Research Group for Molecular Spintronics
Group Leader : Seiji Sakai
Members : Yoshihiro Matsumoto, Shiro Entani, Manabu Ohtomo, Pavel V. Avramov,Shinjiro Arai, Hiroshi Naramoto

Spintronics is an emerging technology taking advantage of thedual property of electrons, i.e., the charge and spin degrees offreedom. Over the past decades, spintronics has been developed
based on inorganic materials like metals and semiconductors.
Recent studies have started to shed light on the spintronics
applications of carbon-based materials (organic molecules andnanocarbons). We call this new field “Molecular Spintronics”.
In molecular spintronics, the efficient control of spin transport
in carbon-based materials is essential for device applications bytaking advantages of their potentials for transfer and storage ofspin information. Our group focuses on establishing theadvantages of nanocarbons like graphene by designing the
hybrid nanostructure. For this purpose, the novel spectroscopic
and fabrication techniques are employed properly, which allow
the microstructure analysis and control at the atomic layer level
to be possible. We successfully made distinguishedachievements on the spintronics application of graphene asfollows. We are now developing efficient spin operation sourcesand exploring new spintronic nanostructures for nanocarbonbased spintronics.
Tailor-made graphene for spintronics applications
After the discovery of the convenient fabrication method by
the exfoliation from graphite (A. Geim and K. S. Novoselov, the2010 Nobel Prize in physics), graphene has attracted worldwide
attention as an innovative nanoelectronic material. In the field ofspintronics, graphene is expected to be an ideal spin transportmaterial owing to the extremely long spin relaxation length andhigh carrier mobility. Exfoliated graphene, however, does notappear to be relevant for device applications due to the size
limitation (less than 10 m in diameter) and the non-uniformity
in the layer number, which make it difficult to control the spintransport properties. Accordingly, a new fabrication method thatenables us the structural tailoring of graphene is indispensablefor graphene-based spintronics.
From this viewpoint, ultrahigh vacuum chemical vapour
deposition can be a promising alternative method because of nosize limitation and structural tunability based on the catalytic
growth on the substrate surface. We focused on the ultrahighvacuum chemical vapour deposition growth of graphene byusing metal thin films as catalytic substrates and hydrocarbon
molecules as precursors [1].

It was successfully demonstrated that single-crystalline
graphene with a controlled layer number can be synthesized byin-situ monitoring the reflection intensity of the electron beamfrom the surface, which reflects the process of the layer-by-layer
growth of graphene on the substrate as shown in Fig. 1. It wasalso found that the electronic uniformity of graphene obtainedby this method could be remarkably high compared to exfoliatedgraphene by controlling the growth condition so that the growthof the respective layers was completed.
This technique for the tailor-made synthesis would facilitate
the development of graphene-based spintronics.
Spin orientation transition across graphene / magnetic metalinterface at atomic layer level
Efficient injection of spin-polarized electrons into graphene is
the most fundamental issue of graphene-based spintronics.
Basically, the spin-injection is performed via the interface
between carbon-based materials and magnetic electrodes, andtherefore the understanding and control of the interface structure
are essential for the device design.
In this study, we first explored the spin-electronic structuresof the interface between graphene and a nickel thin film byapplying the depth-resolved X-ray magnetic circular dichroism
technique with atomic layer resolution. This innovative
approach led to the finding of the abrupt spin orientationtransition across the graphene/nickel interface (2). The spinpolarization direction in the nickel thin film changes from inplane to out-of-plane within the region of several Ni atomiclayers from the interface. The out-of-plane spin-polarization isinduced in monolayer graphene on the interface by contactingwith nickel. Furthermore, the sign of the spin polarization
reverses between the first and the second layer from theinterface in the case of bilayer graphene [3] ( Fig. 2).
The modulation of the spin-electronic structures across thegraphene/magnetic metal interface as found in this study would
be of particular importance not only in controlling the spininjection but also in building up spintronic functions ofgraphene-based devices at the atomic layer level.

Fig. 2 Spin orientation transition across the bilayer graphene/nickel
thin film interface. The arrows indicate the spin direction.

Fig. 1 Reflection intensity change with the benzene exposure on thenickel surface. The growth process of graphene can becontrolled by the exposure amounts. The inset image showsmonolayer graphene obtained under the optimized exposure
condition.

References[1] S. Entani et al., J. Appl. Phys. 111, 064324 (2012).
[2] Y. Matsumoto et al., J. Mater. Chem., in press.
[3] P. V. Avramov et al., J. Appl. Phys. 112, 114303 (2012).