Guided propagation of vortex - Antivortex pairs in permalloy/NdCo/GdCo trilayers

Abstract

Correlated propagation of opposite polarity vortex-antivortex pairs has been obtained in magnetic trilayers combining soft magnetic layers (NiFe, GdCo) and weak perpendicular anisotropy (NdCo) materials. Stripe domains at the central NdCo layer provide both nucleation sites for reversed domains (at bifurcations within the periodic stripe pattern) and a confining potential with a preferred propagation direction. A detailed characterization by magnetic X-ray microscopy and micromagnetic simulations shows that reversed domains in the top NiFe layer nucleate at dislocation cores and expand by the propagation of opposite polarity vortex-antivortex pairs along the lines defined by the stripe pattern. Bifurcation geometry and stray field asymmetries restrict the possible vortexantivortex pairs at the top NiFe layer: vortices always nucleate at the central reversed stripe and, under the effect of an in-plane field, propagate in opposite senses depending on their polarity over μm long distances. This is a robust effect protected by the topological characteristics of stripe pattern dislocations and top/bottom closure domain asymmetry that may have practical applications. The possibility to tune vortex/antivortex chiralities by a proper choice of soft magnetic layers will also be discussed.