Atypical antiferromagnetic ordering in single crystalline quasi-2D honeycomb magnet YbI₃

Here, we study YbI₃, a quasi-2D layered material with Yb atoms arranged on an ideal honeycomb network of edge-sharing YbI₆ octahedra, analogous to the low-temperature phase of α-RuCl₃. High quality single crystals of YbI₃ are grown from Yb and I as starting precursors, using the vapor transport technique. The grown crystals are characterized by single crystal x-ray diffraction, Raman spectroscopy, magnetization, and heat capacity probes. The crystal-field split ground state of Yb³⁺ in ~ is a well-isolated Kramers doublet with an effective moment Jeff = 1/2. Upon cooling, the low-temperature heat capacity of ~ reveals a broad peak at T₁ = 0.95~K due to short-range ordering of the Yb moments, followed by a sharp peak at T₂ = TN = 0.6~K due to long-range ordering. The magnetic behavior is found to be weakly anisotropic with χ^∥ > χ^⊥, where χ^∥ and χ^⊥ refers to the in-plane (H ∥ ab) and out-of-plane (H ⊥ ab) susceptibilities. The 2~K isothermal magnetization saturates at ≈~1.5~μB/Yb³⁺ (in-plane) and ≈~1~μB/Yb³⁺ (out-of-plane), suggesting the anisotropy to be easy-plane type. Low-temperature heat capacity, well below TN, is found to vary as T^α with α~≈~2.5, indicating a possible unconventional magnetic ground state for YbI₃.