Magnetic structure of the layered perovskiteLaSrCoO4

Abstract

By using the unrestricted Hartree-Fock approximation and the real-space recursion method, we have investigated various magnetic structures in an enlarged double cell of layered perovskite ${\mathrm{LaSrCoO}}_{4}.$ These include the low-spin state ${(t}_{2g}^{6}{e}_{g}^{0}),$ the intermediate-spin state ${(t}_{2g}^{5}{e}_{g}^{1}),$ and the high-spin state ${(t}_{2g}^{4}{e}_{g}^{2})$ as well as all possible combinations among these three states. The densities of states are computed and stabilities of various states are analyzed as functions of the model parameters. For a fixed Hund's coupling $j,$ it is shown that the ground state of ${\mathrm{LaSrCoO}}_{4}$ transforms first from the antiferromagnetic high-spin state to the ferromagnetically ordered high-spin low-spin state, and then to the nonmagnetic low-spin state as the crystal-field splitting $\mathrm{Dq}$ increases. The intermediate-spin state never becomes the ground state of ${\mathrm{LaSrCoO}}_{4}.$ In view of the measured effective magnetic moment of ${\ensuremath{\mu}}_{\mathrm{eff}}\ensuremath{\sim}2.6{\ensuremath{\mu}}_{B},$ we conclude that the most probable ground state of ${\mathrm{LaSrCoO}}_{4}$ lies in the regime of ferromagnetically ordered high-spin low-spin state, this state is also consistent with the optical conductivity spectra and neutron-diffraction experiment.