Theoretical Study Of Strain Gradient Effect On Perovskite Ferroelectric And Magnetic Properties

Nanoscale materials such as thin films,nanowires,nanoparticles,and nanocom-posites,can withstand much greater strain or stress(e.g.tensile or shear)than large scale macroscopical materials without breaking.Therefore,scientists have used the strain parameters to expand the space for developing new functional materials.A large elastic strain can be introduced to nanomaterials by epitaxy or external load.It can be artificially controlled to produce uniform or nonuniform elastic strain.This leads to new possibilities for tuning the physical and chemical properties of a material,such as electronic,optical,magnetic,phononic,and catalytic properties,by varying the six-dimensional elastic strain as continuous variables.By controlling the elastic strain field statically or dynamically,a much larger parameter space opens up for optimizing the functional properties of materials.Therefore,the once neglected coupling effect of the strain gradient and the ferroelectric polarization-flexoelectric effect has attracted much attention recently.In particular,the 180° reversal of ferroelectric domain can be made only by inhomogeneous strain.This is expected to be applied to future information storage areas.In addition,recent experiments have found the coexistence of ferromagnetism(FM)and antiferromagnetism(AFM)in the LuMnO3 thin films on(110)oriented YAlO3 substrate.It has a wide range of applications in the field of new multi-functional devices.Great progress has been made in the research of the influence of strain gradient and epitaxial strain on the mechanical-electrical coupling and magnetic properties of materials,but there are still some problems to be solved,such as:(1)So far,there is not yet a universal consensus regarding the size,or even the sign,of the flexoelectric tensor components for any material.There is still a significant variation in the values measured by different experiments and predicted by different models.For perovskite BaTi03 material,the situation is even worse:Experimentally,not all the individual tensor components are available,and the calculations and the measurements disagree by several orders of magnitude.(2)Previous theoretical works mainly focused on the dependence of electric po-larization on local strain gradient in the para-electric phase,the nonlinear nature of the full energy functional on polarization is not considered.Most importantly,because the polarization definition is pseudo-potential dependent,the accuracy of flexoelectric coefficient is in doubt.(3)No specific forms of the energy functional for the coupling of strain gradient and ferroelectric polarization of a given material have been given.(4)Experiments have found the coexistence of ferromagnetism(FM)and antifer-romagnetism(AFM)in the LuMn03 thin films on(110)oriented YA103 substrate,but the coupling of ferromagnetic and ferroelectric was not found.The physical nature of these phenomena is not clear,and the support of theoretical work is urgently needed.Based on the above reasons,this thesis focuses on how to include the flexoelectric effect in energy functional and to obtain the coefficients from the first principle cal-culation on perovskite BaTiO3.With this energy functional,we can study the law of 180° domain reversal process.And focuses on how to obtain the phase diagram of the magnetic states in the epitaxial o-LuMnO3 film under the substrate strain by the theo-retical calculation,and then explain the related experimental phenomena.The details are described below:(1)Based on the first-principles total-energy calculation,we have studied the shear-strain gradient effect on the polarization reversal of ferroelectric BaTi03 thin films.By calculating the energies of double-domain supercells for different electric polarization,shear-strain gradient,and domain-wall displacement we extracted,in ad-dition to the domain-wall energy,the polarization energy,elastic energy,and flexoelec-tric coefficient of single domain.The constructed Landau-Devonshire phenomenologi-cal theory yields a critical shear-strain gradient of 9.091 ×107/m,(or a curvature radius(R)of 110 A)for reversing the 1800 domain at room-temperature,which is on the same order of the experimentally estimated value of 3.33.3 x 107/m(R = 300A).In contrast to the commonly used linear response theory,the flexoelectric coefficient derived from fitting the total-energy to a Landau-Devonshire energy functional does not depend on the specific pseudo-potential.Thus,our method offers an alternative new numerical approach to study the flexoelectric effect.(2)Substrate strain offers an extra dimension in enriching the functionality of per-ovskite thin films.The recent observation of the coexistence of ferromagnetism(FM)and antiferromagnetism(AFM)in the LuMn03 thin films on(110)oriented YA103 substrate illustrated another interesting example.The FM signal arises from a thin atomic layer near YAlO3 substrate.We propose in this thesis that vertical strain re-laxation may stabilize the FM structure in the most compressively strained region near substrate while AFM structure is stabilized in less compressively strained outer sur-faces of thin films.This physical picture was supported by the magnetic phase diagram vs compressive strain derived from comprehensive first-principles calculations.The critical[110]strain separating E-AFM and FM structures is e ?-0.0425 which is close to the value e?-0.047 dictated by YA103 substrate.From the half-metal fea-ture of the FM structure and ferroelectric property of E-AFM structure,the greatly reduced ferroelectric polarization and averaged magnetic moment of 0.5 ?B/Mn can be understood.