ABO₃-type Perovskite Ferroelectric/multiferroic Controls By Weak Compositional And Structural Perturbations

The competition among different interactions is always the immortal soul in the physical world,and so is the ferroelectric and multiferroic physics.Center on this theme,this dissertation is divided into seven interactive components to be investigated and discussed.In Chapter I,we first give an overview of the significance,crystalline structure,electrical and magnetic properties of perovskite-type oxides.The nonstoichiometry effects and property controls of perovskite-type oxides in view of its special crystalline structure are discussed.And then,we focus on the definition and modification of quantum fluctuations in quantum paraelectric SrTiO3(STO).In parallel,the crystalline structure,magnetoelectric phase diagram,physical properties,especially the multiferroicity of the orthorhombic rare-earth manganites RMnO3(o-RMnO3)are addressed.Finally,we illustrate explicitly our research motivations,approaches,and main contents for this dissertation,from the perspectives of competing interactions and in view of the effects of quantum fluctuations on the critical phenomena in titanium ferroelectrics,and the effects of R/Mn nonstoichiometricity on multiferroic performances.We introduce relevant sample preparation methods,structural characterizations,and physical property measurements in sequence in Chapter II.The content in Chapters III,IV,V,and VI is the work we have done centering on the theme,which is just the core of this thesis.STO is considered to be a prototype of marginal quantum paraelectrics.It is a special dielectric whose dielectric constant rapidly increases with decreasing T because of the strong polarization fluctuations,and subsequently comes to its saturated value in a 104 scale in the relative dielectric constant unit at about 4K.Compared to normal ferroelectrics,the dipole-dipole interaction in STO is relatively so weak that the long range dipole ordering may appear only when the thermal fluctuations are significantly suppressed at very low temperature.However,as T approaches the absolute zero Kelvin,another kind of fluctuations,i.e.,quantum fluctuations(QFs)are highlighted so clearly that the stable long range dipole ordering can't come into being till the lowest T.Since the unstable polarization fluctuations and strong QFs coexist in STO at low T,and their magnitudes are similar,the ferroelectric property of STO is sensitive to lattice distortion caused by internal local disorders or external polar ions,and even weak chemical nonstoichiometry.In the first part of our work,we enhance the dipole-dipole interaction in STO by Sr/Ti ratio slightly deviating from 1 and Ba/Ca co-doping with the averaged Sr-site ionic radius identical to the Sr2+ ionic radius,to induce ferroelectric phase transitions.We investigate the impact of QFs on the critical exponents ? and ? of ferroelectric transitions in the modulated SrTiO3 system.The nonstoichiometric STO enhances dipole-dipole interaction while keeps high QFs,and reflects the competition between QFs and ferroelectric ordering.The Ba/Ca co-doped Sr1-x(Ca0.6389Ba0.3611)xTiO3(SCBT)introduces local lattice distortion with high QFs,and shows us mainly the competition between quenched disorders and ferroelectric ordering.It is revealed that both non-stoichiometric STO and SCBT exhibit much bigger critical exponents than the Landau mean-field theory predictions,i.e.?>>1,?>>0.5,which tells us that both strong QFs and prominent quenched disorder can lead to significantly enhanced critical exponents.These phenomena are confirmed by the calculations using a modified transverse Ising model and general mean-field theory applicable to the Ca2+ doped STO,i.e.,Sr1-xCaxTiO3(SCT)at low x level.Furthermore,a quantum-classical mapping is presented to explain that the crossover of quantum and classical ferroelectric phase transitions is bound to bring about the increased effective dimensionality for the order parameter fluctuations and thus the decreased correlation length of the order parameter.This is the origin for the consequently enhanced critical exponents.As the follow-up of Chapter ?,Chapter ? covers the multiple effects bought by a small amount of strong polarity of BaTiO3(BTO)substitution in marginal quantum paraelectric STO.The XRD,dielectric and pyroelectric characterization are carried on the sample BST(x=0.08)fabricated by high-T solid state method and polymeric precursor method.It is revealed that the broad rounded dielectric peaks in the ?'-T curves,multi-peak feature in the?'