Magnetoelectric responses induced by domain rearrangement and spin structural change in triangular-lattice helimagnets NiI2and CoI2

Dielectric and magnetic properties have been investigated for single crystals of triangular-lattice antiferromagnets NiI${}_{2}$ and CoI${}_{2}$. For NiI${}_{2}$, the proper screw spin order with the magnetic modulation vector $q\ensuremath{\sim}(0.138,0,1.457)$ induces electric polarization ($P$) along the in-plane direction with respect to the triangular lattice basal plane. The $P$ shows monotonic increase as a function of the poling magnetic field ($H$) along the in-plane direction, suggesting the $H$-induced rearrangement of the multiferroic domain out of six possible domains. For CoI${}_{2}$, both in-plane and out-of-plane components of $P$ emerge in the helimagnetic ground state, in which two cycloidal magnetic phases with ${q}_{1}=(\frac{1}{12},\frac{1}{12},\frac{1}{2})$ and ${q}_{2}=(\frac{1}{8},0,\frac{1}{2})$ are supposed to coexist. The application of the in-plane $H$ induces two-step metamagneticlike transitions, which probably goes through another ferroelectric helimagnetic phase as well as a paraelectric spin-collinear phase. Such distinctive magnetoelectric responses in the two simple triangular lattice antiferromagnets demonstrate that even a slight difference in the balance of magnetic interactions leads to a dramatic change of resultant magnetoelectric response in frustrated magnets.