Magnetotropic Susceptibility in α-RuCl_3​: Insights into Bond-Dependent Exchange Interactions and Frustrated Magnetism

Bond-dependent anisotropic interactions on a honeycomb lattice are a new route towards inducing frustration and the potential for spin liquid states in quantum magnets. α-RuCl₃ is the prime example, but it orders magnetically between 7 and 14 K, depending upon sample quality and size. For example, it was recently reported that small single crystals remain in the monoclinic C_2/m structure down to the lowest temperatures, whereas larger samples undergo a structural transition to the rhombohedral R3 phase. We measured the magnetotropic susceptibility (k = ∂²F/∂θ²) in small monoclinic crystals with fine in-plane angular precision of the crystal axes with respect to the external magnetic field. We observed asymmetries in k at temperatures above and below T_N (~14 K) when rotating the c-axis across the magnetic field towards different in-plane directions. These asymmetries, which are highly dependent upon the in-plane rotation direction, are an indication of the prominent bond dependent exchange interaction, including Kitaev-like interactions – the magnitude and sign of which could be determined using future numerical approaches. For some planes of rotation, periodic transitions are observed as a function of the out-of-plane field angle from ~10 to 14 T, likely due to the multiple monoclinic domains.