Spin and charge orders and their hole - doping dependence in single layered cobaltate

Neutron scattering experiments were performed on single crystals of layered cobalt-oxides La2−xCaxCoO4 (LCCO) to characterize the charge and spin orders in a wide hole-doping range of 0.3x0.8. For a commensurate value of x=0.5 in (H,0,L) plane, two types of superlattice reflections concomitantly appear at low temperature; one corresponds to a checkerboard charge ordered pattern of Co/Co ions and the other is magnetic in origin. Further, the latter magnetic-superlattice peaks show two types of symmetry in the reflections, suggesting antiferromagnetic-stacking (AF-S) and ferromagnetic-stacking (F-S) patterns of spins along the c direction. From the hole-doping dependence, the in-plane correlation lengths of both charge and spin orders are found to give a maximum at x=0.5. These features are the same with those of x=0.5 in La1−xSr1+xMnO4 (LSMO), a typical checkerboard and spin ordered compound. However, in (H,H,L) plane, we found a magnetic scattering peak at Q=(1/4,1/4,1/2) position below TN . This magnetic peak can not be understood by considering the Co spin configuration, suggesting that this peak is originated from Co spin order. By analyzing these superlattice reflections, we found that they are originated from high-spin state of Co spin order.