Spin and charge orders and their hole-doping dependence in single layered cobaltate La _2-xCa _xCoO ₄ (0.3 ≦ x ≦ 0.8)

Neutron scattering experiments were performed on single crystals of layered cobalt-oxides La _2-xCa _x-CoO ₄ (LCCO) to characterize the charge and spin orders in a wide hole-doping range of 0.3 ≦ x ≦ 0.8. 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 ²⁺/ Co ³⁺ 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 La _i-xSr _1+xMnO ₄ (LSMO), a typical checkerboard and spin ordered compound. However, in (H, H, L) plane, we found a magnetic scattering peak at Q = (0.25,0.25,0.5) position below T _N. This magnetic peak can not be understood by considering the Co ²⁺ spin configuration, suggesting that this peak is originated from Co ³⁺ spin order. By analyzing these superlattice reflections, we found that they are originated from high-spin state of Co ³⁺ spin order.