Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Synthesis, Characterization, and Magnetic Properties of gamma-NaxCoO2
  (0.70 < x <0.84)   Powder Na$_{x}$CoO$_{2}$ ($0.70\leq x\leq 0.84$) samples were synthesized and
characterized carefully by X-ray diffraction analysis, inductive-coupled plasma
atomic emission spectroscopy, and redox titration. It was proved that
$\gamma$-Na$_{x}$CoO$_{2}$ is formed only in the narrow range of $0.70\leq
x\leq 0.78$. Nevertheless, the magnetic properties depend strongly on $x$. We
found, for the first time, two characteristic features in the magnetic
susceptibility of Na$_{0.78}$CoO$_{2}$, a sharp peak at $T_{p}=16$ K and an
anomaly at $T_{k}=9$ K, as well as the transition at $T_{c}=22$ K and the broad
maximum at $T_{m}=50$ K which had already been reported. A type of weak
ferromagnetic transition seems to occur at $T_{k}$. The transition at $T_{c}$,
which is believed to be caused by spin density wave formation, was observed
clearly for $x\geq 0.74$ with constant $T_{c}$ and $T_{p}$ independent of $x$.
On the other hand, ferromagnetic moment varies systematically depending on $x$.
These facts suggest the occurrence of a phase separation at the microscopic
level, such as the separation into Na-rich and Na-poor domains due to the
segregation of Na ions. The magnetic phase diagram and transition mechanism
proposed previously should be reconsidered.

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The categories of this paper are: atom-ph
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The categories of this paper are: mtrl-th
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The categories of this paper are: q-fin
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The categories of this paper are: q-alg
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The categories of this paper are: physics
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The categories of this paper are: bayes-an
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The categories of this paper are: acc-phys
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The categories of this paper are: cmp-lg
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The categories of this paper are: dg-ga
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The categories of this paper are: astro-ph
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The categories of this paper are: eess
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The categories of this paper are: hep-th
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The categories of this paper are: cs
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The categories of this paper are: hep-lat
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The categories of this paper are: hep-ex
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The categories of this paper are: nlin