Multiferroic properties of melanothallite Cu2OCl2

Here we report on P-E hysteresis loop measurements that unravel the ferroelectric nature of melanothallite Cu2OCl2, a new multiferroic material with high critical temperature. Its spin structure was investigated by polarized and unpolarized neutron scattering experiments which reveal a cycloidal magnetic structure with vector chirality (magnetic polarity) that can be inverted by opposite poling of the sample with an inverted electric field. This shows that Cu2OCl2 is a spin-induced ferroelectric material. Finally, we show that the ferroelectric properties of Cu2OCl2 are driven by the inverse Dzyaloshinskii-Moriya interaction mechanism which is also able to predict the observed direction of the ferroelectric polarization properly. The origin of the noncollinear spin structure in melanothallite are competing AFM-FM exchange couplings which we estimate from a combined ab initio + cluster configuration interaction calculation.