Counterroating incommensurate magnetic order and strong quantum fluctuations in the honeycomb layers of $\rm NaNi_2BiO_6$.

We report the magnetic structure of the honeycomb antiferromagnet $\rm NaNi_2BiO_6$. We find magnetic order with moments along the $c$ axis below $T_{c1}=6.3\>{\rm K}$ and then in the honeycomb plane below $T_{c2}=4.8\>{\rm K}$ with a counterrotating pattern and an incommensurate ordering wave vector ${\bf q}=(\frac{1}{3},\> \frac{1}{3},\> 0.154 \pm 0.011)$. Neutron scattering shows half of the expected static magnetic moment remains fluctuating at low temperatures, suggesting proximity to a quantum disordered phase. The ordering wave vector, in-plane magnetic correlations, and superexchange pathways are all consistent with a bond-dependent 120\textdegree\ compass model exchange in $\rm NaNi_2BiO_6$. %lying close to a quantum critical point. %arising through orbital contributions to Ni magnetism.