Interplay between magnetism and sodium vacancy ordering in NaxCoO2

Abstract

Using a combination of low-temperature nanocalorimetry and x-ray diffraction we identify three temperature regimes characterized by distinct Na ordering patterns (low temperature up to 290 K, intermediate 290-340 K, and high above 340 K). Through freezing-in of these patterns we establish the two key roles sodium intercalation plays in the formation of the magnetic ground state: supplying the proper electron count for in-plane ferromagnetic interaction and through the 3D sodium ordering providing the interplane antiferromagnetic exchange path.

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• Magnetic, electrochemical and thermoelectric properties of $P2-N{a}_x(C{o}_{7/8}S{b}_{1/8})O_2$
• Critical fields and fluctuations determined from specific heat and magnetoresistance in the same nanogram SmFeAs(O,F) single crystal