The alkali Iridates A2IrO3 (A=Na,Li) and their celebrated 4d analogue alpha−RuCl3 are popular candidate materials for the realization of the physics of the honeycomb Kitaev model.
These systems are described by effective spin models with dominant Kitaev and subdominant Heisenberg interactions, possibly accompanied by further-neighbor Heisenberg or anisotropic interactions. Despite the immense progress made in the understanding of these systems, the sign of the dominant Kitaev interaction, which determines the class of effective spin models relevant for describing them, remains an open question.
We investigate the magnetization response of Na2IrO3 for magnetic fields up to 60 Tesla, and find a robust peak-dip feature at fields corresponding to the zigzag ordering temperature (around 15 K). We show using exact diagonalization calculations that such a distinctive signature enables us to constrain the effective spin models describing Na2IrO3 to those with a dominant ferromagnetic Kitaev interaction. We further show that the spin-spin correlation functions become very short-ranged as the field exceeds the zigzag ordering scale, indicating a possible field-induced transition to the quantum spin liquid state, similar to the one suggested in alpha−RuCl3. Our studies illustrate the underlying universalities of the Kitaev materials Na2IrO3 and alpha−RuCl3, with the peak-dip feature having been experimentally observed in the latter system recently.