Graphene stands out as a magnificent material for spin transport, exhibiting long spin diffusion lengths even at room temperature. Studies are performed on graphene spin valve devices, which consist of a monolayer or few-layer graphene sheet contacted by ferromagnetic electrodes for injecting and detecting spin currents and nonmagnetic reference electrodes. Shown to the right is an optical micrograph of a typical graphene spin valve. Our group has made important contributions to the study of graphene spin valves.
Tunneling Spin Injection Into Graphene
Our group demonstrated the first tunneling spin injection across MgO barriers (Han et al.) leading to efficient spin injection and enhanced spin lifetimes. 
The use of hexagonal boron nitride (hBN) for tunnel barriers (Singh et al.) demonstrates spin-preserving tunneling across vdW layers. We also found that SrO offers robust barriers that can operate at high biases, leading to record high spin accumulation in graphene (Singh et al.). In the figure to the right, a graphene spin valve consists of four electrodes, where the left pair is used for spin injection from a ferromagnetic cobalt electrode across a SrO barrier and the right pair is used for spin detection. An applied magnetic field is used to switch the Co magnetizations to achieve either the parallel or antiparallel magnetization alignments of the Co spin injector and Co spin detector electrodes. The resulting switching in the spin voltage Vs vs. magnetic field B indicates the presence of spin currents from the injector to the detector. The shown data represents the largest DC spin signals observed to date.
Graphene as an Ultrasensitive Magnetometer
We found that graphene spin valves can act as ultrasensitive magnetometers, where spin currents were used to detect the formation of magnetic moments when hydrogen atoms adsorb to the surface of graphene. As first seen in monolayer graphene (McCreary et al.) and later in bilayer graphene (Katoch et al.), the presence of the characteristic “dip” in the spin valve signal was the tell-tale sign of magnetic moment formation. As shown to the right, the “dip” feature emerges when a graphene spin valve is exposed to atomic hydrogen at low temperatures (and inside an ultrahigh vacuum chamber). The red curve comes from a theoretical model based on the following idea. The hydrogen induces localized fluctuating paramagnetic moments in the graphene. When these moments interact with the spin currents, they will depolarize the spin current and lower the spin voltage. This interaction is strongest at zero field and becomes weaker at larger magnetic field when the DC magnetic field exceeds the effective fluctuating field generated by the paramagnetic moments. This explains why the magnetic moments produce a dip at zero field.
Graphene Spin Transport in Heterostructures
The study of heterostructures is one of the most interesting directions for 2D spintronics. We found that placing graphene on magnetic insulator YIG produces proximity exchange fields that can fully modulate the spin current (Singh et al.). When building a heterostructure of graphene and MoS2, we demonstrated optical spin injection into graphene through helicity-dependent optical spin injection into the MoS2 and subsequent spin transfer into the graphene (see Optical Spin and Valley Excitation). We also found that encapulsating bilayer graphene in hBN and applying top and bottom gates led to the electric field tuning of the spin-orbit coupling (Xu et al.). The key data are shown to the right.
Interestingly, a perpendicular electric field opens a gap in bilayer graphene to expose the spin-valley coupled band structure at the band edge. This produces a perpendicular spin-orbit field to enhance the spin lifetime for perpendicular spins while reducing the spin lifetime for in-plane spins. This produces a gate-tunable spin filter that can selectively transmit the perpendicular component of spin. As shown in the figure, the ratio of perpendicular spin lifetime to in-plane spin lifetime is enhanced with electric field when the Fermi level lies within the gap (i.e. at n ~ 0).
Future studies in van der Waals heterostructures could take advantage of magnetic proximity effects, spin-orbit proximity effects, optical excitation, spin injection, and twist-angle dependence to realize interesting spintronic and optospintronic behavior.
Science | 2D Materials: Spintronics, Magnetism, and Photonics | Spin Transport in Graphene