Anomalous and anisotropic nonlinear susceptibility in the proximate Kitaev magnet α-RuCl3

The leading order nonlinear (NL) susceptibility, χ 3 , in a paramagnet is negative and diverges as T → 0. This divergence is destroyed when spins correlate and the NL response provides unique insights into magnetic order. Dimensionality, exchange interaction, and preponderance of quantum effects all imprint their signatures in the NL magnetic response. Here, we study the NL susceptibilities in the proximate Kitaev magnet α -RuCl 3 , which differs from the expected antiferromagnetic behavior. For T 50 K. In contrast, for B ∥ c -axis, no separate low/high field behaviors are measured and only a much smaller χ 3 is apparent.