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

TL;DR

This paper provides a theoretical analysis of magnetoelectric effects in the spin-spiral multiferroic copper halides CuCl₂ and CuBr₂, elucidating their phase diagrams, polarization mechanisms, and responses to magnetic fields.

Contribution

It introduces a comprehensive theoretical model explaining the magnetoelectric phenomena and phase behavior in CuCl₂ and CuBr₂, highlighting the coupling of spin-density waves beyond Dzialoshinskii-Moriya interactions.

Findings

Electric polarization arises from coupled spin-density waves.

Magnetic field induces decoupling and symmetry changes in spin-spiral phases.

Periodic magnetic susceptibility and polarization dependences are explained.

Abstract

The nature and symmetry of the transition mechanisms in the spin-spiral copper halides CuCl$_2$ and CuBr$_2$ 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 CuCl$_2$ 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.