Magnetic response and electronic states of well defined Graphene/Fe/Ir(111) heterostructure

We investigate a well defined heterostructure constituted by magnetic Fe layers sandwiched between graphene (Gr) and Ir(111). The challenging task to avoid Fe-C solubility and Fe-Ir intermixing has been achieved with atomic controlled Fe intercalation at moderate temperature below 500 K. Upon intercalation of a single ordered Fe layer in registry with the Ir substrate, an intermixing of the Gr bands and Fe $d$ states breaks the symmetry of the Dirac cone, with a downshift in energy of the apex by about 3 eV, and well-localized Fe intermixed states induced in the energy region just below the Fermi level. First principles electronic structure calculations show a large spin splitting of the Fe states, resulting in a majority spin channel almost fully occupied and strongly hybridized with Gr $\pi$ states. X-ray magnetic circular dichroism on the Gr/Fe/Ir heterostructure reveals an ordered spin configuration with a ferromagnetic response of Fe layer(s), with enhanced spin and orbital configurations with respect to the bcc-Fe bulk values. The magnetization switches from a perpendicular easy magnetization axis when the Fe single layer is lattice matched with the Ir(111) surface to a parallel one when the Fe thin film is almost commensurate with graphene.

Figure 1

Low energy electron diffraction (LEED) patterns (electron beam energy 140 eV) for (a) Gr/Ir(111) and (b) Gr/1 ML Fe/Ir(111); (c) atomic structure as deduced by DFT, top view of the moiré pattern of Gr/Fe/Ir(111) with C atoms represented in gray, Fe in red, and Ir in cream; (d),(e) charge difference with respect to the free standing Gr and the pristine Fe/Ir(111) slab, computed with DFT: (d) top view of the charge difference isosurfaces (positive shown in purple, negative in yellow) and (e) side view of the Gr/Fe/Ir(111) structure showing the graphene corrugation and charge difference isosurfaces, using the same colors described for the previous panels.

Figure 2

LEED patterns (electron beam energy 140 eV) and zoomed pattern around the (10) diffraction spot for Gr/7ML Fe/Ir(111) (a), Gr/0.5ML Fe/Ir(111) (b) and Gr/Ir(111) (c); below: sketch of the main symmetry directions and of the diffraction pattern of the Ir(111) surface (red dots); the Surface Brillouin Zone (SBZ) is shown in black. Photoemission energy distribution curves (EDC) taken at 40.814 eV (He ${\mathrm{II}}_{\alpha }$); angular integrated spectra around $\mathrm{\Gamma }$ (d) and $K$ (e) points of the SBZ for different thickness of the intercalated Fe layer between Gr and Ir.

Figure 3

Angular resolved photoelectron spectroscopy $\mathrm{h}\nu =40.814$ eV spectra of Gr/Ir(111) (left panels) and Gr/1 ML Fe/Ir(111) (right panels), around the $K$ point of the SBZ. Electronic band structure vs ${\mathrm{k}}_{//}$ as intensity plot (upper panels); spectral density plotted as sequential EDC curves from the $K$ point towards $\mathrm{\Gamma }$ (bottom panels).

Figure 4

Band structure and projected density of states (pDOS) computed within DFT for Gr/Ir(111) and Gr/1 ML Fe/Ir(111): (a) Gr/Ir(111) DOS projected on C and Ir atomic orbitals and (b) Gr/Ir(111) band structure, unfolded on the $1\times 1$ graphene unit cell as described in the main text; (c) Gr/Fe/Ir(111) majority spin DOS projected on C, Fe, and Ir atomic orbitals; (d) majority and (e) minority spin band structure, unfolded on the $1\times 1$ graphene unit cell; (f) Gr/Fe/Ir(111) minority spin pDOS.

Figure 5

XMCD spectra of Fe ${\mathrm{L}}_{2,3}$ absorption edges for Gr/1.4 ML Fe/Ir(111) (top) and Gr/7ML Fe/Ir(111) (bottom) acquired in remanence at RT in normal (left) and grazing (right) incidence geometries (see sketches for the experimental geometry). The easy magnetization direction switching from perpendicular (top) to parallel (bottom) to the surface plane is indicated by the blue arrows.