Magnetic Studies of Polycrystalline and Single-Crystal NaxCoO2

Original · EN

We have investigated the preparation and bulk magnetic properties of NaxCoO2 in the range 0.5<x<0.95, including those for single crystals grown by the floating-zone method in an image furnace. To characterize the samples and ascertain their quality we performed x-ray powder diffraction, electron-probe microanalysis (EPMA) and magnetization measurements. Based on our findings we are able to report conditions for the preparation of single-phase polycrystalline and single-crystal samples in the range 0.65<x<0.95. For lower concentration samples, however, i.e. x<0.65, residual Co3O4 impurities above the detectable limit remained, and conditions to eliminate this have still to be identified. Investigations of the anisotropic magnetization of NaxCoO2 single crystals show: (i) transitions at 320 K, 275K and 22 K present for all Na doping levels investigated, (ii) a strong increase in magnetization below 8 K, (iii) a crossover in sense for the anisotropy around 8-10 K, and (iv) a hysteresis in the magnetization for certain Na doping levels at temperatures below 15 K. Notably, neither of the high temperature anomalies (275 K, 320 K) could be observed in the powder samples.