Magnetic anisotropy of NaxCoO2 single crystals

We report the magnetic properties of single crystals of NaxCoO2 (x=0.42, 0.82, and 0.87). The magnetic susceptibility measurements revealed considerable anisotropy along H parallel to ab and H parallel to c for the as-grown single crystals. It was found that an antiferromagnetic transition with a Neel temperature T-N=21 K occurred for the x=0.82 sample, and there was a paramagnetic phase for the x=0.87 sample over a wide temperature range from 2 to 300 K, but the sample with x=0.42 shows a monotonic increase of chi with increasing temperature above similar to 100 K. In addition, the x=0.82 sample has the largest derived anisotropic g-factor ratio (gab/gc-1.30), whereas the sample with x=0.42 is nearly isotropic (gab/gc-0.96).