Diverging-converging spin vortex pairs in biquadratically interlayer exchange coupled elements

Spin structures have been an interesting topic of magnetism research for many years. Within this field, magnetic vortices have attracted much attention, due to their non-trivial topology and the various dynamic modes they exhibit [1]. A magnetic vortex consists of a planar, flux-closing magnetization curl that turns out of the plane in the central nanoscopic core region. In a single layer structure, the curl’s radial components typically cancel each other out. Recent investigations show that this also holds true for multilayer vortex systems with bilinear interlayer exchange coupling [2]. Here we report on pairs of diverging-converging spin vortices occurring in biquadratically coupled systems. Using magnetic scanning transmission x-ray microscopy (STXM) we directly observe that the individual vortices of such pairs possess a residual radial magnetization component, i.e. ∇Mxy≠0. This implies an additional perpendicular magnetization divergence ∇Mz, for which we compare a continuous model with discrete micromagnetic simulations.
[1] S.-B. Choe et al., Science 304, 420 (2004). [2] S. Wintz et al., Appl. Phys. Lett. 98, 232511 (2011).