The Investigation On Multiferroicity In Perovskite Titanium Oxides And Heterostructures

In the past decade,multiferroic materials have received widespread attention not only for their application potentials in multi-function memory,storage,sensing,and actuation among mang others.Multiferroic materials have more than one ferroic behavior simultaneously,including ferroelectricity,magnetism,ferroelasticity,and ferrovortex etc.It is known that the coexistence of ferroelectricity and magnetism in transition metal compounds is physically difficult due to the fact that the two ferroicities arise from totally different sources.The ferroelectricity is associated with the physics of empty d-orbitals but magnetism requires partially occupied d-orbitals.In order to overcome this deficiency,one strategy is to explore this coexistence in transition metal oxide heterostructures(superlattices),no matter whether the components themselves have ferroelectricity and/or magnetism or not,which will be the core issued addressed in this thesis.On one hand,we explore the ferroelectric and magnetism as well their coupling across the interfaces of the heterostructures.On the other hand,we are particularly interested in emergent magnetism and magnetoelectric coupling in such heterostructures via the interfacial reconstruction.Finally,we also investigate the emergent ferroelectricity in a naturally-layered doped rare-earth titanate,where the A-site ordered structure is found to benefit to the ferroelectricity.At the same time,the strain induced effect in this titanate compound is examined.The whole thesis is arranged as the following six chapters:In Chapter One,a comprehensive review on multiferroic materials and physics is given,covering a brief description of several basic concepts of ferroelectricity and magnetism.Several types of exchange interactions and microscopic mechanisms associated with multiferroicity will be introduced too.The emergent electronic reconstruction and the impact of ferroelectric polarization in several transition metal oxide heterostructures will be reviewed.Particularly,we discuss potentials to induce additional ferroelectricity in 4f-rare-earth titanates via the A-site ordering sequence and strain effect.In Chapter Two,the basic physics of the first-principles calculations is outlined,including the theoretical framework for density functional expression stemming from the Schrodinger equation for many-body systems.The applications and functionality of the commercial VASP package are also highlighted.Chapter Three focuses on the(YTiO3)2/(BaTiO3)n(YTO/BTO)superlattices,addressing the consequences of biaxial in-plane strain and varying BTO thickness.The interfacial electronic reconstruction and magnetism in response to the varying BTO thickness are discussed.It is revealed that the nonmagnetic YTO layers can be in the A-type antiferromagnetic state under the in-plane compressive strained state.For n=2,the superlattice shows the metallic state,due to the BTO ferroelectricity suppression and quantum tunneling effect.For n=4,the preserved ferroelectricity in BTO layers allow significant modulation of the interfacial electronic structure.Consequently the interfaces to which the polarization points accommodate the 2D electronic gas.More interestingly,one finds the favored ferromagnetic order in the BTO layers near the interfaces.Chapter Four addresses the magnetoelectric properties of the(LaAlO3)m/(PbTiO3)m(LAO/PTO)superlattices with either integer or half-integer m.For the n-p interface at m?4,it is predicted that the LAO polar surface in neighboring with PTO layer will impose sufficient internal electric field which polarizes the PTO layer into a ferroelectric single-domain,no matter the initial state of the PTO layer.In this case,the superlattices have no magnetism.However,for the n-n interface,several emergent phenomena are predicted.First,the internal electric field arisen from the LAO polar surface favors a head-to-head ferroelectric domain in PTO layers if m is no more than 3.5.For m>3.5,the domain structure of the PTO layers is dependent of the initial state.More importantly,clear magnetism is generated and the maximal moment appears at the domain walls.This feature allows a possible control of the magnetism by external electric field via the ferroelectric polarization switching.Here,an additional magnetoelectric mode accounting for the dot product of the polarization spatial gradient and squared magnetic moment is predicted.Chapter Five is devoted to ferroelectric and magnetic properties of Gd1/2Na1/2TiO3(GNTO),where Gd3+ has the 4f element.Our motivation is two-folded.One is to understand the role of the A-site ionic ordering in modulating the ferroelectricity in GNTO.The other is to look into the effect of lattice strain.While so far available data are insufficient to justify the A-site occupation,our calculations seem to indicate that the alternative stacking of the Gd-O layer and Na-0 layer along the b-axis is the ground state.The ferroelectricity and G-type antiferromagnetic order in such A-site ordered structure are predicted.It is revealed that the ferroelectric polarization can be remarkably tuned by lattice strain,while the G-type spin order remains robust.Chapter Six gives the conclusion and perspectives to the future work.