Magnetization Dynamics in Vortex-Imprinted Ni $_{80}$ Fe $_{20}$ /Ir

The dynamic behavior of the vortex magnetization on micrometer-sized, ferromagnetic/antiferromagnetic (FM/AFM) bilayer exchange-coupled square elements is investigated using time-resolved Kerr effect microscopy and micromagnetic modeling. $10 \mu$ m $\times 10 \mu$ m square elements consisting of Ni $_{80}$ Fe $_{20}$ /Ir $_{20}$ Mn $_{80}$ bilayers were prepared and subsequently field cooled at various field strengths $H_{\rm cf}$ , resulting in different exchange-bias field $H_{\rm ex}$ profiles at the FM/AFM interface. With the presence of the $H_{\rm ex}$ , the vortex core is displaced from the center of the square and consequently the fourfold symmetry of the Landau vortex state breaks. The spin-wave eigenmode excited upon application of a magnetic field pulse is strongly dependent on the level of the displacement of the vortex configuration, in which the mode frequency decreases for the growing domain, whereas it increases for the shrinking domain.