Suhl instabilities in a magnetic nanoparticle

We report simulations of large-amplitude responses of a permalloy ellipsoid with z, x, and y diameters of 100×50×50nm³ driven by an algorithm that constrains the H₁ field to lie the x-y plane and perpendicular to the sample-averaged magnetization. For smaller H₁ uniform precession is observed, but as H₁ increases instabilities are encountered. The first, signaled by an abrupt increase in the exchange energy, involves a crossover of the uniform mode and a standing spin wave with a 2Pi phase variation along z, characteristic of the second Suhl¹ processes. The degeneracy arises from a strong dependence of the 2Pi mode frequency on the uniform precession amplitude. Above the crossover these two modes appear to merge into a bound state, in which the spins precess nonuniformly but in phase and the two modes periodically exchange energy back and forth.  At the second instability two low frequency modes with different mode numbers appear with frequencies summing to the bound-state mode, characteristic of an asymmetric parametric instability, a generalized form of the first Suhl processes that satisfies angular momentum conservation.  The system subsequently evolves in a highly irregular manner.