Abstract
The nature and symmetry of transition mechanisms in the spin-spiral
copper halides CuCl2 and CuBr2 are analyzed theoretically. The
magnetoelectric effects observed in the two multiferroic compounds are
described and their phase diagram at zero and applied magnetic fields
are worked out. The emergence of the electric polarization at zero field
below the paramagnetic phase is shown to result from the coupling of two
distinct spin-density waves and to be only partly related to the
Dzialoshinskii-Moriya interactions. Applying a magnetic field along the
two-fold monoclinic axis of CuCl2 yields a decoupling of the
spin-density waves modifying the symmetry of the phase and the
spin-spiral orientation. The remarkable periodic dependences of the
magnetic susceptibility and polarization, on rotating the field in the
monoclinic plane, are described theoretically.
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