We have investigated the magnetic and electric ground states of a quasi-two-dimensional triangular lattice antiferromagnet (TLAF), ${\mathrm{Ba}}_3{\mathrm{CoNb}}_2{\mathrm{O}}_9$, in which the effective spin of Co${}^{2+}$ is 1/2. At zero field, the system undergoes a two-step transition upon cooling at $T_{N2}=1.36$ K and $T_{N1}=1.10$ K and enters a 120${}^{\circ }$ ordered state. By applying magnetic fields, a series of spin states with fractions of the saturation magnetization $M_s$ are observed. They are spin states with 1/3, 1/2, 2/3 (or $\sqrt{3}/3$) $M_s$. The ferroelectricity emerges in all spin states, either with collinear or noncollinear spin structure, which makes ${\mathrm{Ba}}_3{\mathrm{CoNb}}_2{\mathrm{O}}_9$ 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 ${\mathrm{Ba}}_3$NiNb${}_2{\mathrm{O}}_9$ [J. Hwang et al., Phys. Rev. Lett. 109, 257205 (2012)].

• Received 19 August 2013
• Revised 3 February 2014

DOI:https://doi.org/10.1103/PhysRevB.89.104420