Direct evidence of a zigzag spin chain structure in the honeycomb lattice: A neutron and x-ray diffraction investigation on single crystal $\rm Na_2IrO_3$

TL;DR

This study provides direct experimental evidence of zigzag spin chain structures in Na₂IrO₃'s honeycomb lattice, revealing magnetic ordering below 18.1 K and highlighting the importance of structural distortions for understanding its magnetic properties.

Contribution

It offers the first combined neutron and x-ray diffraction analysis confirming zigzag magnetic order and detailed structural features in Na₂IrO₃, contrasting previous theoretical models.

Findings

Magnetic order sets in below 18.1 K with zigzag spin chains.

Ir atoms form nearly ideal 2D honeycomb lattice.

Trigonal distortion of IrO₆ octahedra influences ground state.

Abstract

We have combined single crystal neutron and x-ray diffractions to investigate the magnetic and crystal structures of the honeycomb lattice $\rm Na_2IrO_3$. The system orders magnetically below $18.1(2)$ K with Ir$^{4+}$ ions forming zigzag spin chains within the layered honeycomb network with ordered moment of $\rm 0.22(1) \mu_B$/Ir site. Such a configuration sharply contrasts the N{\'{e}}el or stripe states proposed in the Kitaev-Heisenberg model. The structure refinement reveals that the Ir atoms form nearly ideal 2D honeycomb lattice while the $\rm IrO_6$ octahedra experience a trigonal distortion that is critical to the ground state. The results of this study provide much-needed experimental insights into the magnetic and crystal structure crucial to the understanding of the exotic magnetic order and possible topological characteristics in the 5$d$-electron based honeycomb lattice.