Research Group for Molecular Spintronics
Group Leader: Seiji Sakai
Members: Yoshihito Matsumoto, Shiro Entani, Manabu Ohtomo, Pavel V. Avramov

Precise layer number control of graphene in ultra-highvacuum chemical vapour deposition
After the discovery of the convenient fabrication method by theexfoliation from graphite (A. Geim and K. Novoselov, the 2010Nobel Prize in physics), graphene has been attracting world-wide
attention as an innovative electronic material. In the spintronics,
graphene is expected to be an ideal spin-transport material due tothe extremely long spin diffusion length and high carrier mobility.
However, not only the non-uniformity in the carbon layer
number and in the electronic states but also the polycrystalline
nature of exfoliated graphene are making it impossible to control
the spin transport properties in graphene. It can be said that a
new fabrication method which enables tailoring of graphene isnecessary for the development of spintronic applications.
From the above view point, ultrahigh vacuum chemical
vapor deposition (UHV-CVD) can be an alternative fabrication
method, because single-layer graphene is known to beepitaxially grown on the catalytic metal crystal. In UHV-CVD,graphene grows through the dissociation and polymerization ofhydrocarbon precursors on the metal surfaces.
In this study [1], we focused on a UHV-CVD growth of
microstructure-controlled graphene by using epitaxial metal thinfilms as a catalytic substrate and with in-situ spectroscopy. Itwas successfully demonstrated that single-crystalline graphene(single-layer and bilayer graphene) with the same number ofcarbon layer over the entire area can be obtained by precise
control of the exposure amount of precursors. In addition, it wassuggested that the uniformity of the electronic states in UHVCVD graphene can be remarkably high compared to exfoliatedgraphene (see the Research Highlight for additional details).
Our findings would enable controls of spin transport
properties of graphene and lead to the development of graphenebased spintronics and nano-electronics.

Very high interface spin polarization and its mechanism infullerene-magnetic metal system
Generation of high spin polarization of conduction electrons
at the interface between the magnetic electrode and moleculartransport medium is essential for efficient spin injection. After1988, magnetic metal crystals have been commonly used fora magnetic electrode in spintronic devices. However, it has
been pointed out that much higher spin polarization than thatin magnetic metal crystals or an appropriate barrier material isnecessary to realize high spin injection efficiency overcoming
the conductivity mismatch problem that can limit spinpolarization of injected electrons into a molecular medium [2].
In this study, it was demonstrated that new compounds offullerene and magnetic metal (e.g., C 60-Co compound, seeFig. 1) can be a promising barrier material which gives rise tonearly-complete spin polarization of conduction electrons at theinterface with magnetic metal crystals. In order to elucidate themagnitude of spin polarization at the interface with the magnetic
metal crystal, the characteristics of tunnelling current andmagnetoresistance were analysed for the C60-Co films consistingof a C60-Co compound and a small amount of Co nanocrystals[3, 4]. It was revealed that spin polarization increases as high as
80% at the C60-Co compound/Co crystal interface in comparison
with that in Co crystal (30%). Our theoretical analysis [5]based on the X-ray absorption and magnetic circular dichroism
spectroscopy indicated that the C60-Co compound can work as aspin filtering barrier due to the exchange splitting in the LUMO
region as represented in Fig. 1.

Group Activities

Spintronics is an emerging technology taking advantage ofthe dual freedom of both charge and spin degrees. Since thediscovery of the giant magnetoresistance of metal multilayers in1988 (A. Fert and P. Grünberg, the 2007 Nobel Prize in Physics),
spintronics has been developed based on inorganic substances
like metals and semiconductors. Recent studies have started
to shed light on spintronic applications of molecular materials
including organic molecules and nanocarbons in which the spinsof conduction electrons can be preserved for a long time anddistance. We call this new field “Molecular spintroncis”.
In molecular spintronics, efficient control of spin transportprocesses (spin injection, manipulation and detection) in molecularmaterials are of special importance to realize spintronic applications.
Our group aims at elucidating the advantages of molecular materials
as innovative spintronic materials by exploring novel systems
for efficient spin transport control based on the hybrid systems ofmolecular materials and magnetic metals and also by developing
cutting-edge spectroscopy techniques for the electronic and magnetic
states of the molecular material-based structures and interfaces therein.
Typical progresses in FY2011 are summarized in the following.

Fig. 1 Atomic structure of the C60-Co compound (left) and aschematic view of the spin filtering effect of the C60-Co
compound as a tunnelling barrier for spin injection into a
molecular medium (right) [5, 6]. LUMO/HOMO and up/downarrows in the right figure represent the lowest unoccupied/highest occupied molecular orbital and up/down spin,respectively. The HOMO-LUMO (band) gaps in the C60-Co
compound are smaller for the spin-down electrons (0.8 eV)than that for the spin-up electrons (1.9 eV), which causes spin
filtering.
References[1] S. Entani et al., J. Appl. Phys. 111, 064324 (2012).
[2] G. Schmidt et al., Phys Rev. B 62, R4790 (2000).
[3] S. Sakai et al., Phys. Rev. B 83, 174422 (2011).
[4] S. Sakai et al., J. Magn. Magn. Mater. 324, 1970 (2012).
[5] I. Hojo et al., J. Electro. Spec. Relat. Phenom. 185, 32 (2012).
[6] A. Nakajima et al., J. Phys. Chem. A 104, 176 (2000).

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