Linear scaling between momentum and spin scattering in graphene Jozsa, C.; Maassen, T.; Popinciuc, M.; Zomer, P. J.; Veligura, A.; Jonkman, H. T.; van Wees, B. J.

Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum. In the field of spintronics the "conductivity mismatch" problem remains an important issue. Here the difference between the resistance of ferromagnetic electrodes and a (high resistive) transport channel causes injected spins to be backscattered into the leads and to lose their spin information. We study the effect of the resulting contact-induced spin relaxation on spin transport, in particular on nonlocal Hanle precession measurements. As the Hanle line shape is modified by the contact-induced effects, the fits to Hanle curves can result in incorrectly determined spin transport properties of the transport channel. We quantify this effect that mimics a decrease of the spin relaxation time of the channel reaching more than four orders of magnitude and a minor increase of the diffusion coefficient by less than a factor of two. Then we compare the results to spin transport measurements on graphene from the literature. We further point out guidelines for a Hanle precession fitting procedure that allows the reliable extraction of spin transport properties from measurements.

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Transport measurements on few layer graphene (FLG) are important because they interpolate between the properties of single layer graphene (SLG) as a true 2-dimensional material and the 3-dimensional bulk properties of graphite. In this article we present 4-probe local charge transport and non-local spin valve and spin precession measurements on lateral spin field-effect transistors (FET) on FLG. We study systematically the charge and spin transport properties depending on the number of layers and the electrical back gating of the device. We explain the charge transport measurements by taking the screening of scattering potentials into account and use the results to understand the spin data. The measured samples are between 3 and 20 layers thick, and we include in our analysis our earlier results of the measurements on SLG for comparison. In our room temperature spin transport measurements we manage to observe spin signals over distances up to 10 µm and measure enhanced spin-relaxation times with an increasing number of layers, reaching τs ∼ 500 ps as a maximum, about 4 times higher than in SLG. The increase of τs can result from the screening of scattering potentials due to additional intrinsic charge carriers in FLG. We calculate the density of states (DOS) of FLG using a zone-folding scheme to determine the charge diffusion coefficient DC from the square resistance RS. The resulting DC and the spin-diffusion coefficient DS show similar values and depend only weakly on the number of layers and gate induced charge carriers. We discuss the implications of this on the identification of the spin-relaxation mechanism.

We developed an easy, upscalable process to prepare lateral spin-valve devices on epitaxially grown monolayer graphene on SiC(0001) and perform nonlocal spin transport measurements. We observe the longest spin relaxation times τ S in monolayer graphene, while the spin diffusion coefficient D S is strongly reduced compared to typical results on exfoliated graphene.The increase of τ S is probably related to the changed substrate, while the cause for the small value of D S remains an open question.Spin transport in graphene draws great attention since the observation of spin relaxation lengths of λ S = 2 µm, with spin relaxation times in the order of τ S = 150 ps at room temperature (RT) in