Competition of various spin states of LaCoO 3

The spin-state transition in perovskite ${\mathrm{LaCoO}}_{3}$ has been a controversial topic ever since its discovery. To obtain a better physical understanding of its origin we have studied various magnetically ordered states of an enlarged double cell, including the low-spin state, intermediate-spin state, high-spin state, as well as all combinations among these three states. The calculations were done within the unrestricted Hartree-Fock approximation and the real-space recursion method. The density of states and stability of various states are analyzed as functions of model parameters. Our results show that the ground state of the system depends on the competition between the crystal-field splitting $\mathrm{Dq}$ and the Hund's coupling $j$. For a fixed $j$, the ground state of the system changes from the low-spin state to the low-spin high-spin ordered state (LS-HS) and finally to the high-spin high-spin antiferromagnetically ordered state as $\mathrm{Dq}$ is weakened. Our study suggests that the spin-state transition at 90 K probably takes place from the low-spin state to the LS-HS ordered state.