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Magnetic Studies of Polycrystalline and Single-Crystal NaxCoO2

D. Prabhakaran·A. T. Boothroyd·R. Coldea·L. M. Helme·D. A. Tennant
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Abstract

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.

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D. Prabhakaran, A. T. Boothroyd, R. Coldea, L. M. Helme, D. A. Tennant. 2003-12-18. Magnetic Studies of Polycrystalline and Single-Crystal NaxCoO2. https://arxiv.org/abs/cond-mat/0312493

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