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

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 YBaT2O5 (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 YBaMn2O5, whereas YBaFe2O5 and YBaCo2O5 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 YBaFe2O5 and YBaCo2O5. 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(z2) orbital of Mn in YBaMn2O5 gave rise to G-type ferrimagnetic spin ordering along the c direction and checkerboard-type charge ordering, whereas ordering of the d(x2-y2) orbital of Fe in YBaFe2O5 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 YBaCo2O5 activated spin and charge orderings similar to those in YBaFe2O5.