Anisotropic Nonlinear Magnetic Susceptibility in Quantum Spin Systems

The leading order nonlinear susceptibility, χ₃ , in a paramagnet is negative and diverges as T→0. This divergence is destroyed when spins correlate and the nonlinear response provides unique insights into magnetic order. Dimensionality, exchange interaction, and preponderance of quantum effects all imprint their signatures in the nonlinear magnetic response. We will start by reviewing DC nonlinear susceptibility measurements in insulating and itinerant magnets in general.  We will follow this with a focus on recent nonlinear susceptibility studies in the proximate Kitaev magnet -RuCl₃. The possible simultaneous dual role of the spins, itinerant and localized, will be examined.