Counterroating Magnetic Order in the Honeycomb Layers of NaNi2BiO6−δ

Abstract

We report the magnetic structure and electronic properties of the honeycomb antiferromagnet NaNi2⁢BiO5.66. We find magnetic order with moments along the 𝑐 axis for temperatures below 𝑇𝑐⁢1=6.3⁢(1)⁢K and then in the honeycomb plane for 𝑇<𝑇𝑐⁢2=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 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 NaNi2⁢BiO6−𝛿.