Ab initio investigation of structure, magnetism and ferroelectricity and their interplay in multiferroic bulk materials and films
Zusammenfassung der Projektergebnisse
The main objective of my research stay in the group of Prof. Nicola Spaldin was a theoretical study, based on the density functional theory (DFT), of a possible multiferroic (ferromagnetic and ferroelectric in a single phase) behavior of several members of the class of ferrimagnetic double perovskites A2BCO6. Our interest in this study was based on the recent progress in synthesis of a ferrimagnetic insulating double perovskite compound, Sr2CrOsO6. In the series Sr2CrMO6, where M = W, Re, Os, the ordering temperature rises by approximately 100 K by each added 5d electron, ranging from 500 K in Sr2CrWO8 and 635 K in Sr2CrReO6 to 725 K in Sr2CrOs06. The mechanism for high ordering temperature relies on a kinetic energy gain due to hybridization between the 3d-states of Cr and the 5d-states of the M atom. In Sr2CrOsO6 both Cr and Os have the t2g states of the valent d-shell filled and the eg states empty. This configuration results in the insulating behavior of the compound, since the magnetic moments on the two atoms order in an antiparallel fashion. The total magnetic moment of Sr2CrOs06 is about 0.54 μB and is due to the strong spin-orbit coupling. We focused our efforts on the double perovskite compounds containing one 3d and one 5d transition metal with fully filled t2g, or t2g and eg d-orbitals, in hope that the 3d-5d combination would ensure the high magnetic ordering temperature, while the filled orbitals would yield an insulator. While predicting a magnetic insulating compound with a high ordering temperature would already be a nice result, we aimed for including an additional property, namely we searched for a compound which would at the same time present ferroelectricity. Since both the 3d and the 5d atom placed in the oxygen octahedra are magnetic, their off-centering as a driving force for ferroelectricity of the compound is unlikely. We invoke here another mechanism of inducing proper ferroelectricity: the stereochemically active 6s2 lone pair of electrons. An example of this mechanism is BiMnO3, where Bications move in the direction opposite to oxygen-manganese cage. This mode is induced by the Bi-O covalency, in contrast to the usual perovskite ferroelectrics where it is the covalency of O with the cation situated in the oxygen cage that induces ferroelectricity. We have also placed Bi at the A-site of A2BCO6 double perovskites and predicted two promising multiferroic compounds: Bi2NiReO6 and Bi2MnRe06. The calculations were performed using Vienna Ab-initio Simulation Package (VASP) which is, as a pseudopotential and planewaves code, well-suited for structural optimization. The exchange interactions were estimated in the frozen magnon approach implemented in the full-potential augmented linearized planewave code, FLEUR. In both codes, generalized-gradient approximation (GGA) to DFT was used and the convergence of the results with respect to the number of k-points and basis functions carefully checked. We have also investigated the influence of inclusion of Hubbard U at the Mn, Ni, and Re sites. Since including a U on Re did not bring qualitative changes to our results, we consider further only its influence when included on Mn and Ni site. A Monte Carlo method was used for an estimation of the ordering temperature.
