Spin gap of K2Ni(MoO4)2 having two-dimensional distribution of MoO4-bridged spin-1 Ni dimers
1Center for Condensed Matter Sciences, National Taiwan University, Taipei 10617, Taiwan
2National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan
3Taiwan Consortium of Emergent Crystalline Materials, Ministry of Science and Technology, Taipei 10622, Taiwan
4Center of Atomic Initiative for New Materials, National Taiwan University, Taipei 10617, Taiwan
* Presenter:SENTHIL MURUGAN GANESAN, email:nanosen@gmail.com
The orthorhombic crystal structure of K2Ni(MoO4)2 is constructed by layers of NiO6 octahedra pairs which are bridged by the polyatomic ion groups of MoO42- in the ac-plane with K+ ions sitting in the van der Waals gap between layers. The temperature dependence of magnetic susceptibility [χ(T)] shows both thermally activated spin gap opening of gap size ∆/kB~38 K and a Curie-Weiss paramagnetic behavior of Weiss temperature Θ∼-38.7 K among Ni spins of S=1 above ∼100 K. High field isothermal magnetization study at 1.5 K indicates that all S=1 spins are aligned to saturate at the level of ∼2 µB/Ni above ∼50 Tesla, but a plateau of ∼1 µB/Ni is identified near ∼25 Tesla to suggest a superstructure that only half of the spins are in the excited triplet state. Density functional theory calculations have been applied to estimate the nearest neighbor coupling constant (J1) among Ni S=1 spins to be ∼-10 K, which is consistent to the spin gap size and an order of magnitude higher than all inter-dimer couplings to support the unfrustrated 2D S=1 dimer network consistently.

Keywords: Spin gap, spin-1 dimer, magnetization, plateau, coupling constant