Spin-wave emission from vortex cores under static magnetic bias fields

Abstract

Spin waves are collective excitations of a spin system, where the magnetic moments precess with a spatially periodic phase shift that determines the wavelength [1]. From a technological point of view, the use of spin waves as signal carriers in future spintronic logic and memory devices has the advantages of a potentially lower power consumption and improved miniaturization prospects compared to the present charge-based CMOS technology [2]. For achieving nanoscale integration, topological spin textures such as magnetic vortex cores can be driven by alternating magnetic fields to excite coherent spin waves with short, sub-micrometer wavelengths, as shown recently for both synthetic ferrimagnetic systems and single ferromagnetic layers [3, 4]. In this contribution, we will demonstrate the versatility of this excitation mechanism by showing that the application of a static magnetic bias field makes the vortex core a manipulable spin-wave source that can be continuously translated in space without altering the resulting spin-wave dispersion relation [5]. We employ time-resolved scanning transmission x-ray microscopy (TR-STXM) to directly image vortex-core spin textures as well as the emission and propagation of spin waves from them.By applying a static in-plane magnetic bias field, we can continuously displace the vortex core from its equilibrium position in the center of a structured element. For a synthetic ferrimagnet (Co/Ru/Ni81Fe19), it is possible to shift the core relatively close to the edge while maintaining a radial, isotropic spin-wave emission pattern (see Figure 1). The wavelengths of the excited spin waves are not noticeably changed by the applied magnetic bias field, but remain tunable by the driving frequency in the same way as without bias field.For a single Ni81Fe19 layer with a thickness of the order of 100 nm, we observe a similar behavior for relatively small magnetic bias fields but when the vortex core approaches the edge of the disk, it also expands (see Figure 2). This expansion can be seen as a transformation from a zero-dimensional point source to a one-dimensional curved object where its shape and extent is controlled by the magnitude of the applied magnetic bias field. Such an expansion leads to a directional emission of the spin waves that propagate away from the core in mainly two opposite directions rather than radially as observed for non-expanded cores. For certain combinations of magnetic bias fields and driving frequencies, focusing effects appear. Such effects provide promising means for a controlled and directional propagation of spin waves without the need for additional patterning or waveguides. **