Image-Potential States on a 2D Gr-Ferromagnet Hybrid: Enhancing Spin and Stacking Sensing

Spin-resolved scanning tunneling microscopy and spectroscopy studies of image-potential states on Gr/Fe/Ir(111) show their sensitivity to the spatial variation of the Gr-Fe distance and of the interfacial charge and spin transfer within the moiré unit cell. A stacking contrast between the fcc and hcp sites, indistinguishable in the direct tunneling mode, is provided by the image-potential states in both resonant tunneling topography and spectroscopy images. The spin-polarized measurements reveal a site- and energy-dependent spin-polarization of the image-potential states that is mapped across the moiré unit cell with a spin contrast between hcp and fcc sites unobservable in direct tunneling mode. On the on-top sites, the lowest image-potential states are found to exhibit a high spin-polarization, in contrast to the electronic states around the Fermi energy evanescent into vacuum, and attributed to their interfacial character at the respective Gr-Fe distance. Since the physisorbed graphene is only weakly spin-polarized on these sites, image-potential states reflect the spin density at buried interfaces covered by nonmagnetic passivation layers. The simultaneous imaging of structural, electronic, and magnetic properties, both at the surface and the buried interface, will prove to be invaluable in research of 2D hybrids and heterostructures.