Direct observation of imprinted antiferromagnetic vortex states in CoO/Fe/Ag(001) discs

In magnetic thin films, a magnetic vortex is a state in which the magnetization vector curls around the centre of a confined structure(1). In a thin-film disc, vortex states are characterized by the vortex polarity and the winding number(2,3). In ferromagnetic (FM) discs, these two parameters have been shown to govern many fundamental properties of the vortex, such as its gyroscopic rotation(4), polarity reversal(5-7), core motion(8) and vortex-pair excitation(9). In antiferromagnetic (AFM) discs(10), in contrast, there has been only indirect evidence for a vortex state, obtained through the observation of induced FM-ordered spins in the AFMdisc(11-14). Here we report the direct observation of an AFM vortex state in the AFM layer of an AFM/FM bilayer system. We have fabricated single-crystalline NiO/Fe/Ag(001) and CoO/Fe/Ag(001) discs, and using X-ray magnetic linear dichroism techniques we observe two types of AFM vortex, one of which has no analogue in FM structures. We also show that a frozen AFM vortex can bias an FM vortex at low temperature.