# Universal spin diffusion length in polycrystalline graphene

## TL;DR

This study shows that in polycrystalline graphene, the spin diffusion length is unaffected by grain size and is solely determined by substrate-induced spin-orbit coupling, which is promising for large-scale spintronic applications.

## Contribution

The paper introduces a first-principles derived tight-binding model to evaluate spin transport in polycrystalline graphene with grain boundaries, revealing the independence of spin diffusion length from grain size.

## Key findings

- Spin diffusion length is independent of grain size.
- Spin relaxation is governed by substrate-induced spin-orbit coupling.
- Results support the D'yakonov-Perel' mechanism in the diffusive regime.

## Abstract

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.