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

TL;DR

This study uncovers complex magnetic ordering and strong quantum fluctuations in honeycomb layers of NaNi2BiO6, revealing counterrotating magnetic patterns and bond-dependent interactions near a spin transition.

Contribution

It provides the first detailed characterization of magnetic order and electronic properties in NaNi2BiO6, highlighting the role of Kitaev-$ m extbf{ extGamma}$-Heisenberg interactions in this material.

Findings

Magnetic order with moments along the c axis below 6.3 K.

In-plane magnetic order with counterrotating pattern below 4.8 K.

Evidence of bond-dependent Kitaev-$ m extGamma$-Heisenberg exchange interactions.

Abstract

We report the magnetic structure and electronic properties of the honeycomb antiferromagnet $\rm NaNi_2BiO_{5.66}$. We find magnetic order with moments along the $c$ axis for temperatures below $T_{c1}=6.3(1)\>{\rm K}$ and then in the honeycomb plane for $T < T_{c2}=4.8(1)\>{\rm K}$ with a counterrotating pattern and an ordering wave vector ${\bf q}=(\frac{1}{3},\> \frac{1}{3},\> 0.15(1))$. 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 $\rm NaNi_2BiO_{6-\delta}$.