Transport measurements on few-layer graphene (FLG) are important because they interpolate between the properties of single-layer graphene (SLG) as a true two-dimensional material and the three-dimensional bulk properties of graphite. In this article we present four-probe local charge transport and nonlocal spin-valve and spin-precession measurements on lateral spin field-effect transistors 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 $\mathrm{\mu }\mathrm{m}$ and measure enhanced spin-relaxation times with an increasing number of layers, reaching ${\tau }_s~500$ ps as a maximum, about $4$ times higher than in SLG. The increase of ${\tau }_s$ can result from the screening of scattering potentials due to additional intrinsic charge carriers in FLG. We calculate the density of states of FLG using a zone-folding scheme to determine the charge-diffusion coefficient $D_C$ from the square resistance $R_S$. The resulting $D_C$ and the spin-diffusion coefficient $D_S$ 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.

• Received 2 December 2010

DOI:https://doi.org/10.1103/PhysRevB.83.115410