Large Nonlinear Dynamical Response of Superparamagnets: Interplay between Precession and Thermoactivation in the Stochastic Landau-Lifshitz Equation

The nonlinear dynamical response of classical spins governed by the stochastic Landau-Lifshitz equation is found to be large and very sensitive to the damping in the medium-to-weak damping regime. This result is interpreted in terms of a cooperation, induced by the driving field, between the precession of the spin and its thermoactivation over the potential barrier. The large damping dependence (absent in the linear response) can be used to determine the evasive damping coefficient in superparamagnets, so clarifying the nature of the spin-environment interaction in these systems.