First-Principles Investigation On The Structures And Properties Of Multiferroics

The term multiferroism has been coined to describe materials in which two or all three of ferroelectricity, ferromagnetism and ferroelasticity occur in the same phase. This means that they have a spontaneous magnetization which can be reoriented by an applied magnetic field, a spontaneous polarization which can be reoriented by an applied electric field, and a spontaneous deformation which can be reoriented by an applied stress. In this work we focus on magnetoelectric multiferroics, which are simultaneously ferromagnetic and ferroelectric. Great device applications are suggested for such materials and There has been growing recent interest in them, especially in the origin of ferroelectric and the coupling between ferroelectric and ferromagnetic.In this article, the perovskite-type ferromagnetic ferroelectric BiMnO3, anti-ferromagnetic ferroelectric BiFeO3 and the nonoxidic anti-ferromagnetic ferroelectric BaCoF4 were investigated. The CASTEP module of Materials Studio 4.0 (Accelrys) was utilized to calculate electronic structure of materials concerning to spin-polarized generalized-gradient approximation and plane-wave ultra soft pseudopotential.It plays a key role to study perovskite-type magnetoelectric material BiMnO3 as a"hydrogen atom"for that the first principals research on multiferroics materials begins on this materials. In common sense, the inducing mechanism for coexist of ferromagneticity and ferroelectricity come from the directional localization of 6s lone-pair electron in Bi atom.In this paper, the electronic structure of multiferroic BiMnO3 was calculated based on the first principals, including its high temperature paraelectric paramagnetic Pbnm phase, room temperature ferroelectric paramagnetic monoclinic phase and low temperature ferroelectric ferromagnetic monoclinic phase. The ferroelectric distortion-inducing electric polarization was very low, and its estimation was 0.95μC/cm2,at the same time, low temperature electric polarization is lower than that of room temperature. After the ferroelectric distortion, the charge transfer happened on Bi-6p to strengthen the covalent interaction between Bi-6p obit and O-2p orbit selectively and orientedly to help the appearance of energetic split between bonding orbit and anti-bonding orbit to decrease the system energy. Initiating from that, the change on spatial distribution of electronic density destroyed the equilibrium of static electric interaction with 6s electrons distributing at lower energy ,which polarize toward energetic reduction away from Oxide electrons cloud. The former is the prime factor for ferroelectric distortion; while the latter (polarization of 6s electron) is the subsequent change and helps the Bi–O relative displacement, which profit for stabilizing system to promote ferroelectric distortion futher. The study on orbit order and spin order indicates that the ferroelectric distortion changes the orbit order of Mn-3d for the Jahn-Teller distortion of Mn3+–O octahedron requires the dz2 orbit locating chemical bond length maximum. According to the Goodenough and Kanamori theories, the change of orbit order caused the spin order to change from A type anti ferromagnetic order of paraelectric orthogonal phase to ferromagnetic order containing a little spin frustration of ferroelectric phase, which exhibits the coupling interaction of ferroelectricity and ferromagneticity. At low temperature, spin frustration and lattice order couples in one system, especially at Mn1 position. The competition among the coupling lattice distortion, Jahn-Teller distortion and ferroelectric distortion inhibits the electric polarization very much to make the theoretical electric polarization at low temperature lower than that at room temperature on the countrary.BiFeO3 has the strong spontaneous electric polarization and the Bi atom at A positon as same as BiMnO3, but less inner competition with ferroelectric distortion than BiMnO3, therefore the nature of this kind of materials can be discovered based on its research. We then calculated and analyzed the Born effective charges, Mulliken atomic population, chemical bond overlap population and partial density of states of both ferroelectric phase and possible high temperature paraelectric cubic phase.Bi atom's Born effective charge is much bigger than both other atoms and formal charges itself. Therefore, the most contributors to the system ferroelectric distortion are Bi atoms, or the Bi atom is the fundamental factor for the system ferroelectric distortion. We obtained very large theoretical electric polarization of 85 according to the relationship between Born effective charge and electric polarization:And it certificates very well the results in thin-film test. The calculation on total energy certificates that BiFeO3 ground state was G type anti-ferromagnetic order, the theoretical magnetic moment of Fe ion was 3.80μB close to high spin state, according with experiment values. After ferroelectric phase transformation, the systemic chemical bond changes much following with Bi-6s and Bi-6p state charge transfer, and the effect of Bi-6s state electron manifest. Partial density of states calculation indicates that the energetic split between bonding orbit of Bi-6p state and anti-bonding orbit was bigger than other electronic states, by which covalent bond between Bi-6p state and O-2p state strengthens. Thus the discussion about the BiMnO3was testified. Bi-6s state polarizes due to the static electric repulsion, but does not take part in the hybridization of Bi-6p and O-2pelectrons. The charge transfer originates from the weak covalent interaction between itself and Ot-2s,2p orbits, by which Bi-6s state electronic polarization is reinforced. This kind of covalent interaction helps the relative replacement of Bi–O ,which is the reason for the strong systemic electric polarization comparing with BiMnO3 system. This process exhibits that the ferroelectric distortion is one balance of static electric interaction and covalent interaction. BaCoF 4 is affiliated to BaMF4(M= Mn, Fe, Co, Ni, Zn, Mg)series of non-oxide multiferroics, and the study on its magnetoelectricity starts just now in recent years. Its inducing mechanism of ferroelectric is different from previous perovskite-type materials.The anti ferromagnetic phase of BaCoF4 is stable which was testified via comparing the total energy among paramagnetic phase, high spin ferromagnetic phase, low spin ferromagnetic phase and anti ferromagnetic phase. This character is probably profit to the ferroelectric stability at low temperature. The structure of energy bandes tells us that GGS can be used in such system. The ion bond interaction is the main interation in the BaCoF4 system due to the strong electronegativity of F atoms, based on the Mulliken population analysis, density of state and Co–F electronic density figures. As CoF6 octahedron is concerned, it is entirely ion bond between Co ion and F2 or F3 ions (bc plane), but weak covalent bond between Co ion and F4 ion and more weaker as for F1 ion. It was found that the spontaneous electric polar direction is along [001], when a theoretical model nearest to center symmetry structure Cmcm phase is constructed and its electronic density is compared with above mentioned ahead. Meanwhile, the ferroelectric distortion is only derived from Ba ion and F ion relative displacement along c axis, but not b axis due to the sturctrural symmetry operation. F4 displacement contributes least to ferroelectric phase transformation due to its weak covalent bond interaction with Co making a center symmetry trend array. Three center symmetry trend Ft ions and two polarized Fp ions of Ba on a same bc plane form into a F pentaring center whose static balance is easy to be destroyed. In addition, the energy of s and p orbits of F2 or F3 ion is lower in the ferroelectric phase than in the cented symmetry phase and the covalence character of F1 ion whose contribution to the displacement is largest almost lose, which stabilize the structure of the ferroelectric system. At the same time,the radius of Ba ion is very large, not easy to return to the balance state. Thus the ferroelectric distortion happened. It has an important role to investigate magnetoelectricity of BaMF4 like multiferroics for design novel multiferroics.