  59Co- and 23Na-NMR measurements have been carried out on polycrystalline and
c-axis aligned samples of Na0.5CoO2, which exhibits successive transitions at
temperatures T = 87 K (= Tc1) and T = 53 K (= Tc2). 59Co-NMR has also been
carried out on c-axis aligned crystallites of K0.5CoO2 with similar successive
transitions at Tc1 ~ 60 K and Tc2 ~ 20 K. For Na0.5CoO2, two sets of three NMR
lines of 23Na nuclei explained by considering the quadrupolar frequencies nuQ
~1.32 and 1.40 MHz have been observed above Tc1, as is expected from the
crystalline structure. Rather complicated but characteristic variation of the
23Na-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 Tc1 and
the 591/T1-T curves of Na0.5CoO2 and K0.5CoO2 are also discussed in a
comparative way.

Strongly Correlated Electrons