Long spin lifetimes in graphene spin valves

Graphene is an attractive material for spintronics due to the possibility of long spin lifetimes arising from low intrinsic spin-orbit coupling. However, the longest spin lifetimes measured in single layer graphene (SLG) spin valves are ~500 ps for devices with tunneling contacts and less than 200 ps otherwise. These spin lifetimes are much shorter than theoretical prediction and have motivated intense theoretical investigations on the possible extrinsic sources of spin scattering in graphene. Here, we observe spin lifetimes as long as ~1 ns in SLG and ~6 ns in bilayer graphene (BLG) with tunneling contacts, which is the highest reported value in a graphene spin valve. By studying the spin lifetime and diffusion coefficient in SLG and BLG as a function of temperature and carrier concentration, we observe sharply contrasting behaviors between the two systems. For SLG, the momentum scattering (e.g. impurity scattering) produces the dominant spin relaxation at low temperatures (<=100 K). For BLG, more complicated behavior indicates the relevance of multiple sources of spin relaxation. These results indicate that spin lifetime could potentially become very long in graphene for spintronic applications in logic and storage.