Investigation of the Spin Density Wave in NaxCoO2

TL;DR

This study investigates the magnetic, transport, and thermal properties of NaxCoO2, revealing a spin density wave transition at 22 K, associated electronic gap opening, and complex magnetic behavior influenced by sodium ordering.

Contribution

It provides detailed experimental evidence of SDW transition and electronic structure changes in NaxCoO2, highlighting the anisotropic magnetic and transport phenomena.

Findings

SDW transition at 22 K observed in multiple measurements

Opening of an electronic gap and partial Fermi surface removal

Positive magnetoresistance of about 40% below Tmag

Abstract

Magnetic susceptibility, transport and heat capacity measurements of single crystal NaxCoO2 (x=0.71) are reported. A transition to a spin density wave (SDW) state at Tmag = 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 (~40%) positive magnetoresistance below this temperature. Calculations of the electronic heat capacity gamma above and below Tmag and the size of the jump in C indicate that the onset of the SDW brings about the opening of 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 Tmag. Sodium ordering transitions at higher temperatures are observable as peaks in the heat capacity with a corresponding increase in resistivity.