Synthesis, Characterization, and Magnetic Properties of γ-Na_xCoO_2 (0.70 ≤ x ≤ 0.84)(Condensed Matter : Electronic Structure, Electrical, Magnetic and Optical Properties)

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Powder Na_xCoO_2 (0.70 ≤ x ≤ 0.84) samples were synthesized and characterized carefully by X-ray diffraction analysis, inductive-coupled plasma atomic emission spectroscopy, and redox titration. It was proved that γ-Na_xCoO_2 is formed only in the narrow range of 0.70 ≤ x ≤ 0.78. Nevertheless, the magnetic properties depend strongly on x. We found, for the first time, two characteristic features in the magnetic susceptibility of Na_<0.78>CoO_2, a sharp peak at T_p = 16 K and an anomaly at T_k = 9 K, as well as the transition at T_c = 22 K and the broad maximum at T_m = 50 K which had already been reported. A type of weak ferromagnetic transition seems to occur at T_k. The transition at T_c, which is believed to be caused by spin density wave formation, was observed clearly for x ≥ 0.74 with constant T_c and T_p independent of x. On the other hand, ferromagnetic moment varies systematically depending on x. These facts suggest the occurrence of a phase separation at the microscopic level, such as the separation into Na-rich and Na-poor domains due to the segregation of Na ions. The magnetic phase diagram and transition mechanism proposed previously should be reconsidered.
社団法人日本物理学会の論文
2004-08-15