ApplPhysLett_93_022509

Graphene-protected iron layer on Ni(111)

Yu. S. Dedkov, M. Fonin, U. Rüdiger, and C. Laubschat

Citation: Appl. Phys. Lett. 93, 022509 (2008); doi: 10.1063/1.2953972

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Graphene-protected iron layer on Ni …111…

Yu.S.Dedkov,1,a ?M.Fonin,2U.Rüdiger,2and https://www.360docs.net/doc/325593519.html,ubschat 1

1Institut für Festk?rperphysik,Technische Universit?t Dresden,01062Dresden,Germany 2

Fachbereich Physik,Universit?t Konstanz,78457Konstanz,Germany

?Received 24April 2008;accepted 12June 2008;published online 17July 2008?

Here we report a photoemission study of the Fe intercalation underneath a graphene layer on Ni ?111?.The process of intercalation was monitored by means of x-ray photoemission of corresponding core levels as well as ultraviolet photoemission of the graphene-derived ?states in the valence band.Thin fcc Fe layers ?2–5ML thickness ?at the interface between a graphene capping layer and Ni ?111?form epitaxial ?lms passivated from the reactive environment.?2008American Institute of Physics .?DOI:10.1063/1.2953972?

Magnetic thin ?lms with out-of-plane ?or perpendicular ?magnetic anisotropy play an important role in nanotechnol-ogy.Such systems can be used as perpendicular recording media 1which is predicted to allow information storage den-sities of up to 1Tbit /in.2a quadrupling of today’s highest areal densities.1Along with the widely used materials with out-of-plane magnetic anisotropy such as CoPt or FePt al-loys,face-centered cubic ?fcc ?Fe thin ?lms also showing perpendicular magnetic anisotropy have recently attracted considerable interest as possible candidates for applications in novel magnetic data storage devices.Here Fe which origi-nally has body centered cubic ?bcc ?structure can be stabi-lized in ?phase ?fcc ?at room temperature in thin epitaxial ?lms grown on suitable fcc substrates.

As for the study of magnetic properties,the necessity of a close lattice match between substrate and deposited mate-rial usually results in the choice of nonmagnetic fcc tem-plates such as Cu.2,3However,Fe ?lms grown on different Cu surfaces have been found to show different growth modes and different ranges of thicknesses,in which the ferromag-netic fcc phase with out-of-plane anisotropy persists.Ni is another suitable material due to the small lattice mismatch of +2%,referred to the room temperature lattice parameter of fcc Fe,extrapolated from the high-temperature phase.4–8It was shown 4–6that a fcc Fe layer on top of Ni ?111?/W ?110?induces out of plane to the ?lm magnetic anisotropy for 0.5–3ML thick iron ?lms.At higher Fe coverages,an in-plane magnetization was found,which is proposed to be caused by the fcc to bcc transition in the Fe layer of around 4ML.

The aim of the present work is the preparation of a fcc Fe-based system with potential out-of-plane magnetic aniso-tropy which behaves inert against an aggressive environ-ment.Here we demonstrate the possibility to prepare such a system via intercalation of a thin Fe ?lm underneath a graphene layer formed on a Ni ?111?substrate.Here,we dem-onstrate that graphene behaves like a passivation layer con-serving the underlying epitaxial Fe ?lm.

The investigation of the Fe intercalation process was performed in an experimental setup for photoelectron spec-troscopy consisting of two chambers described in detail elsewhere.9,10An ordered graphene/Ni ?111?system was pre-pared on the W ?110?substrate according to the recipe de-scribed in Refs.11–13.Figures 1?a ?–1?c ?show the LEED images of the main preparation steps.The LEED spots of the graphene/Ni ?111?system reveal a well-ordered p ?1?1?overstructure as expected from the small lattice mismatch of only 1.3%?Fig.1?c ??.After the cracking procedure the Ni ?111?surface is completely covered by the graphene ?lm as was earlier demonstrated in Refs.11and 12.Intercalation of Fe underneath a graphene layer was performed via anneal-

a ?

Author to whom correspondence should be addressed.Electronic mail:

dedkov@physik.phy.tu-dresden.de.

