  A theoretical description of the sequence of magnetic phases in Co3TeO6 is
presented. The strongly first-order character of the transition to the
commensurate multiferroic ground state, induced by coupled order parameters
corresponding to different wavevectors, is related to a large magnetoelastic
effect with an exchange energy critically sensitive to the interatomic spacing.
The monoclinic magnetic symmetry C2' of the multiferroic phase permits
spontaneous polarization and magnetization as well as the linear
magnetoelectric effect. The existence of weakly ferromagnetic domains is
verified experimentally by second harmonic generation measurements.
