Spin correlations in NaxCoO2 with x = 0.8 and 0.5

Abstract

We present magnetization, specific heat, and Hall coefficient measurements on high qualityNaxCoO2 single crystalswith 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 metamagnetictransition around 20 K would occur under a moderate magnetic field of∼9 T inNaxCoO2 withhigh x. For x = 0.5, we carried out elaborate specific heat investigations around 85 K under differentmagnetic fields. The results show an anisotropic nature of the transition at 85 K.This anisotropic property, revealed by both susceptibility and specific heatmeasurements, indicates that the spins are mainly pointing perpendicular to thec axisfor x = 0.5 crystal. Moreover, by fitting the specific heat data below 46 K, a gap in low-lyingexcitation 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 ofNa0.5CoO2.