Magnetic and Metal-Insulator Transitions in β-Na_<0.5>CoO_2 and γ-K_<0.5>CoO_2 : NMR and Neutron Diffraction Studies (Condensed Matter: Electronic Structure, Electrical, Magnetic and Optical Properties)

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Co-oxides β-Na_<0.5>CoO_2 and γ-K_<0.5>CoO_2 have been prepared by the Na de-intercalation from α-NaCoO_2 and by the floating-zone method, respectively. It has been found that successive phase transitions take place at temperatures T_<c1> and T_<c2> in both systems. The appearance of the internal magnetic field at T_<c1> with decreasing temperature T indicates that the antiferromagnetic order exists at T〓T_<c1>, as in γ-Na_<0.5>CoO_2. For β-Na_<0.5>CoO_2, the transition temperatures and the NMR parameters determined from the data taken for magnetically ordered state are similar to those of γ-Na_<0.5>CoO_2, indicating that the difference of the stacking ways of the CoO_2 layers between these systems do not significantly affect their physical properties. For γ-K_<0.5>CoO_2, the quantitative difference of the physical quantities are found from those of β- and γ-Na_<0.5>CoO_2. The difference between the values of T_<ci> (i=1 and 2) of these systems might be explained by considering the distance between CoO_2 layers.
社団法人日本物理学会の論文
2006-03-15