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

Abstract

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 AFM disc 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. © 2011 Macmillan Publishers Limited. All rights reserved.