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 Ba3CoNb2O9 powder, described as a uniform triangular-lattice antiferromagnet (TLAF) with a fictitious spin-1/2. Ba3CoNb2O9 is found to undergo two-step antiferromagnetic transitions at T-N1 = 1.39 K and T-N2 = 1.13 K. As the magnetic field is increased, both T-N1 and T-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 Ba3CoSb2O9