Counterrotating magnetic order in the honeycomb layers of NaNi2BiO6-δ

We report the magnetic structure and electronic properties of the honeycomb antiferromagnet NaNi2BiO5.66. We find magnetic order with moments along the c axis for temperatures below Tc1=6.3(1)K and then in the honeycomb plane for T<Tc2=4.8(1)K with a counterrotating pattern and an ordering wave vector q=(13,13,0.15(1)). Density functional theory and electron spin resonance indicate this is high-spin Ni3+ magnetism near a high to low spin transition. The ordering wave vector, in-plane magnetic correlations, missing entropy, spin state, and superexchange pathways are all consistent with bond-dependent Kitaev-Γ-Heisenberg exchange interactions in NaNi2BiO6-δ.