Counterrotating Magnetic Order in the Honeycomb Layers of NaNi2BiO6-delta

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 T-c1 = 6.3(1) K and then in the honeycomb plane for T < T-c2 = 4.8(1) K with a counterrotating pattern and an ordering wave vector q = (1/3, 1/3, 0.15(1)). Density functional theory and electron spin resonance indicate this is high-spin Ni3+ magnetism 3 1, 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-Gamma-Heisenberg exchange interactions in NaNi2BiO6-delta.