Theoretical Study On The Magnetic Transition Of 3d Transition Metal Oxide

The magnetic phase transition is a very important phenomenon in the study of tran-sition metal oxides.The magnetic phase transition includes the spin state transition and the magnetic order transition.The magnetic phase transition reflects the changes in the competitive relationship of various forms of energy.The competitive relation-ship is determined by many physical factors,such as charge density,bond length,bond angle,crystal symmetry and so on.Therefore,by adjusting the relevant physical quan-tities such as stress,doping,temperature,external field,interface engineering and so on,magnetic properties of the transition metal oxides can be modulated.On the other hand,changes of the magnetic properties means there are relevant changes in elec-tronic structure,resulting corresponding changes in the thermal,mechanical,optical and electrical properties of the materials.So,the understanding of the magnetic phase transition can be the key to understand the mechanism of other macroscopic physical properties.These two points are the starting point of the study in this doctoral thesis.In this thesis,we have studied the magnetic phase transition of two types of 3d transition metal oxides theoretically.One is Bi2A2Co2O8(A=Ca,Sr,Ba)series of lay-ered materials.Although these materials have attracted much attention in recent years because of their excellent thermoelectric properties,theoretical study of the magnetic structure is quite rare.The other one is multiferroic heterostructure composed of two types of ABO3 perovskite materials via interfacial engineering.This kind of material is considered to be the promising multiferroic material.Related experimental research has been very sufficient.While,a series of problems exist in theoretical researches.We use the first-principle calculation as our research method.By comparing the energy of different magnetic structures,we find the ground state and possible magnetic transition.By exploring the magnetic transition,we explain or predict a variety of experimental phenomena.The doctoral thesis is divided into six chapters.In the first chapter,with using ABO3 perovskite materials as the background of the discussion,the magnetic proper-ties of transition metal oxides are introduced.The basic concepts and common models used in the study are also introduced.Then we give brief on the research background and recent progress of the two types of transition metal oxides.The second chapter introduces the theoretical background of the first-principles calculation.From the third to the fifth chapters,three main works of my Ph.D program are elaborated respectively.Finally.a brief summary is made in the sixth chapter and the prospect of the future research is put forward.The summary of the three main works of this doctoral thesis is as follows:(1)We theoretically give the mechanism of the electrical transport properties of Bi2A2Co2O8(A=Ca.Sr,Ba)(BACO=BCCO,BSCO.BBCO),which is dependent on the lattice constant.BACO has experienced insulator-metal transition when the ionic ra-dius of A ion increases or the temperature increases.Around the transition tempera-ture,sharp peak in the specific heat curve can be suppressed by large magnetic field.It means that the change of electrical transport properties is intrinsically related to the magnetic degree of freedom.Considering the insulator-metal transition in LaCoO3 is induced by the spin state transition of Co3+.we assume similar physical mechanism may exist in BACO system with Co3+.With the LSDA+U scheme,we carry out com-prehensive numerical calculations among a series of possible spin states and magnetic order structures,and finally get two possible states that determine the ground state of this series of materials.They are strongly correlated low-spin-state insulator(LS state)and intermediate-spin-low-spin-state metal(IS-LS mixed-state).The LS state explains the insulating properties of BCCO and BSCO in low temperature region.As lattice parameters increase and the crystal field splitting is reduced,the IS-LS mixed-state becomes the ground state,with its semi-metal band structure explaining the insulator-metal transition of BCCO and BSCO in the high temperature,as well as the metal behavior of BBCO.Meanwhile,the spin fluctuations of the IS-LS mixed-state explain a series of physical properties such as the magnetic field suppressed specific heat peak.The hexagonal symmetry of the IS-LS mixed-state accords with the properties of the Fermi surface obtained by ARPES measurement.(2)We theoretically give the mechanism of the thickness-dependent insulators-metal transition of Bi2Ca(Sr)2Co2O8(BSCO)nanosheets.The experiment shows that mechanically exfoliated BSCO nanosheets thinner than 4 blocks show insulating be-havior in 0-300K while nanosheet with 4 blocks shows insulator-metal transition at 140K.Through comprehensive numerical calculations,we confirmed that the insulator-metal transition observed in BSCO nanosheets can naturally be explained by insulating LS state and metallic IS-LS mixed-state.We calculated the energy difference between the two states of nanosheets with different blocks.We assume the transition tempera-ture scale with this energy difference in a linear mode.The energy difference decrease as the thickness increases,thus the corresponding transition temperature gets lower.Our calculated transition temperature of the nanosheet with 4 blocks is 156K,which is in good agreement with the experiment(140K).While,the predicted transition temper-ature of the nanosheet with 1-3 blocks is above room temperature,which is a result that can be used to check our proposed mechanism if relevant experiments can be carried out in the future.(3)We study the magnetic reconstruction in monecell SrMnO3 on ferroelectric BaTiO3 substrate.The ferroelectric/ferromagnetic composite structure is the most pop-ular design scheme to achieve magnetoelectric coupling effect.Doped manganese ox-ide(LAMO)nano films are selected as the most commonly used magnetic layer be-cause of its rich phase diagram and the sensitivity to the fluctuation of charge.While,the choice of LAMO nano films will lead to two problems in theoretical study.On the one hand,experimental results show that the screening length of charge modulation effect is of the order of one-unit cell in LSMO films,therefore multilayer magnetic film involves sizeable carrier inhomogeneity.So the magnetic order can be extremely complicated.On the other hand,doped thin films less than 10 unit cells are suscepti-ble to Anderson's localization,so the actual transport properties can be quite different from the theoretically predicted results.The stoichiometric monocell SrMn03 is a rig-orously carrier homogeneous system,which avoids the two problems.Our calculations on monocell SrMnO3/BaTiO3 composite structure show that magnetic reconstruction takes place between ferromagnetic metal to nearest-neighbor antiferromagnetic insu-lator when the ferroelectric polarization direction of BaTi03 changes from pointing to the SrMnO3 to the opposite.Our research provides a new way for the design of magnetoelectric multifunction devices.