NMR Studies of Successive Phase Transitions in Na_<0.5>CoO_2 and K_<0.5>CoO_2(Condensed matter: electronic structure and electrical, magnetic, and optical properties)

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^<59>Co- and ^<23>Na-NMR measurements have been carried out on polycrystalline and c-axis aligned samples of Na_<0.5>CoO_2, which exhibits successive transitions at temperatures T=87K (=T_<c1>) and T=53K (=T_<c2>). ^<59>Co-NMR has also been carried out on c-axis aligned crystallites of K_<0.5>CoO_2 with similar successive transitions at T_<c1>〜60K and T_<c2>〜20K. For Na_<0.5>CoO_2, two sets of three NMR lines of ^<23>Na nuclei, explained by considering the quadrupolar frequencies ν_Q〜1.32 and 1.40MHz, have been observed above T_<c1>, as is expected from the crystalline structure. A rather complicated but characteristic variation of the ^<23>Na-NMR spectra has been observed with varying T through the transition temperatures, and the internal fields at two crystallographically distinct Na sites are discussed on the basis of the magnetic structures reported previously. The internal fields at two distinct Co sites observed below T_<c1> and the ^<59>1/T_1-T curves of Na_<0.5>CoO_2 and K_<0.5>CoO_2 are also discussed in a comparative way.
社団法人日本物理学会の論文
2008-07-15