Magnetic and electric properties of triangular lattice antiferromagnets Ba3ATa2O9 (AÂ =Â 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 TN ∼ 3.3 K, (ii) a weak spin-flop-like transition in isothermal magnetization curves around 4 T, and the saturation magnetization about 1.6 μB 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 TN, 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)].