High-spin and low-spin mixed state inLaSrCoO4: Anab initiostudy

Abstract

Spin state is an important issue for many cobaltates, and an intermediate spin (IS) state having a half-filled ${e}_{g}$ orbital may well be expected for a ${\text{Co}}^{3+}$ ion in a ${\text{CoO}}_{6}$ octahedron with a remarkable tetragonal distortion. Here the single-layered perovskite cobaltate ${\text{LaSrCoO}}_{4}$, which has a notable tetragonal elongation, is investigated for its spin state and electronic structure, through a set of local-spin-density approximation plus Hubbard $U$ $(\text{LSDA}+U)$ calculations including also the multiplet effect and spin-orbit coupling. Counterintuitively, our calculations evidence that the IS state is not the ground state and it would, even if being so, give rise to a wrong ferromagnetic half-metallic solution. We find that a strong band hybridization significantly suppresses a multiplet energy splitting of the IS state. Instead, a high-spin (HS) and low-spin (LS) mixed state turns out to have the lowest total energy among all possibly combined spin states. Moreover, the mixed $\text{HS}+\text{LS}$ ground state well accounts for the experimental paramagnetic insulating behavior, the effective magnetic moment, and the observed optical spectral features. We also predict that ${\text{LaSrCoO}}_{4}$ in the mixed $\text{HS}+\text{LS}$ ground state has a sizeable out-of-plane orbital moment and a local lattice distortion, which would motivate experimental studies.