Density functional theory studies of spin, charge, and orbital ordering in YBaT2O5 (T = Mn, Fe, Co)

R Vidya, P Ravindran, K Knizek, A Kjekshus, H Fjellvåg
Inorganic Chemistry 2008 August 4, 47 (15): 6608-20
Spin, charge, and orbital orderings are influenced by electron/hole doping, cation radii, oxygen stoichiometry, temperature, magnetic field, and so on. In order to understand the role of electron/hole doping, we have studied variations in spin, charge, and orbital ordering in terms of d-band filling for YBaT 2O 5 (T = Mn, Fe, Co). The calculations were performed using density functional theory as implemented in the full-potential linearized augmented-plane-wave method. We have carried out calculations for nonmagnetic, ferromagnetic, and antiferromagnetic configurations. A ferrimagnetic ground state was established for YBaMn 2O 5, whereas YBaFe 2O 5 and YBaCo 2O 5 have antiferromagnetic ground states; all of these results are in agreement with experimental findings. The effects of spin-orbit coupling, the Hubbard U parameter, and orbital polarization on the magnetic properties were also analyzed. The electronic band characteristics were analyzed using total as well as site- and orbital-projected densities of states. Inclusion of spin-orbit coupling and Coulomb correlation effects in the calculations was found to be important in order to reproduce the experimentally established semiconducting behaviors of YBaFe 2O 5 and YBaCo 2O 5. In order to quantify the charges at each atomic site, we made use of the Bader "atom-in-molecule" concept and Born effective-charge (BEC) analyses. The structural optimizations and BEC tensor calculations were performed using the VASP-PAW method. The different types of charge and orbital orderings in these compounds were visualized using the energy-projected density matrices of the d electrons. Substantial differences in ordering patterns with respect to d-band filling emerged. Ordering of the d z (2) orbital of Mn in YBaMn 2O 5 gave rise to G-type ferrimagnetic spin ordering along the c direction and checkerboard-type charge ordering, whereas ordering of the d x (2) - y (2) orbital of Fe in YBaFe 2O 5 caused Wollan-Koehler G-type antiferromagnetic spin ordering along the b direction and stripe-type charge ordering. Similarly, a complex pattern of orbital ordering in YBaCo 2O 5 activated spin and charge orderings similar to those in YBaFe 2O 5.

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