Magnetic-Field Induced Competition of Two Multiferroic Orders in a Triangular-Lattice Helimagnet Mn I2

Magnetic and dielectric properties with varying magnitude and direction of magnetic-field H have been investigated for a triangular-lattice helimagnet MnI2. 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 degrees rotation of H around the [ 001] axis, discontinuous 120 degrees 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 parallel to < 1 (1) over bar0 > to q parallel to < 110 > leading to a change of P-flop patterns under rotating H. At the critical field region (similar to 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.