High-field torque response as evidence for strong ferromagnetic Kitaev correlations in the alkali Iridate Na₂IrO₃

The alkali Iridate Na₂IrO₃ has received much attention, theoretically and experimentally, as a prime candidate for realizing Kitaev physics. Despite the progress made, the sign of the dominant Kitaev interaction remains an open question. The high field magnetization response is potentially an important tool for addressing this question. The magnetization response of Na₂IrO₃ has been investigated for magnetic fields up to 60 Tesla, and find a robust peak-dip feature in the torque response at fields corresponding to the zigzag ordering (≈ 15K) temperature. Using exact diagonalization calculations it is shown that such a distinctive signature enables us to constrain the effective spin models describing Na₂IrO₃ to those with a dominant ferromagnetic Kitaev and subdominant antiferromagnetic Heisenberg exchange, accompanied by smaller competing interactions. In contrast, competing models with an antiferromagnetic Kitaev interaction do not exhibit such a characteristic peak-dip feature in the transverse magnetization, irrespective of the nature of the accompanying interactions