Magnetic Correlation of Na_xCoO_2 and Successive Phase Transitions of Na_<0.5>CoO_2 : NMR and Neutron Diffraction Studies(Condensed Matter : Electronic Structure, Electrical, Magnetic and Optical Properties)

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Data taken by various methods including NMR/NQR and neutron scattering are presented for Na_xCoO_2, particularly Na_<0.5>CoO_2. 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<x_c〜0.6 is emphasized. For samples with x≤0.6, signals from two kinds of Co sites with different quadrupole frequencies ν_Q of〜4.1 and 〜2.8MHz were observed, and the former ν_Q, which is nearly equal to that of the superconducting system Na_<0.3>CoO_2・yH_2O, becomes dominant with decreasing x to 0.3. For Na_<0.5>CoO_2, the Co sites with the larger ν_Q have larger magnetic moments. They align antiferromagnetically at T_<c1>〜87K with their direction within the plane, while the Co sites with the smaller ν_Q 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 ν_Q at the second transition temperature T_<c2>〜53K, any other anomaly of the ordered magnetization has not been observed at T_<c2> 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 Na_<0.3>CoO_2・yH_2O is discussed.
社団法人日本物理学会の論文
2005-11-15

Magnetic Correlation of Na_xCoO_2 and Successive Phase Transitions of Na_<0.5>CoO_2 : NMR and Neutron Diffraction Studies(Condensed Matter : Electronic Structure, Electrical, Magnetic and Optical Properties)

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