Role of MgO barriers for spin and charge transport in Co/MgO/graphene nonlocal spin-valve devices

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 k Omega mu 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.