Spin-wave caustic formation by higher-harmonic radiation from a localized spinwave mode in a Heusler waveguide

Cobalt-based full Heusler compounds are very promising candidates for future magnon-spintronic devices as well as for the observation of novel phenomena in magnon transport in magnetic microstructures. The reason for the anticipated advantages is their decreased Gilbert damping in comparison with most conventional metallic 3d-ferromagnets and, in particular, in comparison with the widely used Ni81Fe19.

As shown recently, the decay length of propagating spin waves in spin-wave waveguides made of the Heusler compound Co2Mn0.6Fe0.4Si (CMFS) shows a significant increase in comparison with wave propagation Ni81Fe19. This observation reflects the decreased Gilbert damping of α=0.003 in CMFS with respect to the damping constant of α=0.008 in Ni81Fe19.

The decreased losses in CMFS not only lead to an increase of the decay length but also to the pronounced occurrence of nonlinear effects in the spin system. In this talk, we report the nonlinear emission of spin-wave beams with a well-defined propagation direction from the directly-excited spin-wave mode.

The overall process that led to our observation comprises three interesting phenomena of spin dynamics: the localization of a spin-wave mode due to a field gradient, higher harmonic generation, and the formation of spin-wave caustic beams. Even though, each of the constituent phenomena stimulated serious research efforts in the field of magnon spintronics on its own, their complex interplay was observed for the first time in a CMFS spin-wave waveguide just recently. This highlights the advantage of the Heusler material compared to the commonly used 3d-ferromagnets.

Subsequently, all three phenomena will be addressed in this talk. The localization of the directly-excited mode is described on the basis of a micromagnetic simulation as well as a dispersion calculation. This localized spin-wave mode can be identified as the source for the generation of the second and third harmonic. Finally, the radiation characteristics of the higher harmonics are described quantitatively by an analytical calculation based on the anisotropic dispersion relation of spin waves in magnetic thin films.