We have investigated cobaltite relatives of the layered perovskite cuprates and nickelates, ${\text{Pr}}_{2-x}{\text{Ca}}_x{\text{CoO}}_4$ $\left(0.39\le x\le 0.73\right)$ and ${\text{La}}_{2-x}{\text{Sr}}_x{\text{CoO}}_4$ $\left(x=0.4,0.61\right)$, using elastic neutron scattering. We have discovered doping-dependent incommensurate short-range ordering of charges and magnetic moments, which in cobaltites occur in the nonitinerant polaron phase, for $0.5\lesssim x\lesssim 0.75$. The charge order exists already at room temperature and shows no change on cooling. The incommensurability of its propagation vector, ${\mathbf{Q}}_c=\left({ϵ}_c,0,l\right)$, roughly scales with the concentration of ${\text{Co}}^{2+}$ ions, ${ϵ}_c\sim \left(1-x\right)$. Magnetic order follows at low $T\lesssim 40\text{K}$ and has twice larger periodicity, indicating a dominant antiferromagnetic correlation between the nearest ${\text{Co}}^{2+}$ spins.

• Received 7 October 2008

DOI:https://doi.org/10.1103/PhysRevB.78.180406