The isolation of graphene has triggered an avalanche of studies into the spin-dependent physical properties of this material, as well as graphene-based spintronic devices Here we review the experimental and theoretical state-of-art concerning spin injection and transport, defect-induced magnetic moments, spin-orbit coupling and spin relaxation in graphene Future research in graphene spintronics will need to address the development of applications such as spin transistors and spin logic devices, as well as exotic physical properties including topological states and proximity-induced phenomena in graphene and other 2D materials

Graphene Spintronics

Graphene and topological insulator two-dimensional electron systems are described by massless Dirac equations. Although the two systems have similar Hamiltonians, they are polar opposites in terms of spin–orbit coupling strength. The status of efforts to achieve long spin-relaxation times in weakly spin–orbit-coupled graphene, and large current-induced spin-polarizations in strongly spin–orbit-coupled topological insulator surface states are reviewed in this Progress Article.

Spintronics and pseudospintronics in graphene and topological insulators.

We investigate spin relaxation in graphene spin valves and observe strongly contrasting behavior for single-layer graphene (SLG) and bilayer graphene (BLG). In SLG, the spin lifetime (τ(s)) varies linearly with the momentum scattering time (τ(p)) as carrier concentration is varied, indicating the dominance of Elliot-Yafet (EY) spin relaxation at low temperatures. In BLG, τ(s) and τ(p) exhibit an inverse dependence, which indicates the dominance of Dyakonov-Perel spin relaxation at low temperatures. The different behavior is due to enhanced screening and/or reduced surface sensitivity of BLG, which greatly reduces the impurity-induced EY spin relaxation.

Spin relaxation in single-layer and bilayer graphene

Graphene may be set to revolutionize electronics, but its small spin–orbit coupling limits its potential in spintronics. It is now shown, however, that adding hydrogen atoms can greatly enhance the magnetic properties of graphene. This then enabled the observation of the spin Hall effect, essential for controlling spin currents. Graphene’s extremely small intrinsic spin–orbit (SO) interaction1 makes the realization of many interesting phenomena such as topological/quantum spin Hall states2,3 and the spin Hall effect4 (SHE) practically impossible. Recently, it was predicted1,5,6,7 that the introduction of adatoms in graphene would enhance the SO interaction by the conversion of sp2 to sp3 bonds. However, introducing adatoms and yet keeping graphene metallic, that is, without creating electronic (Anderson) localization8, is experimentally challenging. Here, we show that the controlled addition of small amounts of covalently bonded hydrogen atoms is sufficient to induce a colossal enhancement of the SO interaction by three orders of magnitude. This results in a SHE at zero external magnetic fields at room temperature, with non-local spin signals up to 100 Ω; orders of magnitude larger than in metals9. The non-local SHE is, further, directly confirmed by Larmor spin-precession measurements. From this and the length dependence of the non-local signal we extract a spin relaxation length of ∼1 μm, a spin relaxation time of ∼ 90 ps and a SO strength of 2.5 meV.

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Colossal enhancement of spin-orbit coupling in weakly hydrogenated graphene

Controlled spin transport in graphene and other two-dimensional materials has become increasingly promising for applications in devices. Of particular interest are custom-tailored heterostructures, known as van der Waals heterostructures, that consist of stacks of two-dimensional materials in a precisely controlled order. This Colloquium gives an overview of this developing field of spintronics and outlines the experimental and theoretical state of the art.

Colloquium: Spintronics in graphene and other two-dimensional materials

We report on the first systematic study of spin transport in bilayer graphene (BLG) as a function of mobility, minimum conductivity, charge density, and temperature. The spin-relaxation time τ(s) scales inversely with the mobility μ of BLG samples both at room temperature (RT) and at low temperature (LT). This indicates the importance of D'yakonov-Perel' spin scattering in BLG. Spin-relaxation times of up to 2 ns at RT are observed in samples with the lowest mobility. These times are an order of magnitude longer than any values previously reported for single-layer graphene (SLG). We discuss the role of intrinsic and extrinsic factors that could lead to the dominance of D'yakonov-Perel' spin scattering in BLG. In comparison to SLG, significant changes in the carrier density dependence of τ(s) are observed as a function of temperature.

J. Samm

Papers

Observation of long spin-relaxation times in bilayer graphene at room temperature.