  Spin-wave amplification techniques are key to the realization of magnon-based
computing concepts. We introduce a novel mechanism to amplify spin waves in
magnonic nanostructures. Using the technique of rapid cooling, we create a
non-equilibrium state in excess of high-energy magnons and demonstrate the
stimulated amplification of an externally seeded, propagating spin wave. Using
an extended kinetic model, we qualitatively show that the amplification is
mediated by an effective energy flux of high energy magnons into the low energy
propagating mode, driven by a non-equilibrium magnon distribution.
