Magnetic and transport properties are investigated for single crystals of layered perovskite cobalt oxides, ${\mathrm{La}}_{2\mathrm{-}\mathrm{x}}$${\mathrm{Sr}}_{\mathrm{x}}$${\mathrm{CoO}}_4$ (0.4⩽x⩽1.0). The effective magnetic moment ${\mathrm{\mu }}_{\mathrm{eff}}$ is found to decrease with increasing x (or increasing the number of ${\mathrm{Co}}^{3+}$ ions) from ≈4.0${\mathrm{\mu }}_{\mathrm{B}}$ for x=0.4–0.5 to ≈2.6${\mathrm{\mu }}_{\mathrm{B}}$ for x=0.8–1.0, accompanied by a steep reduction of the resistivity ${\mathrm{\rho }}_{\mathrm{ab}}$ and Wiess temperature Θ. These drastic changes of the magnetic and electronic properties suggest a spin-state transition of the ${\mathrm{Co}}^{3+}$ ions from the high-spin to the intermediate-spin state.

DOI:https://doi.org/10.1103/PhysRevB.55.R14725