Spin polarization at the graphene-ferromagnetic metal interface
S. Entani1), Y. Yamauchi2), X. Sun2), M. Kurahashi2), M. Ohtomo1) ,Y. Matsumoto1) , H. Naramoto1) and S. Sakai1)1) : Research Gr. for Molecular Spintronics, JAEA 2): National Institute for Materials Science.

Graphene attracts much attention as the most promisingmaterial for realizing molecular spintronic devices, due to thepossible extremely long spin relaxation length which reflects theweak spin-orbit interaction, hyperfine interaction and thespecific  band structure. In the graphene spintronic devices, thecontrol of the spin injection/detection processes through thecontact between graphene and ferromagnetic metal (FM)electrodes are crucial for the device operation. Therefore, theelucidations of the electronic structures of the graphene/FMinterface including the spin polarization in graphene are
essentially important for designing molecular spintronic devices
based on graphene.
In order to investigate the interface electronic structures, weneed to address the following two issues. The first issue is theformation of a well-defined interface between graphene and FM.
So far, several fabrication methods have been developed for thedeposition of graphene on various substrates, such as
micromechanical exfoliation (ME) from bulk graphite [1] andultrahigh vacuum chemical vapor deposition (UHV-CVD) [2].
ME results in small flakes of graphene with significant
inhomogeneity in the atomic-layer thickness and electronic state.
Moreover,contaminationandoxidizationofthegraphene/substrate interface are unavoidable in practice during
the fabrication processes. On the other hand, UHV-CVD can be
an alternative fabrication method to overcome the aboveproblems. The chemical growth of graphene on the cleancatalytic metal surface in UHV enables the interface to be kept
free from contamination. In the present study, UHV-CVD wasadopted for the deposition of graphene on the FM substrates.
The second issue is to employ an appropriate spectroscopic
technique which allows highly-sensitive detection of atomicallythin graphene. The conventional techniques such as spinresolved ultraviolet photoelectron spectroscopy are notsatisfactory due to the superposition of large background signals
from the substrate and small signals from graphene. We havedirected our attention toward spin-polarized metastable-atom
deexcitation spectroscopy with the extremely high sensitivity to
the topmost atomic layer on the sample surface, which makes it
possible to selectively detect the spin-resolved electronic
structure of graphene contacted with FM substrates.
The graphene with coverage ranging from 0% to 100% wasdeposited on the ferromagnetic nickel, Ni(111), surface bycontrolling the exposure amount of benzene, which is aprecursor for the graphene growth, in UHV-CVD [3]. The spinresolved electronic structure of graphene was investigated by
SPMDS as follows: The secondary electron emission wasdetected during irradiation of the spin-polarized triplet
metastable helium atom, He*(23S), beam with a thermal velocity
onto the sample surface. The ejected electron intensity (SPMDSspectrum) depends on degree of the spin polarizatin of thesurface electrons since only a surface electron with the same
sign of spin as that of the 1s hole of He*(23S) can be transferred
to and fill the 1s hole following the Pauli’s exclusion principle[4].
Figure 1 shows the spin asymmetry (Ip – Ia)/(Ip + Ia) obtained
from the SPMDS measurements, where Ip and Ia represent thesecondary electron intensities obtained under irradiation of thespin-polarized He*(23S) beams with electron spins parallel andantiparallel to the majority spin of the Ni substrate, respectively.
The sign and magnitude of the spin asymmetry change with thecoverage of graphene, and, at the coverage of 100%, theasymmetry becomes approximately -2% at the Fermi energy ofgraphene. This negative spin asymmetry of graphene is different
from the positive asymmetry of approximately +7% at the Fermi
energy of Ni measured for the Ni(111) clean surface at thecoverage of 0%. The above results lead to the conclusion thatthe conduction electrons in graphene at the graphene/Ni(111)interface have the opposite sign of the spin polarization withrespect to those in Ni(111) [5]. It is suggested that the opposite
spin polarization as clarified in the present study possibly resultsin the degradation of the spin polarization of the charge current
passing through the interface between graphene and FM in spin
valve devices.
The present study has, for the first time, provided direct
information on how the conduction electrons in graphene areaffected by the contact with FMs. Our research achievement isexpected to greatly contribute to the designing of spin-related
properties of molecular spintronic devices by taking advantage
of graphene and other two-dimensional materials.

Fig.1 Spin asymmetries of graphene/Ni(111) obtained from theSPMDS measurements with the He*(23S) beams aligning the spinparallel and anti-parallel to the majority spin of Ni(111).

References[1] K.S. Novoselov et al., Science 306, 666 (2004).
[2] C. Oshima & A. Nagashima, J. Phys. Condens. Mater 9, 1(1997).
[3] S. Entani et al., J. Appl. Phys. 111, 064324 (2012).
[4] Y. Yamauchi et al., J. Surf. Sci. Soc. Jpn. 26, 151 (2005).
[5] S. Entani et al., Carbon 61, 134 (2013).