Atomistic Simulation Study Of Multiferroic Rare-earth Manganites

Rare-earth manganites RMnO3 based on rare-earth ions with small ionic radii are well known examples of a multiferroic, which has great potential interest in sensors, memory devices, etc. In the present thesis, an atomistic simulation method was employed to investigate the crystal structural features and defect behaviors of the last several members of the rare-earth manganites series. This work provides a further understanding of the promising properties of the related materials and the mechanism behind, and is helpful for finding modified RMnO3 based multiferroics with desired performance.Firstly, a short-range Buckingham interaction potential together with a core-shell model were developed for hexagonal h-YMnO3 and h-HoMnO3, respectively, which were able to accurately describe their structure properties. The potentials were further employed to investigate the defects behavior therein. The calculation results showed that among all possible intrinsic defects, O Frenkel disorder was the most energetically favorable one; besides, planar oxygen vacancies had a lower energy compared with their apical counterparts; charge compensation upon doping was more likely to occur via electronic defect formation; and in a perfect bulk, the dopants tended to enter Mn site while in oxygen nonstoichiometric material, the defect energy of cation substitution especially rare-earth substitution was lowered significantly. The distortion of MnO5 bipyramid was expected to be enhanced with dopants incorporation, however, the tilting and buckling would be depressed, and consequently, affected relevant ferroelectric polarization and magnetic properties.Secondly, the proposed potential set was applied to model the neutral 180°transverse domain wall of two configurations in both perfect YMnO3 and YMnO3-δ. In the former, there was no obvious energy difference between the two different domain walls. The domain wall width was found to be narrow and the lattice distortion was significantly depressed near domain center. Besides, Mn3+ ions and Y3+ ions formed a double arc pattern around the center area. In YMnO3-δ, the energy difference between two configurations was broadened, and the oxygen vacancies tended to accumulate and order along the domain wall, which further suppressed the structural distortion, but smoothened the abrupt structural change in the vicinity, thus enhanced the domain wall conductivity.Thirdly, we developed interatomic potentials for metastable orthorhombic o-RMnO3(R=Dy, Y, Ho), respectively. The reproduced structures of the original materials and Ca2+ doped systems accorded well with experiment observations. Schottky disorders were found to be the dominate intrinsic disorder in these materials, and the anti-site defects involving Mn3+ ion and R3+ ion were also energetically favorable. The introduced Ca2+ dopants tended to form chemically and structurally CaMnO3-like clusters in the lightly doped R1-x CaxMnO3, while in half doped R0.5Ca0.5MnO3, a charge ordering state with RMnO3-like and CaMnO3-like layer stripes pattern was favored. Furthermore, mangnites with a smaller rare-earth ion were more likely to form charge ordered stripes.Finally, based on the radial Coulomb interaction and Buckingham potential, we introduced a new potential based on Angular Overlap model, which was capable of describing the non-spherical interaction of transition metals. The newly suggested model successfully reproduced the structure of both hexagonal and orthorhombic structure of YMnO3. The intrinsic defects were examined by supercell method with a correction of the interaction between a point defect and its image. The oxidation reactions for h-MnO3 and o-YMnO3 were found to occur via oxygen interstitial and yttrium vacancy formation, respectively, and the ability of the orthorhombic phase to accommodate oxidative nonstoichiometry was to be expected.