  Graphene grown by chemical vapor deposition (CVD) is the most promising
material for industrial-scale applications based on graphene monolayers. It
also holds promise for spintronics; despite being polycrystalline, spin
transport in CVD graphene has been measured over lengths up to 30 $\mu$m, which
is on par with the best measurements made in single-crystal graphene. These
results suggest that grain boundaries (GBs) in CVD graphene, while impeding
charge transport, may have little effect on spin transport. However, to date
very little is known about the true impact of disordered networks of GBs on
spin relaxation. Here, by using first-principles simulations, we derive an
effective tight-binding model of graphene GBs in the presence of spin-orbit
coupling (SOC), which we then use to evaluate spin transport in realistic
morphologies of polycrystalline graphene. The spin diffusion length is found to
be independent of the grain size, and is determined only by the strength of the
substrate-induced SOC. This result is consistent with the D'yakonov-Perel'
mechanism of spin relaxation in the diffusive regime, but we find that it also
holds in the presence of quantum interference. These results clarify the role
played by GBs and demonstrate that the average grain size does not dictate the
upper limit for spin transport in CVD-grown graphene, a result of fundamental
importance for optimizing large-scale graphene-based spintronic devices.

Mesoscale and Nanoscale Physics