Doping-dependent charge and spin superstructures in layered cobalt perovskites

We have investigated cobaltite relatives of the layered perovskite cuprates and nickelates, Pr(2-x)Ca(x)CoO(4) (0.39 <= x <= 0.73) and La(2-x)Sr(x)CoO(4) (x=0.4, 0.61), 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 <= x <= 0.75. The charge order exists already at room temperature and shows no change on cooling. The incommensurability of its propagation vector, Q(c)=(epsilon(c), 0, l), roughly scales with the concentration of Co(2+) ions, epsilon(c) similar to (1-x). Magnetic order follows at low T less than or similar to 40 K and has twice larger periodicity, indicating a dominant antiferromagnetic correlation between the nearest Co(2+) spins.