Magnetism in the S=1 triangular dimer lattice antiferromagnet K 2 Ni 2 (SeO 3 ) 3

We report an S = 1 antiferromagnet K 2 Ni 2 (SeO 3 ) 3 crystalizing in the hexagonal space group P 6 3 / mmc . The structure consists of the perpendicular -to -plane Ni 2 O 9 dimers in the triangular layers. Surprisingly, K 2 Ni 2 (SeO 3 ) 3 exhibits a sharp upturn of magnetic susceptibility below T N = 5 . 8 K, a single or two-step antiferromagnetic transitions at T N1 and T N2 , whose values are magnetic -field dependent, and a sign of 1 / 3 -like magnetization plateau at 2 K, all of which are typical features for a triangular -lattice antiferromagnet with small easy -axis anisotropy rather than the S = 1 dimer compound. The density functional theory calculations prove the small intradimer exchange ( J 0 / k B = - 2 . 11 K) and the large intraplane interdimer exchanges ( J 1 / k B = - 13 . 27 K); the latter originates from the two Ni-O-Se-O-Ni super-superexchange paths within the triangular plane. This feature makes K 2 Ni 2 (SeO 3 ) 3 a triangular -lattice antiferromagnet with small easy -axis anisotropy, rather than a spin-dimer antiferromagnet.