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Abstract

Na(10)Co(4)o(10) was investigated by neutron powder diffraction at 230,
70, and 4K. The crystal structure, determined previously by X-ray
diffraction on single crystals, was confirmed. Na10Co4O10 orders
magnetically below 37 K. All observed magnetic reflections could be
indexed by integers (hkl) with respect to the chemical unit cell and
the magnetic propagation vector q=0. The refinement was performed in
the Shubnikov space group C2/c and indicated a collinear
antiferromagnetic spin structure. The determined spin arrangement is
consistent with the magnetic intratetramer interactions suggested
previously from the analysis of magnetic susceptibility data: the
magnetic moments of the central Co-III ions of the Co4O10 tetramer lie
parallel to each other and couple in an antiparallel fashion to the
terminal Co-II moments. The Rietveld analysis shows that the net
moments of 0.64 mu(B) per tetramer form ferromagnetic layers parallel
to the ab plane. Adjacent layers are coupled antiferromagnetically
along c.The spins are aligned in the ac plane along the line connecting
adjacent Co-II and Co-III ions of the tetramer. We have determined
unusually low values for the ordered magnetic moments of 2.43(5) mu(B)
and 2.11(6) mu(B) for Co-III and Co-II respectively. The occurrence of
spontaneous magnetization below 37 K indicates a slight canting of 2.2
degrees of the antiferromagnetic structure. A representation analysis
shows that a weak ferromagnetic component along b is compatible with
the determined antiferromagnetic structure.