FIG.1.?Color online ?LEED images of ?a ?W ?110?,?b ?Ni ?111?/W ?110?,?c ?graphene/Ni ?111?/W ?110?systems,and ?d ?the system obtained after inter-calation of 2ML of Fe underneath the graphene layer on Ni ?111?.All im-ages are collected at a primary electron energy of 99eV.?e ?Schematics of a possible crystallographic structure of the graphene/2ML Fe ?111?/Ni ?111?system.The corresponding distances between Ni ?Fe ?atoms in ?111?plane and the lattice constant of the graphene layer are indicated on the left-hand side of the ?gure.

APPLIED PHYSICS LETTERS 93,022509?2008?

0003-6951/2008/93?2?/022509/3/$23.00?2008American Institute of Physics

93,022509-1

ing of the 2–5ML thick predeposited Fe ?lm.Photoemission spectra monitoring the process of intercalation were recorded at 21.2eV,40.8eV ?He I ?,He II ?,ultraviolet photoemission spectroscopy ?UPS ??as well as 1253.6eV,1486.6eV ?Mg K ?,Al K ?,x-ray photoemission spectroscopy ?XPS ??photon energies using a hemispherical energy analyzer SPECS PHOIBOS 150.The energy resolution of the ana-lyzer was set to 50and 500meV for UPS and XPS,respec-tively.

Figure 1shows LEED images of successive steps of the preparation of the intercalationlike system on the basis Fe and graphene.The predeposited Fe ?lm with the nominal thickness of 2ML on top of the graphene/Ni ?111?system was annealed at ?600K leading to Fe intercalation,which forms a commensurate ?1?1?close-packed fcc structure ob-served by LEED ?Fig.1?d ??.The same LEED spots ?with slightly increased background ?were also observed after in-tercalation of 5ML of Fe underneath a graphene layer on Ni ?111?.Figure 1?e ?shows the possible crystallographic structure of the graphene/Fe ?111?/Ni ?111?system obtained after intercalation.

The investigation of intercalation was performed for 2ML and 5ML thick Fe layers on top of the graphene/Ni ?111?system.For both Fe thicknesses we found that iron is com-pletely intercalated underneath the graphene layer.In the fol-lowing we will focus on the intercalation process of the 2ML thick Fe ?lm.Figure 2shows different preparation steps of the Fe-based intercalationlike system monitored by core-level XPS at ?a ?Ni 2p 3/2,1/2,?b ?C 1s ,and ?c ?Fe 2p 3/2,1/2.In Fig.2?a ?all Ni 2p emission spectra consist of a spin-orbit doublet ?2p 3/2,1/2?and a well-known satellite struc-ture.Modi?cations of the Ni 2p XPS spectra upon the graphene/Ni ?111?system formation ?spectra 1and 2in Fig.2?a ??were discussed earlier in Ref.11.For the graphene/Ni ?111?system the C 1s XPS spectrum ?open circles in Fig.2?b ??shows a single emission line demonstrating only one carbon phase at the surface ?graphene ?.Deposition of the 2ML thick Fe ?lm on top of the graphene/Ni ?111?system leads to a reduction in the Ni 2p 3/2,1/2?spectrum 3in Fig.2?a ??and the C 1s ?solid line in Fig.2?b ??emission lines intensities.The XPS Fe 2p 3/2,1/2spectrum in this case has a typical shape characteristic for metallic Fe ?dashed line in Fig.2?c ??.

Annealing of the 2ML Fe/graphene/Ni ?111?system at 600K leads to a successful intercalation of the Fe layer un-derneath the graphene layer.This fact is supported by the respective changes in the monitored XPS spectra:the inten-sity of the Ni 2p 3/2,1/2spectra almost does not change ?spec-trum 4in Fig.2?a ??,the intensity of the C 1s spectra is restored and coincide with the one for the graphene/Ni ?111?spectra ?open circles and dash-dot line in Fig.2?b ??,and the intensity of the Fe 2p 3/2,1/2emission line is reduced in accor-dance with the assumption that the graphene layer stays on top of the system and the mean free path for electrons emit-ted from Fe 2p level has to be taken into account.However,these changes in intensities of the emission lines cannot be simply considered as a proof of a successful Fe intercalation underneath the graphene layer because almost the same changes in the intensities of XPS lines can be ascribed to the formation of high iron islands on top of the graphene layer.In order to rule out this assumption test experiments on the investigation of inert properties of the graphene layer were performed as suggested in Ref.11.Two systems under study were exposed to oxygen at a partial pressure p O 2=1?10?6mbar for 20min:2ML of Fe on top of graphene/Ni ?111?and the system obtained after intercalation of iron atoms between the graphene layer and Ni ?111?.The results are shown in Fig.2?c ?where Fe 2p 3/2,1/2XPS spectra for these two cases are shown ?open squares and open circles in Fig.2?c ??.The 2ML thick Fe ?lm on top of graphene/Ni ?111?is completely oxidized after such oxygen treatment and the spectrum line shape is similar to the one of magnetite.14In contrast to that the oxygen exposure of the system obtained by intercalation of Fe underneath

