Magnetic-field induced competition of two multiferroic orders in a triangular-lattice helimagnet MnI2

Magnetic and dielectric properties with varying magnitude and direction of magnetic-field H have been investigated for a triangular-lattice helimagnet MnI_{2}. The in-plane electric polarization P emerges in the proper screw magnetic ground state below 3.5 K, showing the rearrangement of six possible multiferroic domains as controlled by the in-plane H. With every 60° rotation of H around the [001] axis, discontinuous 120° flop of the P vector is observed as a result of the flop of magnetic modulation vector q. With increasing the in-plane H above 3 T, however, the stable q direction changes from q‖(110[ ¯over 0]) to q‖(110), leading to a change of P-flop patterns under rotating H. At the critical field region (∼3  T), due to the phase competition and resultant enhanced q flexibility, the P vector smoothly rotates clockwise twice while the H vector rotates counterclockwise once.