Magnetic Phase Diagram of the S=1/2 Triangular-Lattice Heisenberg Antiferromagnet Ba3CoNb2O9

We report the results of low-temperature thermal and magnetic measurements on Ba${}_3$CoNb${}_2$O${}_9$ powder, described as a uniform triangular-lattice antiferromagnet (TLAF) with a fictitious spin-1/2. Ba${}_3$CoNb${}_2$O${}_9$ is found to undergo two-step antiferromagnetic transitions at $T_{\mathrm{N}1}=1.39\mathrm{K}$ and $T_{\mathrm{N}2}=1.13\mathrm{K}$. As the magnetic field is increased, both $T_{\rm N1}$and$T_{\rm N2}$ monotonically decrease. The magnetic field vs temperature phase diagram indicates that the exchange interactions are nearly of the Heisenberg type with weak easy-axis anisotropy and that the exchange interaction between triangular lattices is crucial, in contrast to the case of the quasi-two-dimensional TLAF Ba${}_3$CoSb${}_2$O${}_9$ [Susuki {\it et al}., Phys. Rev. Lett. $\mathbf{\text{110}}$, 267201 (2013)].