NMR study of the spin correlations in the $S=1$ armchair chain Ni${}_2$NbBO${}_6$

We report our nuclear magnetic resonance (NMR) study on the structurally spin chain compound Ni${}_2$NbBO${}_6$ with complex magnetic coupling. The antiferromagnetic transition is monitored by the line splitting resulting from the staggered internal hyperfine field. The magnetic coupling configuration proposed by the first-principle density functional theory (DFT) is supported by our NMR spectral analysis. For the spin dynamics, a prominent peak at $T\sim 35$ K well above the N'{e}el temperature ($T_N\sim 20$ K at ${\mu }_{0}H=10$ T) is observed from the spin-lattice relaxation data. As compared with the dc-susceptibility, this behavior indicates a antiferromagnetic coupling with the typical energy scale of $\sim 3$ meV. Thus, the Ni${}_2$NbBO${}_6$ compound can be viewed as strongly ferromagnetically coupled armchair spin chains along the crystalline $b$-axis. These facts place strong constraints to the theoretical model for this compound.