Anisotropic magnetic ground state of single-crystalline quasi-two-dimensional honeycomb antiferromagnet YbI 3

Here we study ${\mathrm{YbI}}₃$, a quasi-two-dimensional layered material with Yb atoms arranged on an ideal honeycomb network of edge-sharing ${\mathrm{YbI}}₆$ octahedra, analogous to the low-temperature phase of $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}₃$. High-quality single crystals of ${\mathrm{YbI}}₃$ are grown from Yb and ${\mathrm{I}}₂$ as starting precursors, using the vapor transport technique. The as-grown crystals are characterized by single-crystal x-ray diffraction, Raman spectroscopy, magnetization, and heat capacity probes. The crystal-field split ground state of ${\mathrm{Yb}}³⁺$ in ${\mathrm{YbI}}₃$ is a well-isolated Kramers doublet with an effective moment ${J}_{\mathrm{eff}}=1/2$. On cooling, the low-temperature heat capacity of ${\mathrm{YbI}}₃$ reveals a broad peak at ${T}₁=0.95\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ due to short-range ordering of the Yb moments followed by a sharp peak at ${T}₂={T}_N=0.6\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ due to long-range ordering. The magnetic behavior is found to be weakly anisotropic with ${\ensuremath{\chi}}^{\ensuremath{\parallel}}>{\ensuremath{\chi}}^{\ensuremath{\perp}}$, where ${\ensuremath{\chi}}^{\ensuremath{\parallel}}$ and ${\ensuremath{\chi}}^{\ensuremath{\perp}}$ refers to the in-plane $(H\ensuremath{\parallel}ab)$ and out-of-plane $(H\ensuremath{\perp}ab)$ susceptibilities. The 2 K isothermal magnetization saturates at $\ensuremath{\approx}1.5\phantom{\rule{0.28em}{0ex}}{\textmu{}}_B/{\mathrm{Yb}}³⁺$ (in-plane) and $\ensuremath{\approx}1\phantom{\rule{0.28em}{0ex}}{\textmu{}}_B/{\mathrm{Yb}}³⁺$ (out-of-plane), suggesting the anisotropy to be easy-plane type. The low-temperature heat capacity, well below ${T}_N$, is found to vary as ${T}^{\ensuremath{\alpha}}$ with $\ensuremath{\alpha}\phantom{\rule{0.16em}{0ex}}\ensuremath{\approx}\phantom{\rule{0.16em}{0ex}}2.5$, indicating a possible unconventional magnetic ground state for ${\mathrm{YbI}}₃$.