-T and pyroelectric current I-t curves for a series of BaxSr1-xTiO3(BST)at low doping level x?0.10 are the intrinsic properties for this STO-based system.XRD.SEM and high resolution TEM tests suggest that Sr2+ and Ba2+is homogeneous in this system.The broad rounded dielectric peak should originate the quenched disorders caused by Ba2+ doping.We attribute the multi-peaks in ?'—T and I—T curves to the structural phase transitions in the low-T range and the highlighted ferroelectric phase of STO by suppressing the quantum fluctuations.The strong coupling between spin,orbital,charge and lattice degrees of freedom in the orthorhombic rare-earth manganites RMnO3(o-RMnO3)gives rise to a complex phase diagram.These manganites play as central figures for unveiling the microscopic mechanisms underlying the magnetically induced ferroelectricity.Two kinds magnetic structures in o-RMnO3—nonlinear cycloidal spin(CS)orders and collinear ???? E-type antiferroic magnetic(E-AFM)orders,can break the space inverse symmetry and generate ferroelectric polarization.The ferroelectricity of CS orders are associated with the inverse Dzyaloshinskii-Mariya(DM)interactions/spin-orbital coupling.And the ferroelectricity in E-AFM orders comes from symmetric exchange striction associated with the spin-lattice coupling.Especially for R=Gd3+,Dy3+,Ho3+,the R3+spin ordering induced by the interplay between R3+and Mn3+ magnetism have been proven to have a significant effect on the multiferroic properties of o-RMnO3.It is noted that the specific spin structures of o-RMnO3 multiferroics are highly frustrated.We can modify their physical properties by R/Mn nonstoichiometry.In addition,the nonstoichiometry can simply change the balance of the internal interactions without additional effect caused by substituting species if any.The R-R,Mn-Mn,and Mn-R interactions can be reshuffled by tiny adjustment of the R/Mn stoichiometry.Consequently,the spin order and ferroelectricity can be modulated.o-DyMnO3 stands as a typical example of the be CS order multiferroics.Its ferroelectricity mainly comes from the Mn3+ CS orders.Meanwhile,the Dy spin order coherent with the Mn spin order emerges at temperature lower than TFE and higher than TDy.This coherence is believed to be induced by the strong Dy-Mn spin interaction,and play an important role in ferroelectricity generation.We address the simple Dy/Mn nonstoichiometry(Dy1-xMn1+xO3)as a measure to modulate the multifold competing interactions and reshuffle the spin frustration in multiferroic DyMnO3.The effects of chemical off-stoichiometry on the multiferroic properties are then examined.It is indicated that the measured polarization exhibits different T dependences on the Mn deficiency from that on the Dy deficiency:For the Mn-deficient case,the crystal structure and multiferroic properties remain relatively stable.On the other hand,for the Dy-deficient case,the Dy-deficiency disturbs the Mn3+ CS orders,suppresses the "induced" Dy3+CS orders,and destroys the independent commensurate Dy3+spin ordering.So far,we come to the conclusion that the Dy/Mn nonstoichiometricity in DyMnO3 is detrimental to the multiferroicity.o-HoMnO3 is regarded as the E-AFM order multiferroic representative.In the ferroelectric phase,the spin wave vector of the Mn3+E-AFM order is about 0.4b but its measured polarization points along the c-axis.This two seeming contradictory experimental results lie in the fact that the Mn3+E-AFM order leads to antiferroelectric orders along the a-axis,meanwhile the "induced" Ho3+noncollinear spin order by Ho-Mn exchange striction generates ferroelectric order along the c-axis.This problem is compounded by the emerging of independent commensurate Ho-Ho AFM interaction below the Ho3+ Neel temperature,THo.This independent Ho-Ho interaction competes with the Ho-Mn exchange striction.In this part,we investigate the consequences of the Ho-deficient non-stoichiometry in orthorhombic HoMnO3 in terms of microscopic mechanisms for ferroelectricity modulation.It is suggested that the Ho-deficiency(then Mn excess)results in Ho-vacancies and then Mn occupation of the Ho-site with increasing non-stoichiometry.The Ho-deficiency enhances the Mn-Mn symmetric exchange striction by suppressing the independent Ho-Ho interaction,and thus benefits to the induced Ho spin ordering against the independent Ho spin ordering.The symmetric Ho-Mn exchange striction is thus enhanced by this induced Ho spin ordering,leading to remarkably enhanced ferroelectric polarization as observed.This work presents an alternative scheme to modulate the multiferroicity in rare-earth manganites of strong 4f-3d coupling.Chapter VII presents the conclusion and perspectives.