  Realisation 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 injection of spin-polarized electrons from ferromagnetic contacts
via a tunnel barrier, with Al2O3 and MgO 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 ultra-thin tunnel barrier contacts to graphene. We show that
laser-assisted chemical functionalization of graphene with sp3-bonded phenyl
groups effectively provides a seed layer for growth of ultrathin Al2O3 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, 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 new route towards circumventing the
resistance mismatch problem in graphene-based spintronic devices based on the
easily available and highly stable Al2O3, and facilitate a step forward in the
development of their practical applications.
