  A complex magnetic order of the multiferroic compound Co3TeO6 has been
revealed by neutron powder diffraction studies on ceramics and crushed single
crystals. The compound adopts a monoclinic structure (s.g. C2/c) in the studied
temperature range 2 K - 300 K but exhibits successive antiferromagnetic
transitions at low temperature. Incommensurate antiferromagnetic order with the
propagation vector k1 = (0, 0.485, 0.055) sets in at 26 K. A transition to a
second antiferromagnetic structure with k2 = (0, 0, 0) takes place at 21.1 K.
Moreover, a transition to a commensurate antiferromagnetic structure with k3 =
(0, 0.5, 0.25) occurs at 17.4 K. The magnetic structures have been determined
by neutron powder diffraction using group theory analysis as a preliminary
tool. Different coordinations of the Co2+ ions involved in the low-symmetry
C2/c structure of Co3TeO6 render the exchange-interaction network very complex
by itself. The observed magnetic phase transformations are interpreted as an
evidence of competing magnetic interactions. The temperature dependent changes
in the magnetic structure, derived from refinements of high-resolution neutron
data, are discussed and possible mechanisms connected with the spin
reorientations are described.
