Two-magnon scattering in permalloy thin films due to rippled substrates

Abstract

We report on the influence of correlated substrate roughness on the two-magnon scattering in 30 nm Ni${}_{81}$Fe${}_{19}$ thin films. Using ion beam erosion, periodically modulated substrates (ripple) were produced with wavelengths between 20 and $432\phantom{\rule{0.28em}{0ex}}\mathrm{nm}$. This surface corrugation is adopted by magnetic layers grown on top yielding dipolar stray fields if magnetization and ripple ridges are aligned perpendicular to each other. In case of $\ensuremath{\lambda}\ensuremath{\ge}222\phantom{\rule{0.28em}{0ex}}\mathrm{nm}$, the evolving periodic field pattern triggers two-magnon scattering, which depends strongly on the direction of magnetization with respect to the ripple pattern. In-plane broadband ferromagnetic resonance reveals prominent peaks in the frequency-dependent linewidth measured perpendicularly to the ripple ridges. These peaks can be switched off if the magnetization is aligned along the ripple ridges. Our results are compared to predictions obtained from recent theory on spin waves in periodically perturbed films.