Spin states of Co ions in La1.5Ca0.5CoO4 from first principles

The spin states and electronic structure of layered perovskite La1.5Ca0.5CoO4 are investigated using full-potential linearized augmented plane-wave method. All the computational results indicate that the Co2+ ion is in a high-spin (HS, t(2g)(5)e(g)(2)) state and the Co3+ in a low-spin (LS, t(2g)(6)) state. The Co2+ t(2g) orbitals with a small crystal-field splitting are mixed by spin-orbit coupling, which accounts for the observed easy in-plane magnetism. The nonmagnetic LS-Co3+ state, which is stabilized by a strong crystal field, provides a natural explanation for the observed low magnetic ordering temperature and a spin-blockade phenomenon of the electron hopping. Furthermore, we find that the intermediate-spin (IS, t(2g)(5)e(g)(1)) state of Co3+ has a large multiplet splitting. But the lowest-lying IS state of Co3+ is still higher in energy than the LS ground state by a few hundred millielectron volts and the HS state of Co3+ is even less stable, both in sharp contrast to a recent experimental study which suggested the HS+IS mixed Co3+ ground state. We note that either the IS-Co3+ or HS-Co3+ states or their mixture would produce a wrong out-of-plane magnetic anisotropy and a much higher magnetic-ordering temperature than observed. Thus, the present work sheds light on this material concerning its electronic and magnetic structure, and it would stimulate different experiments to settle this intriguing spin-state issue.