The realization of practical spintronic devices relies on the ability to create and detect pure spin currents. In graphene-based spin valves, this is usually achieved by the injection of spin-polarized electrons from ferromagnetic contacts via a tunnel barrier, with ${\mathrm{Al}}_2{\mathrm{O}}_3$ and $\mathrm{Mg}\mathrm{O}$ used most widely as barrier materials. However, the requirement to make these barriers sufficiently thin often leads to pinholes and low contact resistances, which, in turn, results in low spin-injection efficiencies, typically 5% at room temperature, due to the so-called resistance mismatch problem. Here, we demonstrate an alternative approach to fabricate ultrathin tunnel-barrier contacts to graphene. We show that laser-assisted chemical functionalization of graphene with $s{p}^3$-bonded phenyl groups effectively provides a seed layer for the growth of ultrathin ${\mathrm{Al}}_2{\mathrm{O}}_3$ films, ensuring smooth high-quality tunnel barriers and an enhanced spin-injection efficiency. Importantly, the effect of functionalization on spin transport in the graphene channel itself is relatively weak, so that the enhanced spin injection dominates and leads to an order of magnitude increase in spin signals. Furthermore, the spatial control of functionalization using a focused laser beam and lithographic techniques can, in principle, be used to limit functionalization to contact areas only, further reducing the effect on the graphene channel. Our results open a route towards circumventing the resistance mismatch problem in graphene-based spintronic devices based on easily available and highly stable ${\mathrm{Al}}_2{\mathrm{O}}_3$ and facilitate a step forward in the development of their practical applications.

• Received 15 January 2021
• Revised 1 April 2021
• Accepted 16 April 2021

DOI:https://doi.org/10.1103/PhysRevApplied.15.054018