Magnetic Correlation of Na<sub><i>x</i></sub>CoO<sub>2</sub>and Successive Phase Transitions of Na<sub>0.5</sub>CoO<sub>2</sub>–NMR and Neutron Diffraction Studies–

  • Yokoi Mai
    Department of Physics, Division of Material Science, Nagoya University
  • Moyoshi Taketo
    Department of Physics, Division of Material Science, Nagoya University
  • Kobayashi Yoshiaki
    Department of Physics, Division of Material Science, Nagoya University
  • Soda Minoru
    Department of Physics, Division of Material Science, Nagoya University
  • Yasui Yukio
    Department of Physics, Division of Material Science, Nagoya University
  • Sato Masatoshi
    Department of Physics, Division of Material Science, Nagoya University
  • Kakurai Kazuhisa
    Advanced Science Research Center, Japan Atomic Energy Research Institute

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  • Magnetic Correlation of NaxCoO2 and Successive Phase Transitions of Na0.5CoO2-NMR and Neutron Diffraction Studies-

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Data taken by various methods including NMR/NQR and neutron scattering are presented for NaxCoO2, particularly Na0.5CoO2. Attention has also been paid to the x dependence of the electronic nature of this system. The pseudo-gap-like behavior observed in the region of x<xc∼0.6 is emphasized. For samples with x≤0.6, signals from two kinds of Co sites with different quadrupole frequencies νQ of ∼4.1 and ∼2.8 MHz were observed, and the former νQ, which is nearly equal to that of the superconducting system Na0.3CoO2·yH2O, becomes dominant with decreasing x to 0.3. For Na0.5CoO2, the Co sites with the larger νQ have larger magnetic moments. They align antiferromagnetically at Tc1∼87 K with their direction within the plane, while the Co sites with the smaller νQ have smaller moments. They align in the direction parallel to the c axis. The ordered moments at the two distinct sites exhibit the same T-dependence, indicating that the existence of these sites is not due to macroscopic phase separation. Although we have observed the splitting of the zero-field NMR signals from Co sites with smaller νQ at the second transition temperature Tc2∼53 K, any other anomaly of the ordered magnetization has not been observed at Tc2 in the T-dependences of neutron and NMR data. As x decreases, the two-dimensional nature of the electrons is enhanced and the antiferromagnetic correlation increases. On the basis of these results, the superconducting pair state in Na0.3CoO2·yH2O is discussed.

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