Study Of The Superlattice Structures And Physical Properties Based On Perovskite-type Oxide

Multiferroic materials have potential application prospects in information storage and read-write,wireless microwave,wireless sensor network,and spintronic devices.However,single-phase multiferroic materials with coexisting magnetic and electric orders under room temperature are rare.Multiferroic materials have a weak magnetoelectric coupling effect.These shortcomings restrict the applications of multiferroic materials.Experimental and theoretical studies have reported that physical properties superlattice of multiferroic materials different from corresponding bulk,which may expand the application ranges of materials.Therefore,designing novel ferromagnetic/ferroelectric superlattice materials and exploring the microphysical mechanism between superlattice structure and performance can provide theoretical references for the experimental preparation of high-performance magnetoelectric materials as well as their practical applications.In this study,four types of superlattice materials based on perovskite-type oxide were designed from the perspective of ferromagnetic/ferroelectric superlattice design.A theoretical analysis of the geometric and electronic structures of all designed superlattice materials was conducted using the first-principle software VASP of the density functional theory（DFT）.The energy band characteristics and the magnetic,ferroelectric,half-metallic,and magnetoelectric coupling effects were analyzed thoroughly.The CaMnO₃/BaTiO₃ superlattice along the[001]surface was simulated by alternate piling of cubic CaMnO₃ and tetragonal ferroelectric materials BaTiO₃.Moreover,the geometric and electronic structures,magnetism,electric polarization strength,half-metallic property,and structural stability of the CaMnO₃/BaTiO₃ superlattice were investigated.Results indicated that cubic CaMnO₃ is in a metastable phase with half-metallic property and the magnetic moment is 3.00μ_B of the integer Bohr magneton.Furthermore,the magnetic moment of cubic CaMnO₃ is sensitive to volume expansion or compression.The CaMnO₃/BaTiO₃superlattice exhibits a ferromagnetic property and maintains a stable ferromagnetic half-metallic property.The magnetic moment is 12.00μ_B of the integer Bohr magneton.The half-metallic property is mainly attributed to the strong hybridization between electrons from the Mn-3d and O-2p orbits.The electric polarization strength of the CaMnO₃/BaTiO₃superlattice is 0.34 C/m²,thus showing good magnetoelectric properties.This CaMnO₃/BaTiO₃ superlattice structure has the pinning effect on the metastable phase of the cubic CaMnO₃ structure.The CaFeO₃/BaTiO₃ superlattice along the[001]surface was simulated through alternate piling of cubic CaFeO₃ and tetragonal BaTiO₃.The geometric and electronic structures,magnetism,electric polarization strength,and half-metallic property of the CaFeO₃/BaTiO₃superlattice were discussed.The results showed that cubic CaFeO₃ is ferromagnetic and the magnetic moment is 4.00μ_B of the integer Bohr magneton.Cubic CaFeO₃ has half-metallic properties,and its magnetic moment is relatively sensitive to the changes of the lattice constant.Cubic CaFeO₃ can only maintain the half-metallic property in the variation range of the lattice constant from 3.75?to 3.91?.The magnetic moment of the CaFeO₃/BaTiO₃superlattice is 16.00μ_B of the integer Bohr magneton.The CaFeO₃/BaTiO₃ superlattice keeps a ferromagnetic half-metallic property,which is mainly attributed to the strong hybridization between electrons from the Fe-3d and O-2p orbits.The comparison of the calculation methods of GGA and GGA+U shows that the accurate introduction of the Coulomb effect（U）of Fe atoms plays an important role in the half-metallic property of the CaFeO₃/BaTiO₃ superlattice.The electric polarization strength of the CaFeO₃/BaTiO₃ superlattice is 0.36 C/m²,indicating the strong magnetoelectric properties of the superlattice structure.The M/BiFeO₃（M=Co,Fe,Ni）superlattice along the[001]surface was constructed by alternate piling of ferromagnetic metal M（i.e.,Co,Fe,and Ni）and tetragonal multiferroic materials BiFeO₃.The geometric and electronic structures,atomic magnetic moment on the interface,and magnetoelectric coupling effect of the M/BiFeO₃（M=Co,Fe,Ni）superlattice were discussed.The results showed that a significant magnetization difference exists between upper and lower interfaces in the M/BiFeO₃ superlattice structure.Strong hybridization of electrons from the epitaxial ferromagnetic atom M-3d and O-2p orbits in the BiFeO₃ layer,the interface layer interact with each other mainly through the charge transfer mediated by O atoms.Given the electric polarization,the bonding of interface atoms and the redistribution of charge transfers cause a strong magnetoelectric coupling effect when an external electric field exists.The magnetoelectric coupling coefficients of Co/BiFeO₃,Fe/BiFeO₃,and Ni/BiFeO₃superlattice structures areα_s≈4.2×10^-10 G cm²/V,α_s≈3.8×10^-10 G cm²/V,andα_s≈3.5×10^-10 G cm²/V,respectively.The magnetoelectric coupling coefficient of Co/BiFeO₃superlattice interfaces is stronger than that of Fe/BiFeO₃ and Ni/BiFeO₃,thus showing good magnetoelectric coupling effect.The Co₃O₄/BiFeO₃ superlattice along the[001]surface was constructed by alternate piling of the cubic antiferromagnetic semiconductor Co₃O₄ and tetragonal multiferroic materials BiFeO₃.The geometric and electronic structures,magnetism,and crystal structural stability of two types of interface were discussed.The reasons for the magnetism variation of magnetic thin film materials were investigated.The results indicated that the crystal bonding energy of the Co^A-BiO and Co₂^BO₄-FeO₂ interface is 0.124 eV than that of the Co^A-FeO₂ and Co₂^BO₄-BiO interface.The Co₃O₄/BiFeO₃ superlattice structure,which is formed by alternate piling of Co^A-BiO and Co₂^BO₄-FeO₂,is stable.The Co₃O₄/BiFeO₃ superlattice structures of two types of interface exhibit ferrimagnetism.Under epitaxial stress,the atomic bonding and charge transfer at the interface can induce the transition of the composite system from antiferromagnetism to ferrimagnetism effectively.The internal exchange effect of the magnetic nano-film materials makes the atomic magnetic moment arrange in parallel or antiparallel order,which eventually leads to spontaneous magnetization between magnetic atoms.