Non-Gilbert-damping Mechanism in a Ferromagnetic Heusler Compound Probed by Nonlinear Spin Dynamics

The nonlinear decay of propagating spin waves in the low-Gilbert-damping Heusler film Co2Mn0.6Fe0.4Si is reported. Here, two initial magnons with frequency f0 scatter into two secondary magnons with frequencies f1 and f2. The most remarkable observation is that f1 stays fixed if f0 is changed. This indicates, that the f1 magnon mode has the lowest instability threshold, which, however, cannot be understood if only Gilbert damping is present. We show that the observed behavior is caused by interaction of the magnon modes f1 and f2 with the thermal magnon bath. This evidences a significant contribution of the intrinsic magnon-magnon scattering mechanisms to the magnetic damping in high-quality Heusler compounds.