Phys. Rev. B 90, 165403 (2014)
By successive oxygen treatments of graphene nonlocal spin-valve devices we achieve a gradual increase of the contact-resistance–area products (R_cA) of Co/MgO spin injection and detection electrodes and a transition from linear to nonlinear characteristics in the respective differential dV−dI curves. With this manipulation of the contacts, both spin lifetime and the amplitude of the spin signal can significantly be increased by a factor of seven in the same device. This demonstrates that contact-induced spin dephasing is the bottleneck for spin transport in graphene devices with small R_cA values. With increasing R_cA values, we furthermore observe the appearance of a second charge neutrality point (CNP) in gate-dependent resistance measurements. Simultaneously, we observe a decrease of the gate voltage separation between the two CNPs. The strong enhancement of the spin-transport properties as well as the changes in charge transport are explained by a gradual suppression of a Co-graphene interaction by improving the oxide barrier during oxygen treatment.