  We present an experimental study of spin transport in single layer graphene
using atomic sheets of hexagonal boron nitride (h-BN) as a tunnel barrier for
spin injection. While h-BN is expected to be favorable for spin injection,
previous experimental studies have been unable to achieve spin relaxation times
in the nanosecond regime, suggesting potential problems originating from the
contacts. Here, we investigate spin relaxation in graphene spin valves with
h-BN barriers and observe room temperature spin lifetimes in excess of a
nanosecond, which provides experimental confirmation that h-BN is indeed a good
barrier material for spin injection into graphene. By carrying out measurements
with different thicknesses of h-BN, we show that few layer h-BN is a better
choice than monolayer for achieving high non-local spin signals and longer spin
relaxation times in graphene.
