Spin-polarized hybrid states in epitaxially-aligned and rotated graphene on cobalt

Markings produced by dragging an ordinary pencil on paper can generate great elation in an infant learning how to draw. It is amusing to think that the layered nature of graphite responsible for those markings has stirred an equal amount of excitement in some of the best minds in the physics community in the last decade. The possibility to dissect graphite into individual single atomic layers, famously termed as graphene, gave rise to a plethora of surprising observations in part due to graphene’s exceptional electronic properties. Our research aims at understanding the spin degree of freedom in the electronic structure of graphene, especially when it is interfaced to a ferromagnetic metal.

Regarding spin-related phenomena, the graphene-Co interface is particular as it gives rise to efficient spin injection, spin reorientation transitions in Co, and a Rashba-type DMI interaction. The basis of such spin effects may be sought in the Gr-Co orbital hybridization, which results in a spin-polarized state near the Fermi level, termed as minicone in the literature. On the other hand, all studies to date focused on those cases in which the graphene’s crystallographic orientation matches that of the Co support. Instead, in order to understand more realistic systems, in the present work, we have investigated the spin-polarized electronic structure of azimuthally-rotated Gr-Co interfaces. High-resolution angle-resolved photoemission (ARPES) data collected at the BaDElPh and NanoESCA beamlines were complemented by the Spectroscopic photoemission and low energy electron microscopy (SPELEEM) measurements carried out at the Nanospectroscopy beamline.

From the structural point of view, we identified the most prominent graphene rotational domains on Co(0001). As seen in Fig. 1a, ARPES data near the Fermi level shows the presence of several rotated patterns with hexagonal symmetry. The same can be observed in low-energy electron diffraction (LEED) patterns as seen in Fig. 1b. A detailed analysis of the LEED profile identifies several azimuthal angles (Fig. 1c). These angles nearly coincide with commensurate Gr-Co superlattices, which are also used in the theoretical modelling of each rotational angle.

Figure 1. a) ARPES pattern showing both rotated and epitaxially-oriented graphene phases, along with the primitive cell and supercell overlays. b) LEED pattern from the same surface. c) LEED intensity profile along the azimuthal direction showing distinct rotational domains.

The electronic structures of epitaxially-oriented and rotated graphene domains were studied both experimentally by ARPES and theoretically by density functional theory (DFT). Fig. 2 summarizes the most important findings. Most importantly, Fig. 2a and 2b clearly show the same minicone band feature both for both azimuthal alignments. The binding energy of the minicone state is found unchanged between rotated and epitaxially-oriented domains, whereas the Fermi velocity is slightly higher for the rotated domains. Moreover, the π band apex of rotated graphene is shifted towards lower binding energies. All experimental observations are confirmed by our DFT calculations and point to a slightly weaker average C-Co bond in the case of rotated domains compared to the epitaxially-oriented ones.

Figure 2. Binding energy-dependent ARPES profiles are shown for a) aligned and b) rotated graphene domains along the direction shown by a dashed red line in the inset. c) ARPES pattern near the Fermi level. Spin-polarized ARPES is shown on the left half. d)-e) DFT calculated electronic structure is shown for both spin states for the 19° rotation after unfolding to the primitive cell.

Importantly, the minicone band feature is found to be highly spin-polarized for all azimuthal rotations, as seen in Fig. 2c. The theoretical confirmation is given in Fig. 2d, in which the same band feature appears only in one spin state for the 19° azimuthal rotation case. Notably, the same theoretical observation is made for all other commensurate Gr/Co structures studied.

The correspondence between the band structure of epitaxially-aligned and azimuthally-rotated graphene domains can be understood by the primitive-cell character calculated for each rotation. Even in the rotated phases, the primitive cell character is found to be about 60%. In all the rotated phases considered, the states at K of the graphene primitive cell are unfolded exclusively from the K points of the superstructure unit cell. This results in the appearance of the minicones both at the primitive and supercell K points and explains the observation that the minicone appears at the K point of the primitive cell even in the rotated phases. Considering the presence of different rotational alignments of graphene is unavoidably expected, especially on ultrathin Co films, our findings provide a solid experimental and theoretical basis for exploiting CVD synthesized graphene/Co(0001) interfaces in spintronics.

This research was conducted by the following research team:

Matteo Jugovac^1,3, Edward Danquah Donkor², Paolo Moras³, Iulia Cojocariu^1,4, Francesca Genuzio¹, Giovanni Zamborlini⁴, Giovanni Di Santo¹, Luca Petaccia¹, Nataša Stojić², Vitaliy Feyer^4,5, Claus Michael Schneider^4,5, Andrea Locatelli¹and Tevfik Onur Menteş¹

¹Elettra - Sincrotrone Trieste S.C.p.A., Trieste, Italy
²Abdus Salam International Centre for Theoretical Physics, Trieste, Italy
³Istituto di Struttura della Materia-CNR (ISM-CNR), Trieste, Italy

Contact person:
Local contact person: