Graphene grown on Co(0001) films and islands: Electronic structure and its precise magnetization dependence

We achieve epitaxial growth of graphene on the ferromagnetic substrate Co(0001) by chemical vapor deposition using propylene. Structural and electronic properties are revealed by means of low-energy electron diffraction, scanning tunneling microscopy, and spin- and angle-resolved photoelectron spectroscopy. The results are compared to those of graphene/Ni(111) for which a magnetization-dependent Rashba effect has recently been reported. Bulklike Co(0001) films on W(110) can be separated into islands by heating, and subsequently they can be covered by graphene. Such system is used to present unambiguous proof that in graphene/Co(0001) there is no Rashba effect and no intrinsic dependence of the band dispersion on the magnetization occurs. The high spin polarization of secondary electrons reported for graphene/Ni(111) is confirmed for graphene/Co(0001) with higher values $\left(\sim 25\%\right)$ due to the larger magnetic moment of Co.

Figure 1

(Color online) Preparation and structure of graphene on Co(0001). LEED patterns from (a) bare W(110), (b) 15 ML Co/W(110), and (c) graphene/15-ML Co/W(110). (d) The STM characterization of graphene/Co(0001) reveals the formation of high-quality graphene with (e) pronounced threefold symmetry in the spatial electronic structure.

Figure 2

(Color online) (a) Structural model of graphene on Co(0001).(b) SBZ of graphene and directions of Co magnetization. (c) Angle-resolved photoemission of the band structure of graphene/Co(0001) along $\overline{\Gamma K}$ as compared to the band structure of (d) graphene on Ni(111).

Figure 3

(Color online) Characterization of a Co film collapsed into three-dimensional islands. (a) LEED and (b) STM from an overannealed 15 ML Co film. LEED (c) and STM (d) from the same sample after graphene formation.

Figure 4

(Color online) [(a)–(d)] Comparison of photoemission from the sample with continuous and broken-up Co layer. For the continuous Co film, the (a) $4f$ core-level spectrum of the W substrate reveals an intense interface component, and the (c) valence band shows graphene and Co states only. (b) When Co is overannealed and collapses into three-dimensional islands, the interface component of $\text{W}4f$ is not observed any longer and (d) in the valence band, W-derived peaks appear. For this broken and graphene-covered Co film, [(e)–(h)] spin-resolved spectra of the energy range of $\text{Co}3d$ states measured for opposite magnetization directions confirm reversal of the remanent magnetization.

Figure 5

(Color) Photoemission verification of the $k_{\parallel }$ shift of the graphene $\pi$ band upon magnetization reversal. (a) Raw data and (b) first derivative of valence-band photoemission intensity measured from 15 ML Co/W(110) graphitized and collapsed into three-dimensional islands. Dispersing bands of bulk Co and bulk W are clearly visible and are used as reference for angular and momentum distribution of photoelectrons from graphene. (c) The measured band structure of clean W(110) confirms the correct identification of $\text{W}sp$ states in (a) and (b). (d) $E\left(k_{\parallel }\right)$ diagram containing fitted positions of photoemission peaks of graphene, Co, and W measured for opposite directions of Co magnetization. No $k_{\parallel }$-shift effect is detected.

Figure 6

(Color online) Spin-resolved photoemission of secondary electrons of graphene/Co(0001) as a continuous film. High spin-polarization values of up to $\sim 25\%$ are observed near the cutoff and are due to the Co underneath the graphene.