Low-temperature crystal and magnetic structure of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>α</mml:mi><mml:mo>−</mml:mo><mml:msub><mml:mi>RuCl</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:mrow></mml:math>

Abstract

Single crystals of the Kitaev spin-liquid candidate $\ensuremath{\alpha}\ensuremath{-}{\mathrm{RuCl}}_{3}$ have been studied to determine the low-temperature bulk properties, the structure, and the magnetic ground state. Refinements of x-ray diffraction data show that the low-temperature crystal structure is described by space group $C2/m$ with a nearly perfect honeycomb lattice exhibiting less than 0.2% in-plane distortion. The as-grown single crystals exhibit only one sharp magnetic transition at ${T}_{N}=7$ K. The magnetic order below this temperature exhibits a propagation vector of $k=(0,1,1/3)$, which coincides with a three-layer stacking of the $C2/m$ unit cells. Magnetic transitions at higher temperatures up to 14 K can be introduced by deformations of the crystal that result in regions in the crystal with a two-layer stacking sequence. The best-fit symmetry-allowed magnetic structure of the as-grown crystals shows that the spins lie in the $ac$ plane, with a zigzag configuration in each honeycomb layer. The three-layer repeat out-of-plane structure can be refined as a ${120}^{\ensuremath{\circ}}$ spiral order or a collinear structure with a spin direction of ${35}^{\ensuremath{\circ}}$ away from the $a$ axis. The collinear spin configuration yields a slightly better fit and also is physically preferred. The average ordered moment in either structure is less than 0.45(5) ${\ensuremath{\mu}}_{B}$ per ${\mathrm{Ru}}^{3+}$ ion.