Nonlinear spin dynamics across Néel phase transition in ferromagnetic/antiferromagnetic multilayers

We observe strongly nonlinear spin dynamics in ferromagnetic/antiferromagnetic multilayers, controlled by the number of bilayers in the system, layer thicknesses, and temperature, peaking in magnitude near the Néel point of the antiferromagnetic layers just above room temperature. Well above the Néel transition, the individual ferromagnetic layers are exchange decoupled and resonate independently. As the temperature is lowered toward the Néel point, the ferromagnetic proximity effect through the thin antiferromagnetic spacers transforms the system into a weakly coupled macrospin chain along the film normal, which exhibits pronounced standing spin-wave resonance modes, comparable in intensity to the uniform resonance in the ferromagnetic layers. These findings are supported by our micromagnetic simulations showing clear spin-wave profiles with precessional phase lag along the macrospin chain. Well below the Néel transition, the FeMn layers order strongly antiferromagnetically and exchange-pin the ferromagnetic layers to effectively make the multilayer one macrospin. The appearance and intensity of the high-frequency spin-wave modes can thus be conveniently controlled by thermal gating the multilayer. The nonlinearity in the microwave response of the demonstrated material can approach 100%, which is large compared to nonlinear materials used in, e.g., optics, with second-harmonic generation often at the single percentage level.