Magnetoelectric Effects in the Spiral Magnets CuCl$_{2}$ and CuBr$_{2}$

摘要

The nature and symmetry of the transition mechanisms in the spin-spiralcopper halides CuCl$_2$ and CuBr$_2$ are analyzed theoretically. Themagnetoelectric effects observed in the two multiferroic compounds aredescribed and their phase diagram at zero and applied magnetic fields areworked out. The emergence of the electric polarization at zero field below theparamagnetic phase is shown to result from the coupling of two distinctspin-density waves and to be only partly related to the Dzialoshinskii-Moriyainteractions. Applying a magnetic field along the two-fold monoclinic axis ofCuCl$_2$ yields a decoupling of the spin-density waves modifying the symmetryof the phase and the spin-spiral orientation. The remarkable periodicdependences of the magnetic susceptibility and polarization, on rotating thefield in the monoclinic plane, are described theoretically.