Series of phase transitions and multiferroicity in the quasi-two-dimensional spin-12triangular-lattice antiferromagnetBa3CoNb2O9

Abstract

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${}^{\ensuremath{\circ}}$ 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)].