Frequency and field control of the magnetic relaxation in 2D magnonic crystals

We present a method of changing the magnetic damping by more than 500% in 2D magnonic crystals. These magnonic crystals have been prepared either by direct nanopatterning of the magnetic layer using electron beam lithography or by nanostructuring the substrates prior to deposition by ion beam erosion.
It was shown theoretically that magnetic defect structures lead to an increased magnetic damping due to two-magnon scattering [1]. This two-magnon scattering contribution to the magnetic relaxation can be easily determined e.g. from frequency dependent ferromagnetic resonance measurements (FMR) [2]. It is characterized by a non-linear but monotonous increase of the resonance linewidth with excitation frequency. Recent extension of this theory to films with a periodic defect structure, like 2D magnonic crystals, shows that the two-magnon scattering increases the damping only at certain frequencies related to the structural dimensions [3].
This in turn means that by slightly changing the excitation frequency by a few GHz two-magnon scattering enhanced damping can be switched on and off in addition to the intrinsic Gilbert damping that is always present. The frequency where this happens can be preselected by the periodicity of the magnonic crystal [3,4]. Moreover, it is possible to control the damping by the direction of the in-plane magnetic field. For application in spintronic devices it could be very interesting to have a selectively higher or lower damping at certain frequencies. In conventional materials only a monotonous increase of damping with frequency is achievable.
This work was supported by the DFG grant FA 314/6-1.