Controlling spin relaxation in hexagonal BN-encapsulated graphene with a transverse electric field.

We experimentally study the electronic spin transport in hexagonal BN encapsulated single layer graphene nonlocal spin valves. The use of top and bottom gates allows us to control the carrier density and the electric field independently. The spin relaxation times in our devices range up to 2 ns with spin relaxation lengths exceeding 12 µm even at room temperature. We obtain that the ratio of the spin relaxation time for spins pointing out-of-plane to spins in-plane is τ(⊥)/τ(||) ≈ 0.75 for zero applied perpendicular electric field. By tuning the electric field, this anisotropy changes to ≈ 0.65 at 0.7 V/nm, in agreement with an electric field tunable in-plane Rashba spin-orbit coupling.

Spin-dependent quantum interference in nonlocal graphene spin valves.

Up to date, all spin transport experiments on graphene were done in a semiclassical regime, disregarding quantum transport properties such as phase coherence and interference. Here we show that in a quantum coherent graphene nanostructure the nonlocal voltage is strongly modulated. Using nonlocal measurements, we separate the signal in spin-dependent and spin-independent contributions. We show that the spin-dependent contribution is about 2 orders of magnitude larger than the spin-independent one, when corrected for the finite polarization of the electrodes. The nonlocal spin signal is not only strongly modulated but also changes polarity as a function of the applied gate voltage. By locally tuning the carrier density in the constriction via a side gate electrode we show that the constriction plays a major role in this effect. Our results show the potential of quantum coherent graphene nanostructures for the use in future spintronic devices.

Spin transport in high-quality suspended graphene devices.

We measure spin transport in high mobility suspended graphene (µ ≈ 10(5)cm(2)/(V s)), obtaining a (spin) diffusion coefficient of 0.1 m(2)/s and giving a lower bound on the spin relaxation time (τ(s) ≈ 150 ps) and spin relaxation length (λ(s) = 4.7 µm) for intrinsic graphene. We develop a theoretical model considering the different graphene regions of our devices that explains our experimental data.