Charge and spin ordering in La2−xSrxCoO4 (0.4<x<0.6)

Neutron scattering experiments were performed on the single crystals of layered-perovskite cobaltate La2−xSrxCoO4. For a commensurate value of x=0.5, magnetic reflections due to high-spin (HS) state of Co2+ are observed in (H, 0, L) plane. Further, in (H, H, L) plane, we found super-lattice reflections with wave vector qm=(1/4, 1/4, 1). The latter magnetic peaks cannot be understood by the Co2+ spin configurations, indicating that these peaks are originated from Co3+ spin ordering. From the hole-doping dependence, short ranged charge and spin orderings were observed below x=0.5, while those of peaks were suppressed at x=0.6. Our results suggest that double exchange interaction between the HS state of Co2+ and the intermediate spin (IS) state of Co3+ mediate the IS state in this system.