Series of phase transitions and multiferroicity in the quasi-two- dimensional spin- 12 triangular-lattice antiferromagnet Ba3CoNb2 O9

We have investigated the magnetic and electric ground states of a quasi-two-dimensional triangular lattice antiferromagnet (TLAF), Ba3CoNb2O9, in which the effective spin of Co2+ is 1/2. At zero field, the system undergoes a two-step transition upon cooling at TN2=1.36 K and TN1=1.10 K and enters a 120 ordered state. By applying magnetic fields, a series of spin states with fractions of the saturation magnetization Ms are observed. They are spin states with 1/3, 1/2, 2/3 (or 3/3) Ms. The ferroelectricity emerges in all spin states, either with collinear or noncollinear spin structure, which makes Ba3CoNb2O9 another unique TLAF exhibiting both a series of magnetic phase transitions and multiferroicity. We discuss the role of quantum fluctuations and magnetic anisotropy in contributing more complex phase diagram compared to its sister multiferroic TLAF compound Ba3NiNb2O9 [J. Hwang, Phys. Rev. Lett. 109, 257205 (2012)PRLTAO0031-900710.1103/PhysRevLett.109.257205].