Investigation of the spin density wave in NaxCoO2

Magnetic susceptibility, transport and heat capacity measurements for single-crystal NaxCoO2 (x = 0.71) are reported. A transition to a spin density wave (SDW) state at T-mag = 22 K is observable in all measurements, except chi(ac) data in which a cusp is observed at 4 K and attributed to a low temperature glassy phase. M(H) loops are hysteretic below 15 K. Both the SDW transition and low temperature hysteresis are only visible along the c-axis. The system also exhibits a substantial (similar to40%) positive magnetoresistance below this temperature. Calculations of the electronic heat capacity gamma above and below T,nag and the size of the jump in C. indicate that the onset of the SDW brings about the opening of a gap and the removal of part of the Fermi surface. Reduced in-plane electron-electron scattering counteracts the loss of carriers below the transition and as a result we see a net reduction in resistivity below T-mag. Sodium ordering transitions at higher temperatures are observable as peaks in the heat capacity with a corresponding increase in resistivity.