X-ray imaging of the dynamic magnetic vortex core deformation

Magnetic thin-film square-or disc-shaped nanostructures with adequate dimensions exhibit a magnetic vortex state: the magnetization vectors lie in the film plane and curl around the structure centre. At the very centre of the vortex, a small, stable core exists where the magnetization points either up or down(1,2). The discovery of an easy core reversal mechanism(3) did not only open the possibility of using such systems as magnetic memories, but also initiated the fundamental investigation of the core switching mechanism itself(4-15). Theoretical modelling predicted that the reversal is mediated by the creation and annihilation of a vortex-antivortex pair(3,4,16), but experimental support has been lacking until now. We used high-resolution time-resolved magnetic X-ray microscopy to experimentally reveal the first step of the reversal process: the dynamic deformation of the vortex core. In addition, we have measured a critical vortex velocity above which reversal must occur(5,17). Both observations support the previously proposed reversal mechanism.