  We performed 23Na NMR and magnetization measurements on an S = 1, quasi-2D
honeycomb lattice antiferromagnet Na3Ni2BiO6. A large positive Curie-Weiss
constant of 22.9 K is observed. The NMR spectra at low fields are consistent
with a "zigzag" magnetic order, indicating a large easy-axis anisotropy. With
field applied along the c* axis, the NMR spectra confirm the existence of a
1/3-magnetization plateau phase between 5.1 T and 7.1 T. The transition from
the zigzag order to the 1/3-magnetization plateau phase is also found to be a
first-order type. A monotonic decrease of the spin gap is revealed in the
1/3-magnetization plateau phase, which reaches zero at a quantum critical field
Hc = 8.35 T before entering the fully polarized phase. These data suggest the
existence of exchange frustration in the system along with strong ferromagnetic
interactions, hosting the possibility for Kitaev physics. Besides, well below
the ordered phase, the 1/T1 at high fields shows either a level off or an
enhancement upon cooling below 3 K, which suggests the existence of low-energy
fluctuations.
