Nonlinear Multi-Magnon Scattering in Ensembles of Nanomagnets

Arrays of nanomagnets known as artificial spin ice have recently been discussed in the context of functional magnonic materials, where an interplay between geometry, material properties, and reconfigurability determines the magnon spectrum. Magnons are the fundamental quantum-mechanical bosonic excitations of magnetic solids, whose number does not need to be conserved in scattering processes. Microwave-induced parametric magnon processes have been believed to occur in magnetic thin films only, where quasi-continuous magnon bands exist. Here, we demonstrate the existence of such nonlinear multi-magnon scattering processes and their coherence in artificial spin ice. These systems exhibit effective scattering processes akin to those observed in continuous magnetic thin films. The experimental investigations using microwave and Brillouin light scattering spectroscopy are combined with numerical simulations to understand the evolution of modes. Our results show that harmonic magnon modes can be excited in artificial spin ice subject to sufficiently strong microwave power. This excitation leads to a spatial modification of modes enabling nonlinear processes that is unique to artificial spin ice. Our results suggest that tunable directional scattering is possible in these structures opening new avenues for manipulating magnon populations in reconfigurable nanomagnetic networks.