Theoretical Study Of Tuning The Magnetoelectric Properties Of Perovskite Multiferroic Compounds

With the rapid development of information industry, people usually pursuit devices of abetter performance with large capacity, high speed and low energy cost. Multiferroic materialshold both the charge and spin orders and their coupling effect, possessing more freedoms todesign high quality devices to meet the needs of modern information industry. Multiferroicmaterials, with two or more ferroic orderings, e.g.,（anti）ferromagnetic, ferroelectric,ferrotorriodic and ferroelastic orderings, in the same phase, also raise a series of newchallenges for the experimental and theoretical studies. The single phase multiferroicmaterials have been studied widely, as their magnetism and ferroelectricity can be stronglycoupled. However, coexistence of ferromagnetic and ferroelectricity in single-phasemagnetoelectric materials is relatively rare, while most of magnetoelectric materials showantiferromagnetic coupling, not conducive for practical application.This dissertation is aimed to design new types of magnetoelectric materials and analysistheir magnetic and ferroelectric coupling mechanism, mainly exploring ABO₃perovskitematerials with coexistence of ferromagnetim and ferroelectricity by first principles calculation.The dopant, stain, defect and high pressure effects are studied to improve the magnetoelectricproperties of single phase ABO₃perovskite.In chapters3and4, we discuss the doping, strain and oxygen vacancy effects on BiCoO₃.Doped Cr and Fe ions could be FM ordered in BiCoO₃and provide a net magnetic moment inBiCoO₃, while the strong ferroelectricity of pure BiCoO₃is almost maintained. Thepolarization of BiCoO₃decreases with the tensile strain while the magnetic property is littleaffected by the tensile strain. The oxygen vacancy significantly changes the local magneticmoment of Co ions and possibly provides net magnetic moments to the BiCoO₃system.Those phenomenons can be explained by a simple picture with the crystal field theory andlocal interactions between oxygen ions and the magnetic cations. If the collective magnetismcan be realized, it will enrich the physics of BiCoO₃as a multiferroic material and broaden itspossible application.In chapters5and6, we study the A or B site doping and oxygen defects effect on PbVO₃to improve its magnetoelectric properties. It is found that the antiferromagnetic coupling is maintained and the ferroelectric property is absence when La or Bi is doped at A site ofPbVO₃. The3d transition elements doping at B site of PbVO₃could bring remnant magneticmoments and preserve the large ferroelectric polarization except Ni and Cu. The ferroelectricpolarization and magnetic property have been little affected by the oxygen vacancy. Thefluorides substituted oxygen defect possibly provides1μBnet magnetic moments in additiontothe enhancement of the ferroelectric polarization of PbVO₃.At the last chapter, we have investigated the magnetic property of cubic SrMnO₃with andwithout La or Ba doping under high pressures. Cubic SrMnO₃keeps G-AFM magnetic orderunder the positive pressure and rich magnetic phase transition behaviors are observed undernegative pressures. The four kinds of magnetic phase transitions can also be found in Badoped SrMnO₃with a reduced negative critical transition pressure. However, the originalground state of G-AFM could not be stabilized in the La doped SrMnO₃, in line with recentexperimental reports.