Research Group for Molecular Spintronics
Group Leader: Seiji Sakai
Members: Shiro Entani, Yoshihiro Matsumoto, Takuya Hattori, Shintaro Yotsuya, Hiroshi Naramoto

Spintronics is an emerging technology taking advantage of thedual property of electrons; the charge and spin degrees offreedom. Recent studies have started to shed light on thespintronics applications of molecular materials includingorganic molecules and nanocarbons. In the molecularspintronics, the effective controls of spin and electronic
information via the magnetic heterostructures and interfaces areimportant for achieving the spintronic applications. Our group
aimed at elucidating the interface-related properties of themagnetic heterostructures composed of graphene and its relatedmaterials as the fundamental component of the graphene-based
spintronic devices.
Contact-induced Spin Polarization in Monolayer Hexagonal
Boron Nitride/Ferromagnetic Metal Heterostructure
Hexagonal boron nitride (h-BN), an isomorph of graphene, isbecoming the focus of attention as an insulating material forgraphene spintronics. It was recently demonstrated thatelectrical spins can be injected into graphene by using
monolayer h-BN as an ultra-thin tunneling barrier between
graphene and a ferromagnetic metal electrode [1]. We
successfully clarified the spin-dependent electronic structures ofthe monolayer h-BN contacted with a ferromagnetic Ni(111) byemploying spin-polarized metastable-atom deexcitation
spectroscopy (SPMDS) with spin-polarized metastable helium
atom (He*) as a surface spin detector [2].
A monolayer h-BN/Ni(111) heterostrucuture was synthesizedby chemically depositing h-BN on the Ni(111) surface using
borazine as precursor. The evolution of the surface electronicstructure with the growth of h-BN was in-situ observed in therange of the h-BN coverage (θ) from θ = 0 to 1 by measuring
the kinetic energy distribution of ejected electrons in the spindependent He* deexcitation process on the sample surface.
Figure 1 shows the spin asymmetry spectra representing thespin-dependent change of the kinetic energy distribution of theejected electrons. For the Ni(111) surface (θ = 0), the spin
asymmetry of 9% was observed around the Fermi level (~14eV). According to the principle of SPMDS, a positive (negative)spin asymmetry indicates a negative (positive) spin polarizationof the surface electrons. Therefore, the positive spin asymmetryat θ = 0 shows negative spin polarization of the Ni(111) surface.

states with the negative asymmetry appeared and increased withincreasing the h-BN coverage-fraction. Under the fullcoverage (θ = 1), the asymmetry became as large as − 8%,suggesting large positive spin polarization of h-BN. The
positively spin-polarized states are attributed to the metallic-like
in-gap states of h-BN generated by the π-d hybridization.
The present result clarified that, in the h-BN/Ni(111)heterostructure, insulating h-BN turns to be metallic-like andpositively spin-polarized in contrast to the negative spinpolarization of Ni(111). The present study demonstrated that theapplication of h-BN in combination with graphene could offer apromising way to optimize the operation of graphene-based
spintronic devices.
Strong Physical Interface Interactions in Graphene/Sapphire
Heterostructure
The large area growth of graphene on insulating substrate isone of the central issues to be addressed for the development ofelectronic and spintronic applications. In graphene/insulator
heterostructures, the charge and spin transport properties ofgraphene could be affected by the interactions between
graphene and insulators at the interface. In this study, theinterface interaction in the monolayer graphene/sapphire(0001)
heterostructure was assessed by using the normal-incidence Xray standing wave (NIXSW) spectroscopy [3].
We demonstrated that unusually strong physical interaction
exists between graphene and sapphire from the small interlayer
distance between the graphene and the sapphire surface (Fig. 2).
The interfacial interaction is shown to be dominated by theelectrostatic forces involved in the graphene π-system and theunsaturated electrons of the topmost O layer of sapphire rather
than the van der Waals interaction. Such interaction causes
graphene the p-type doping and enables the graphene to easily
slide on the sapphire surface despite the small interlayer
distance with the energetic stability.
The present study provides a basis for manufacturing highperformance electronic and spintronic devices.

Fig. 2 Atomic structure of the monolayer graphene/sapphire(0001)
heteorstructure elucidated with NIXSW spectroscopy. The crosssectional view at the left and the top view at the right. C, O and Al
atoms are marked by black, red and pink colors, respectively.

Fig. 1 Spin asymmetry obtained from the monolayer h-BN/Ni(111)heterostructure at different h-BN coverages (θ= 0, 0.14, 0.7 and 1).

The h-BN growth resulted in a significant change in the spin
asymmetry in the region around the Fermi level (~16 eV). New

References[1] T. Yamaguchi et al., Appl. Phys. Express 6, 073001 (2013).
[2] M. Ohtomo et al., Appl. Phys. Lett. 104, 051604 (2014).
[3] S. Entani et al., Nano Research 8, 1535 (2015).