Two-magnon scattering and mode-splitting in 1-dimensional quasi-magnonic crystals

The magnetic relaxation in quasi 1-dimensional periodic nanostructures (magnonic crystals) is investigated by ferromagnetic resonance (FMR). In thin ferromagnetic films, the magnetization dynamics are governed by intrinsic effects like Gilbert damping and spin-pumping but also by extrinsic effects like two-magnon scattering due to inevitable defect structures. By using nanoscale periodically modulated magnetic films we are able to artificially create and thus control those defect structures necessary to induce two-magnon scattering. The results are compared to available analytical theory [1].
The magnetic modulation was created by lithographically defined stripes and subsequent ion beam irradiation. The ion beam energy was chosen such that the ions create a magnetic perturbation at the surface. This slightly reduces the saturation magnetization in the irradiated stripes and hence the effective magnetic thickness. These stripe defects resemble a periodic dipolar scattering potential, which couples the uniform with the final-state magnons in the two-magnon scattering process.
Broadband ferromagnetic resonance is used to measure the resonance field Hres and linewidth ΔH for different field directions and frequencies. The frequency-dependent measurements with the external magnetic field aligned parallel to the stripes show only a single resonance mode and linear increase of ΔH. Therefore the magnetic relaxation is purely Gilbert-like. With the magnetic field aligned perpendicular to the periodic structure the frequency dependence exhibits a rich mode-splitting, which can be calculated analytically.