Counterroating incommensurate magnetic order and strong quantum fluctuations in the honeycomb layers of $\mathrm{NaN}{\mathrm{i}}_2\mathrm{Bi}{\mathrm{O}}_6$

Abstract

We report the magnetic structure and electronic properties of the honeycomb antiferromagnet $\mathrm{NaN}{\mathrm{i}}_2\mathrm{Bi}{\mathrm{O}}_{5.66}$. We find magnetic order with moments along the $c$ axis for temperatures below $T_{c1}=6.3(1)\mathrm{K}$ and then in the honeycomb plane for $T<T_{c2}=4.8(1)\mathrm{K}$ with a counterrotating pattern and an ordering wave vector $\mathbf{q}=(\frac{1}{3},\frac{1}{3},0.15(1))$. Density functional theory and electron spin resonance indicate this is high-spin Ni${}^{3+}$ 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-$\Gamma$-Heisenberg exchange interactions in $\mathrm{NaN}{\mathrm{i}}_2\mathrm{Bi}{\mathrm{O}}_{6-\delta }$.

12 pages, 11 figures, 8 pages of appendices