24-μm spin relaxation length in boron nitride encapsulated bilayer graphene

We have performed spin and charge transport measurements in dual gated high mobility bilayer graphene encapsulated in hexagonal boron nitride. Our results show spin relaxation lengths lambda(s) up to 13 mu m at room temperature with relaxation times tau(s) of 2.5 ns. At 4 K, the diffusion coefficient rises up to 0.52 m(2)/s, a value five times higher than the best achieved for graphene spin valves up to date. As a consequence, lambda(s) rises up to 24 mu m with tau(s) as high as 2.9 ns. We characterized three different samples and observed that the spin relaxation times increase with the device length. We explain our results using a model that accounts for the spin relaxation induced by the nonencapsulated outer regions.