Research paperEffectiveness of Co intercalation between Graphene and Ir(1 1 1)
Graphical abstract
Introduction
Among their unique and promising properties, graphene (Gr) has an exceptional chemical stability which can be used to passivate highly reactive surfaces [1], [2]. This is a valuable benefit for thin films presenting perpendicular magnetic anisotropy (PMA) that, once stabilized against oxidation, become suitable candidates to develop ultra-high density magnetic recording devices [3], [4].
Large-area high-quality epitaxial Gr can be grown via chemical vapor deposition (CVD) [5], [6] on transition metal substrates. On Ir(1 1 1), CVD growth of Gr has been successfully performed in a range of growth temperatures from 600 to 1000 °C [7]. Thin films intercalation below Gr on Ir(1 1 1) has already been proven effective against deterioration for several systems [8], [9], [10], [11], [12], [13]: among these, intercalated Co films (Gr/Co/Ir) have raised particular interest [14], [15], [16], [17], [18], [19], [20], [21] due to an unexpected PMA enhancement with respect to bare Co films grown on Ir(1 1 1) [22].
This work explores the thickness-dependent efficiency of the Co intercalation and the effects of air exposure at ambient pressure on Co films intercalated between Gr and Ir(1 1 1). By exploiting the chemical selectivity and the sensitivity to the coordination chemistry of X-ray Absorption Fine Structure (XAFS) spectroscopy, the stability of the film against oxidation is probed. The amount of oxidised Co is then used to evaluate the intercalation effectiveness. A thermal method for the Co oxide reduction is described. This method induces also the final intercalation of Co which becomes protected against further oxidation.
Section snippets
Experimental
The Gr/Co/Ir samples were prepared in the UHV chamber available at the ID03 preparation laboratory (ESRF – Grenoble, France), at a base pressure of mbar, following a well established procedure [19], [21]. The Ir(1 1 1) substrate was cleaned through several cycles of Ar+ sputtering at 1 kV, and thermal annealing at 850 °C. The surface quality was inspected by X-ray Photoelectron Spectroscopy (XPS), and Low Energy Electron Diffraction (LEED). The substrate was considered clean when XPS showed
Results and discussion
The spectra collected after the first, , and the second annealing treatment, , are shown in Fig. 2a and b for the thick and thin film, respectively. At the bottom of every panel, the difference is reported for each polarization to facilitate the identification of the thermally-induced modifications.
Upon annealing and further air exposure, the two samples undergo a similar evolution and two major changes (labels A and B in Fig. 2) emerge in the first
Conclusion
We have investigated the effectiveness of Co intercalation below Gr. Two films of 5 and 10 ML were intercalated between Gr and Ir(1 1 1) by annealing at 300 °C for 5 min. Upon 15 min of air exposure under ambient pressure, a disordered Co oxide phase was found on both the films, being more abundant in the thickest film. This has been interpreted as a sign of incomplete and thickness-dependent intercalation process. The disordered Co oxide was removed by annealing at 500 °C. A second exposure to
Acknowledgements
The authors wish to thank the staff of ID03 and BM23 beamlines, in particular, F. Carlà, T. Dufrane, H. Isern, S. Pascarelli, and O. Mathon for the scientific and technical support provided during the experiments.
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