Graphene spintronics: The current experimental status
Bart van Wees, Paul Zomer, Marcos D. Guimaraes, and Niko Tombros
Physics of Nanodevices, Zernike Institute of Advanced materials,
University of Groningen, The Netherlands
b.j.van.wees@rug.nl
Abstract
The first results on graphene spintronics were obtained in 2007 [1,2,3]. It was demonstrated that
electron spins can be injected into a graphene layer using ferromagnetic (tunnel) contacts. With the
so-called non local technique the electron spins can diffuse towards a ferromagnetic detector contact,
and can give rise to an electrical voltage. This voltage only depends on the spin degree of freedom,
since there is no charge current in this part of the device. The presence of spin accumulation at this
contact can be detected by the dependence of this voltage on parallel/antiparallel orientations of the
electrode magnetizations (spin valve effect). Information about the spin relaxation time and spin
diffusion time can also be obtained by the application of a perpendicular magnetic field, leading to the
so called Hanle signal. Record room temperature spin relaxation lengths of 10 micrometer or more
have already been achieved. However, the basic (relaxation) mechanisms which control spin transport
in graphene are still not understood.
Two types of spin relaxation mechanisms can be distinguished [2,3]. The first is the Elliot-Yafet type,
where spin relaxation is induced by momentum scattering. The second is the Dyakonov-Perel
mechanism, where electrons moving in a particular direction experience a so-called spin orbit effective
field, which causes precession of the electron magnetic moment around it. Momentum scattering
changes the direction of this spin-orbit field, and therefore leads to randomization, and slow down of
the precession. These resulting spin relaxation times depend linearly on the momentum scattering
time for the EY mechanism, but are inversely proportional for the DP mechanism.
These two mechanisms can be distinguished because of their difference dependence on the graphene
mobility. This triggered the study of spin relaxation in high quality suspended graphene . However, the
presence of non-suspended low mobility regions limited the spin relaxation length, so only a lower
bound for it could be established [4]. Experiments on spin valve structures made on a crystalline boron
nitride (BN) gate insulator layer substantially increase the spin relaxation time, and in particular spin
relaxation length, reaching values of 7 micrometer at room temperature and even longer at 4.2 K [5].
The use of BN insulator layers also allows to make sandwich devices with both a bottom as well as a
top gate. This allows the study of spin transport in (single) graphene layers as a function of both the
carrier density, as well as the electric field perpendicular to the layer. The latter can be used to tune
the presence of the Rashba effect on the spin relaxation, since this depends on the asymmetry (e.g.
induced by a perpendicular electric field) of the graphene layer. Recent results show indeed that the
spin relaxation times and lengths can be influenced by the applied electric field, and that now record
values of 10 micrometer spin relaxation lengths at room temperature are obtained [6].
We have also studied spin transport in graphene where damage is introduced intentionally due to an
exposure to an hydrogen/argon plasma. As expected the mobility was reduced. However, the spin
relaxation time increased up to values of several ns [7]. Very recently we studied spin transport in few
layer graphene spin valve devices, which have been irradiated with high energy protons. As a result of
the induced defects the mobility was reduced by more than an order of magnitude , but again the spin
relaxation time was found to increase into the nanosecond range. The implications of these
experiments are not yet clear [8].
An interesting development is the modification of spin transport by the interaction between the
graphene and electronic states present in the layers above or below the graphene. We observed that
for graphene grown epitaxially on silicon carbide (SiC), a dramatic change in spin transport occurred,
leading to a narrowing of the Hanle signal by more than one order of magnitude [9]. A tentative
explanation was given, which involved the presence of interface states in the buffer layer in between
the graphene and the SiC substrate [10]. In agreement with this explanation for the anomalous spin
tranport was not observed in epitaxial graphene where this buffer layer was absent. An interesting
connection exists with the recently reported observation of ferromagnetism in hydrogenated epitaxial
graphene [11]. The origin of this room temperature ferromagnetism was tentatively attributed to the
presence of states in the buffer layer, in particular the silicon dangling bond states. This opens up a
possibility for “alll graphene” spintronics, where an entire spin valve structure can be made in a single
graphene layer by selectively making it ferromagnetic.
References
[1] N. Tombros et al., Nature 448, (2007) 571
[2] C. Josza, and B.J. van Wees, Graphene spintronics, in Handbook of Spin Transport and Magnetism,
eds. E.Y. Tsymbal and I. Zutic, CRC Press (2011)
[3] P. Seneor at al., Spintronics with graphene, MRS Bulletin 37 (2012) 1247
[4] M. H. D. Guimaraes , A. Veligura, P.J. Zomer, T. Maassen, I.J. Vera Marun, N. Tombros, and B.J. van
wees, Nano Lett. 12 (7) (2012) 3512
[5] P.J. Zomer, M.H.D. Guimaraes, N. Tombros, and B.J. van Wees, Phys. Rev. B86 (2012) 161416
[6] M.H. D. Guimaraes et al., in preparation.
[7] M. Wojtaszek et al., Phys. Rev. B87 (2013) 081402
[8] I.J. Vera Marun, in preparation.
[9] T. Maassen et al., Nano Lett. 12 (3) (2012) 1498
[10] T. Maassen et al., Phys. Rev. Lett. 100 (2013) 067209
[11] A.J.M. Giesbers et al. , Phys. Rev. Lett. 111 (2013) 166101