of the system.Different from the traditional route,the cubic classical paraelectric SrTiO3 and antiferromagnetic NiO are selected as the research objects from the perspective of new material design.The variable component SrTiO3-NiO composite films were prepared.By X-ray diffraction and transmission electron microscopy,lattice coupling between the two phases of ordered vertical nanopillar growth was observed,which produces a tetragonal ferroelectric phase with ultrahigh stable temperature and novel ferromagnetism,and improving magnetoelectric coupling compared to the above single-phase system.This study provides some inspiration for the design of novel functionality of composite material systems.This paper delves into the structure-property relation of perovskite type multiferroics(including single-phase o-REFeO3 films and self-assembled SrTiO3NiO nanocomposite films)at multiple scales.Based on experimental and theoretical results,the strategies for improving ferroelectric and ferromagnetic properties of multiferroics have been successfully developed.The present results will provide certain guidance and reference significance for the modulation of physical properties of multiferroics and the exploration of new systems in the future.