Theoretical study of the spin and orbital ordered states ofLaBaMn2O5.5

Abstract

The electronic and magnetic structures of an ordered oxygen-deficient perovskite, ${\mathrm{LaBaMn}}_{2}{\mathrm{O}}_{5.5}$, have been investigated using the unrestricted Hartree-Fock approximation and the real-space recursion method. Among the four metastable spin states found in this paper, the ferromagnetic spin state and the spin-ladder antiferromagnetic state with ferromagnetic coupling along the ladder ($b$ and $c$ axes) and antiferromagnetic coupling perpendicular to the ladder ($a$ axis) are found to have almost the same lowest energies. However, the spin-ladder antiferromagnetic state is further stabilized by the Jahn-Teller distortion of octahedra along the $b$ axis, and the resulting orbital ordering of Mn(1) pyramid/Mn(2) octahedra takes the form of the ${d}_{3{z}^{2}\ensuremath{-}{r}^{2}}∕{d}_{3{y}^{2}\ensuremath{-}{r}^{2}}$ pattern. Our results are consistent with the experimental findings.