Magnetic and electric properties of triangular lattice antiferromagnets Ba₃ATa₂O₉ (A = Ni and Co)

We investigated magnetic and electric properties of two triangular lattice antiferromagnets (TLAFs), Ba₃ATa₂O₉ (A = Ni and Co). Ba₃NiTa₂O₉ shows (i) a single antiferromagnetic phase transition to 120 degree ordered state at T_N ∼ 3.3 K, (ii) a weak spin-flop-like transition in isothermal magnetization curves around 4 T, and the saturation magnetization about 1.6 μ_B per Ni²⁺ above 9.2 T, and (iii) a weak magnetoelectric effect could be removed without ferroelectricity. Comparing to the isostructural TLAF, Ba₃NiNb₂O₉ [Phys. Rev. Lett. 109, 257205 (2012)], which exhibits successive magnetic phase transitions and ferroelectricity, Ba₃NiTa₂O₉ shows lower T_N, smaller saturation magnetization, no successive phase transition, and no ferroelectricity. These differences indicate that the higher order superexchange interaction involving Ta atomic orbitals in Ba₃NiTa₂O₉ disrupts not only the exotic magnetic phases, also ferroelectricity. We show that this scenario can be also applied to explain the differences between Ba₃CoTa₂O₉ and Ba₃CoNb₂O₉ [Phys. Rev. B 89, 104420 (2014)].