We report the 59Co NMR studies of the in-plane anisotropy of bilayer
hydrated NaxβCoO2ββ yH2βO using a oriented
powder sample by a magnetic field in Fluorinert FC70. We found for the first
time the ab-plane anisotropy of the 59Co NMR Knight shift K, the
nuclear spin-lattice relaxation rate 1/T1β and the nuclear spin-spin
relaxation rate 1/T2β at a magnetic field HβΌ 7.5 T up to 200K. Below
75 K, the anisotropy of K is large compared with that at high temperatures.
The hyperfine coupling constants seem to change around the temperature 150 K,
in which the bulk susceptibility Ο shows broad minimum, suggesting a
change of the electronic state of CoO2β plane. 1/T1β also shows a
significant anisotropy, which cannot be explained only by the anisotropy of the
hyperfine coupling constants nor the anisotropic uniform spin susceptibility.
The difference in the in-plane anisotropy of T1β from that of K indicates
that the magnetic fluctuation at a finite wave vector qβξ =0 is also
anisotropic and the anisotropy is different from that at qβ=0.Comment: 4 pages, 5 figure