Local spin structure of the $\alpha$-RuCl$_3$ honeycomb-lattice magnet observed via muon spin rotation/relaxation

TL;DR

This study uses muon spin rotation/relaxation to investigate the local spin structure of single-crystalline $ ext{α-RuCl}_3$, revealing phase separation due to stacking faults and proposing a modulated zig-zag spin structure with reduced magnetic moments.

Contribution

It provides the first muon spin rotation/relaxation analysis of $ ext{α-RuCl}_3$, identifying phase separation effects and proposing a specific spin structure consistent with experimental data.

Findings

Multiple magnetic domains with different $T_N$ due to stacking faults.

Identification of the muon site and hyperfine parameters.

Proposal of a modulated zig-zag spin structure with reduced magnetic moments.

Abstract

We report a muon spin rotation/relaxation ($\mu$SR) study of single-crystalline samples of the $\alpha$-RuCl$_3$ honeycomb magnet, which is presumed to be a model compound for the Kitaev-Heisenberg interaction. It is inferred from magnetic susceptibility and specific-heat measurements that the present samples exhibit successive magnetic transitions at different critical temperatures $T_{\rm N}$ with decreasing temperature, eventually falling into the $T_{\rm N}=7$ K antiferromagnetic (7 K) phase that has been observed in only single-crystalline specimens with the least stacking fault. Via $\mu$SR measurements conducted under a zero external field, we show that such behavior originates from a phase separation induced by the honeycomb plane stacking fault, yielding multiple domains with different $T_{\rm N}$'s. We also perform $\mu$SR measurements under a transverse field in the paramagnetic phase to identify the muon site from the muon-Ru hyperfine parameters. Based on a comparison of the experimental and calculated internal fields at the muon site for the two possible spin structures inferred from neutron diffraction data, we suggest a modulated zig-zag spin structure for the 7 K phase, with the amplitude of the ordered magnetic moment being significantly reduced from that expected for the orbital quenched spin-1/2 state.