the

FIG.2.?Color online ?XPS monitor-ing of Fe intercalation underneath graphene on Ni ?111?and inert proper-ties of the graphene-based intercala-tionlike system:?a ?Ni 2p 3/2,1/2,?b ?C 1s ,?c ?graphene/Ni ?111?/W ?110?,and Fe 2p 3/2,1/2,and ?d ?O 1s XPS emis-sion lines of the systems under study ?see text ?.Spectra are shifted with re-spect to each other for clarity.

graphene layer does not lead to any visible changes in the Fe 2p 3/2,1/2XPS spectra.The intensity of the O 1s photoemis-sion signal of the graphene/2ML Fe ?111?/Ni ?111?system after the oxygen exposure is very weak as compared to the one of the 2ML Fe/graphene/Ni ?111?system upon the same treatment ?compare spectra 1and 2in Fig.2?d ??.These facts together with the observation of well-ordered LEED spots con?rm the formation of a graphene-protected intercalation-like system:graphene/2ML Fe ?111?/Ni ?111?.

The same scenario is applicable to the description of the UPS spectra of the valence band of the investigated system.These results are presented in Fig.3where valence band photoemission spectra are shown for pure Ni ?111??spectrum 1?,graphene/Ni ?111??spectrum 2?,2ML Fe/graphene/Ni ?111??spectrum 3?,and graphene/2ML Fe ?111?/Ni ?111??spectrum 4?,as well as the systems after exposure of both latter samples to oxygen at the same conditions as mentioned above ?spectra 5and 6?.The spectra were obtained with He II ?radiation in normal emission geometry.Present results for Ni ?111?and for the graphene/Ni ?111?system are in good agreement with the previous data.11–13Deposition of a 2ML thick Fe ?lm leads to a decrease in the graphene-derived ?state intensity and modi?cation of the emission in the va-lence band region close to the Fermi level where Ni and Fe 3d overlap ?compare spectra 2and 3in Fig.3?.Annealing of this system leads to the complete restoring of the graphene-derived ?state intensity and new modi?cation of the photo-electron emission in the region of 3d emission ?compare spectra 2–4in Fig.3?.Inert properties of the graphene/2ML Fe ?111?/Ni ?111?system,as described in the previous para-graph,were tested by exposing this system to oxygen at the

same conditions.Almost no changes in the photoemission spectra of the valence band region of this system after oxy-gen adsorption are visible,whereas the UPS spectrum of the system obtained after oxygen exposure of 2ML Fe/graphene/Ni ?111?shows strong contribution of the O 2p emission and drastic changes in the valence band region close to the Fermi level.

In conclusion,we studied the intercalation of thin Fe layers with 2and 5ML thicknesses underneath graphene and formation of the graphene-protected fcc Fe ?lms on Ni ?111?.The process of intercalation was monitored via XPS of cor-responding core levels and UPS of the graphene-derived ?states in the valence band.Exposure of the formed graphene/2ML Fe ?111?/Ni ?111?system to large amounts of oxygen does not lead to any changes in XPS as well as UPS spectra of the system con?rming a successful Fe intercala-tion and inert properties of the Fe-based intercalationlike system.However,magnetic properties of the graphene-protected Fe ?lms on Ni ?111?still remain to be tested by means of,for example,magneto-optical Kerr effect or x-ray magnetic circular dichroism using synchrotron radiation.We suggest that such inert systems may be implemented as chemically stable elements in the future magnetoelectronic devices.Moreover,fcc Fe layers underneath a graphene layer are of special interest for the studies of electronic and mag-netic properties of graphene in out-of-plane local magnetic ?eld.

This work was funded by the Deutsche Forschungsge-meinschaft ?DFG ?through SFB 463,Project B4and SFB 767,Project C5.

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FIG.3.?Color online ?Valence band UPS spectra of intercalation of Fe underneath graphene on Ni ?111?and inert properties of the graphene-based intercalationlike system.Spectra are shifted with respect to each other for clarity.