Polarization control in magnonic vortex crystals

Summary form only given. Wave transmission media created by a periodically modulated magnetic material are referred to as magnonic crystals [1]. The dynamics of magnonic crystals are described by common concepts of solid state physics, i.e., group velocity, density of states, and band structure. We investigate so-called magnonic vortex crystals created via rectangular arrangements of magnetic vortices. Here we aim at the control of vortex-core polarizations by perpendicularly aligned bias fields and use magnetic force microscopy and broadband-ferromagnetic transmission spectroscopy. In the first step, arrays of CoPt-multilayer disks arranged in a checkerboard pattern are prepared by electron-beam lithography, sputter deposition, and lift-off processing, compare figure 1(a). Two layer types A (Pt/0.7 nm Co/Pt) and B (Pt/2x(0.8 Co/1.1 nm Pt)/Pt) of the pattern differ in the magnetic anisotropy and thus yield different switching fields of the perpendicularly magnetized disks [2]. The switching fields are investigated by Kerr microscopy. After saturation in negative field direction, disks of type B start to switch at a field strength of μ₀ H = +29 mT visible in the hysteresis loop shown in figure 1(b). The switching is completed at +37 mT where the greyscale intensity stays constant. A stable state of antiparallel magnetization of both types of disks persists up to +56 mT, where disks of type A start to switch. At +68 mT the magnetizations of disks of type A and B are aligned parallel with the positive field direction, compare figure 1(c). The switching of the multilayer disks follows a normal distribution and shows no dependence on the interdisk distance. In the next step, permalloy disks are prepared on top of the CoPt disks by electron-beam lithography, thermal evaporation, and lift-off processing. A thin Si interlayer is used to avoid direct contact of the perpendicularly and in-plane magnetized ferromagnets. The magnetization of the CoPt disks i- adjusted using a perpendicularly aligned magnetic field. Subsequently an in-plane field is used to nucleate vortices in the permalloy disks. It is expected that due to stray field coupling the polarization of the vortex core in each permalloy disk is determined by the subjacent CoPt disk and coincides with the direction of the magnetization of the CoPt disk. The vortex-core polarizations are investigated using magnetic force microscopy (not shown). Analytical calculations [3] show that the dispersion relations and the density of states of magnonic vortex crystals depend crucially on the polarization pattern of the vortices. It has also been shown experimentally [4] that the polarization patterns in magnonic vortex crystals are determined by the frequency of excitation and the strength of the dipolar interaction between the vortices. Here we aim at the definition of the properties of the magnonic vortex crystal, i.e., its band structure via the polarization control of its vortex-core polarizations. We have realized three different polarization patterns of the vortex cores: uniform polarization of all disks, a checkerboard pattern and a stripe pattern. The density of states of these magnonic vortex crystals with its enforced polarization pattern is determined by broadband-ferromagnetic transmission spectroscopy. Financial support of the Deutsche Forschungsgemeinschaft via the Sonderforschungsbereich 668, the Graduiertenkolleg 1286, and the excellence cluster “The Hamburg Centre for Ultrafast Imaging - Structure, Dynamics and Control of Matter on the Atomic Scale” is gratefully acknowledged.