Theoretical Study On The Multiferroic Properties Of Perovskite Superlattices

Multiferroic materials, which possess simultaneously ferroelectric, magnetic, and/or ferroelastic long range orders, have attracted a great deal of attention recently not only because of their vast potential application in information storage, low energy dissipa-tion devices, fast read-write-devices and so on, but also because of their fundamental importance in unveiling the competition among charge, spin, orbital degrees of free-dom. The motivation for searching for the multiferroic material is its ability to cross-control its physical properties by external field. However, the development of multi-ferroic materials have been hindered because of the "incompatible" requirements for the magnetism and the ferroelectricity. The field was steered to a rapid path recently since the discoveries of TbMnO3, BiFeO3, and EuTiO3, in which the ferroelectrici-ty is induced by spin density waves (SDWs), lone-pair electrons, and epitaxial strain, respectively, thus the "incompatible" issue between the magnetism and the ferroelec-tricity have been overcome. Although great progresses have been made in the field of multiferroic materials, many problems still need to be resolved before their applica-tions. To name a few:Although the magnetoelectric coupling is strong in the SDWs induced multiferroic in TbMnO3, the electric polarization is rather small; While the po-larization induced by the lone-pairs electron of Bi-ion is large in BiFeO3, the magnetic moment comes from Fe-ions. This makes the magnetoelectric coupling small; In addi-tion, most of current multiferroic materials are accompanied by the antiferromagnetic ordering instead of the ferromagnetic ordering, thus cross-control using external field becomes difficult; the critical temperature for either magnetic or ferroelectric ordering is low and is not suitable for application.As the thin-film deposition methods, such as molecule beam epitaxy and pulsed laser deposition, become mature technologies, it is possible to control the deposition rate on atomic scale. Using interface engineering one can combine atomic layers with different properties to form superlattices with improved functionalities. This opens up a new pathway to develop the multiferroic materials. In this thesis, we mainly focused on the physical properties of short period superlattices constructed with perovskites based on different transition metal oxides. In SrTiO3/SrCoO3 superlattices, we explored the possibility to enhance the ferromagnetic superexchange coupling via interface enegi-neering. The ferroelectric and ferromagnetic multiferroic phase is obtained for suitable epitaxia strain and strong interface mediated magnetoelectric coupling is also achieved. Then in SrMnO3/SrCoO3 superlattices, we use the Kanamori-Goodenough rule to im-prove the ferromagnetic superexchange coupling between different magnetic Mn and Co ions, direct magnetoelectric coupling and reduced epitaxial strain for multiferroic phase is realized. Finally, we have reinvestigated the physical mechanism for ferro-electricity using a phenomenological ionic model. The inflexion point of the Madelung potential is found to play a major role, thus the ionic binding instead of d0-ness is be-lieved to the be necessary condition for ferroelectric instability. This guidance is used to explore the possibility of multiferroic in BaFeO3/BaMnO3 superlattice. The details are described below:(1) We have studied the phase diagram of the SrTiO3/SrCoO3 superlattices us-ing the first principle calculations. The possible structures of superlattices was de-rived from the soft-phonon modes of the superlattice. The ferromagnetic (FM) and G-antiferromagnetic (G-AFM) structures are considered. The complete phase diagram is obtained as a function of epitaxial strain. A ferroelectric ferromagnetic multiferroic is found in crystal Pc structure with strong magneto-electric coupling mediated via interface strain. The polarization mainly comes from SrTiO3 layers while the magne-tization mainly comes form SrCoO3 layer. The yz/zx orbital ordering between neigh-boring Co-ions plays an important role in stabilizing the ferromagnetism. In addition, the half-metal property and metal-insulator transition phenomenon are also found in other part of the epitaxial strain range.(2) To improve the magnetoelectric coupling, we have studied the SrMnO3/SrCoO3 superlattices with both transition metal ions contributing to magnetism. The choice of Mn and Co based perovskites is made because of the Kanamori-Goodenough rule which stipulates the magnetic coupling between different transition metal ions. The fer- romagnetic (FM) and G-antiferromagnetic (G-AFM) C-antiferromagnetic (C-AFM), and A-antiferromagnetic (A-AFM) structures are considered. The enhanced ferromag-netic superexchange interaction between Co-Mn not only helps to increase the critical temperature of ferromagnetism up to room temperature, but also help to reduce the ten-sile strain required by the ferromagnetic phase to+1.80% in comparison with+3.40% in SrMnO3 thin films. The detailed analysis reveals that the polarization mainly come from the SrMnO3 layer, and the yz/zx orbital ordering between nearest-neighbour Co-ions are responsible for the ferromagnetism in SrCoO3 layer. Comparing with the SrTiO3/SrCoO3 superlattices in which the magnetic order is of quasi-two-dimensional nature, the magnetic order is three-dimensional in SrMnO3/SrCoO3 superlattices.(3) By studying the Peierls structural instability in a one-dimensional ionic chain, we find that the inflexion point in the Madelung potential plays a crucial role in i-dentifying the transition point. This suggests that the ionic binding instead of the d0 electronic structure fulfills the necessary condition for polar distortion. As the rela-tive weight of ionic binding increases with increasing bond length, it is possible to cross the inflexion point of the Madelung potential if ion with large ionic radius is used or large tensile strain is imposed externally. Therefore, the same mechanism should work in magnetic compounds if covalent bonding is made as small as possible. To test this idea, we have studied the stability of ionic crystal, NaCl, against polar distortion under either in-plane compressive and tensile, or uniform volume tensile strain, the ferroelectricity is always achieved if suitable strain is applied. This phenomenological picture has guided us to study BaFeO3/BaMnO3 superlattices by replacing the small Sr ions with larger Ba ions so as to make the bond length crosses the inflexion point of Madelung potential. Our first-principles calculation shows that ferroelectric and ferromagnetic multiferroic property do realize in BaFeO3/BaMnO3 superlattices with strong magnetoelectric coupling.