Spin gap of K₂Ni(MoO₄)₂ having two-dimensional distribution of MoO₄-bridged spin-1 Ni dimers

SENTHIL MURUGAN GANESAN^1*, FANG-CHENG CHOU^1,2,3,4

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 MoO₄^2- 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