Coexistence of successive antiferromagnetic transitions and spin frustration in triangular-lattice antiferromagnet...

We have comprehensively investigated the low-temperature magnetic and specific-heat properties of millimeter-sized, high-quality Ba₃CoNb₂O₉single crystals, a representative triangular-lattice antiferromagnet. Two successive magnetic transitions were observed atT_N1≈ 0.95 K andT_N2≈ 0.50 K for magnetic fields applied along both theb- andc-axes. While the transition atT_N1corresponds to the onset of long-range antiferromagnetic order, the lower-temperature anomaly atT_N2is 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 theb- andc-axes, and importantly, the absence of a 1/3 magnetization plateau suggests that the interlayer and intralayer exchange interactions are comparable. Furthermore, the frustration parameterf= |θ_CW|/T_N1= 73.7, together with the magnetic entropy of 4.0 J mol^-1K^-1at the paramagnetic state being significantly smaller than the theoretical value of Rln2, indicates that strong magnetic frustration persists in the Ba₃CoNb₂O₉system even within its long-range antiferromagnetic ground state.