Magnetoelectric responses induced by domain rearrangement and spin structural change in triangular-lattice helimagnets NiI2 and CoI2

Dielectric and magnetic properties have been investigated for single crystals of triangular-lattice antiferromagnets NiI2 and CoI2. For NiI2, the proper screw spin order with the magnetic modulation vector q similar to (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 CoI2, 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) = (1/12, 1/12, 1/2) and q(2) = (1/8, 0, 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. DOI: 10.1103/PhysRevB.87.014429