Spin and Charge Orders and Their Hole-Doping Dependence in Single Layered Cobaltate La_<2-x>Ca_xCoO_4 (0.3≦x≦0.8)(Condensed matter : electronic structure and electrical, magnetic, and optical properties)

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Neutron scattering experiments were performed on single crystals of layered cobalt-oxides La_<2-x>Ca_<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_<1-x>Sr_<1+x>MnO_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.
社団法人日本物理学会の論文
2007-11-15