Dynamics of Interlayer Coupled Magnetic Vortex Pairs

A magnetic vortex structure consists of a planar magnetization curl with a perpendicularly magnetized nanoscopic core in its center. As a consequence of the different combinations possible for the curl’s rotation sense (circulation: c ∈ {+1,-1}) and the orientation the core (polarity: p ∈ {+1,-1}), magnetic vortices occur with opposite handednesses. When excited by magnetic fields or spin polarized currents, magnetic vortices exhibit different kinds of eigen modes of which the gyrotropic core mode is most prominent. It corresponds to an orbiting of the core around its equilibrium position, where the sense of motion is determined by p as a topological charge only[1,2]. The static and dynamic properties of single layer vortices have been the subject of numerous fundamental investigations during the past decades[e.g.1,2,3], which also led to the proposal of their application for memory devices[4] and spin-torque oscillators[5].
On the technological context as well as from a basic perspective, the coupling between spatially confined vortices is a key issue. Here we report on the magnetization dynamics of coupled vortex pairs, separated by a nonmagnetic spacer in a stacked geometry. Besides magnetodipolar interaction, interlayer exchange coupling (IEC) can be present in such a system, depending on the spacer material and thickness. We have experimentally identified two nongenerate configurations for both, IEC and purely dipolar coupled vortex pairs regarding their relative circulation orientations by means of scanning transmission x-ray microscopy (STXM)[6]. Time-resolved STXM furthermore allows to probe the layer-specific response of coupled vortex pairs to magnetic excitations. By this, we have observed a principally different core gyration behavior for the different circulation configurations. Also, a collective reorientation of the core polarities has been detected for sufficiently strong excitations.