Coexistence of successive antiferromagnetic transitions and spin frustration in triangular-lattice antiferromagnet Ba3CoNb2O9 single crystal

We have comprehensively investigated the low-temperature magnetic and specific-heat properties of millimeter-sized, high-quality Ba3CoNb2O9 single crystals, a representative triangular-lattice antiferromagnet (TLAF). Two successive magnetic transitions were observed at TN1 ≈ 0.95 K and TN2 ≈ 0.50 K for magnetic fields applied along both the b- and c-axes. While the transition at TN1 corresponds to the onset of long-range antiferromagnetic order, the lower-temperature anomaly at TN2 is associated with a spin reorientation or a secondary magnetic transition within the ordered state. From the magnetization curves measured at 0.4 K, it is evident that the magnetic moment increases monotonically with the applied field along both the b- and c-axes, and importantly, the absence of a 1/3 magnetization plateau suggests that the interlayer and intralayer exchange interactions are comparable. Furthermore, the frustration parameter f = |θCW|/TN1 = 73.7, together with the magnetic entropy of 4.0 J·mol-1·K-1 at the paramagnetic state being significantly smaller than the theoretical value of Rln2, indicates that strong magnetic frustration persists in the Ba3CoNb2O9 system even within its long-range antiferromagnetic ground state.