Spin correlations in NaxCoO2 with x = 0.8 and 0.5

We present magnetization, specific heat, and Hall coefficient measurements on high quality NaxCoO2 single crystals with x = 0.8 and 0.5. Similar to Na0.85CoO2, a metamagnetic transition at about 20 K with was observed in Na0.8CoO2 under a magnetic field of 9 T. This suggests that the common metamagnetic transition around 20 K would occur under a moderate magnetic field of ~9 T in NaxCoO2 with high x. For x = 0.5, we carried out elaborate specific heat investigations around 85 K under different magnetic fields. The results show an anisotropic nature of the transition at 85 K. This anisotropic property, revealed by both susceptibility and specific heat measurements, indicates that the spins are mainly pointing perpendicular to the c axis for x = 0.5 crystal. Moreover, by fitting the specific heat data below 46 K, a gap in low-lying excitation spectra is found and the gap value is estimated to be about 7.6 meV. This opening of the gap would account for the insulating ground state of Na0.5CoO2.