We investigate spin and charge transport in both single and bilayer graphene nonlocal spin-valve devices. An inverse dependence of the spin lifetime ${\tau }_s$ on the carrier mobility $\mu$ is observed in devices with large contact-resistance-area products ($R_{c}A>1\mathrm{k}\Omega \mu {\mathrm{m}}^2$). Furthermore, we observe an increase of ${\tau }_s$ with increasing $R_{c}A$ values, demonstrating that spin transport is limited by spin dephasing underneath the electrodes. In charge transport, we measure a second contact-induced Dirac peak at negative gate voltages in devices with larger $R_{c}A$ values, demonstrating different transport properties in contact-covered and bare graphene parts. We argue that the existence of the second Dirac peak complicates the analysis of the carrier mobilities and the spin scattering mechanisms.

• Received 28 May 2013

DOI:https://doi.org/10.1103/PhysRevB.88.161405