Comparison between charge and spin transport in few-layer graphene

Abstract

Transport measurements on few layer graphene (FLG) are important because they interpolate between the properties of single layer graphene (SLG) as a true 2-dimensional material and the 3-dimensional bulk properties of graphite. In this article we present 4-probe local charge transport and non-local spin valve and spin precession measurements on lateral spin field-effect transistors (FET) on FLG. We study systematically the charge and spin transport properties depending on the number of layers and the electrical back gating of the device. We explain the charge transport measurements by taking the screening of scattering potentials into account and use the results to understand the spin data. The measured samples are between 3 and 20 layers thick, and we include in our analysis our earlier results of the measurements on SLG for comparison. In our room temperature spin transport measurements we manage to observe spin signals over distances up to 10 µm and measure enhanced spin-relaxation times with an increasing number of layers, reaching τs ∼ 500 ps as a maximum, about 4 times higher than in SLG. The increase of τs can result from the screening of scattering potentials due to additional intrinsic charge carriers in FLG. We calculate the density of states (DOS) of FLG using a zone-folding scheme to determine the charge diffusion coefficient DC from the square resistance RS. The resulting DC and the spin-diffusion coefficient DS show similar values and depend only weakly on the number of layers and gate induced charge carriers. We discuss the implications of this on the identification of the spin-relaxation mechanism.

Spin Relaxation in Single-Layer Graphene with Tunable Mobility

“…Recent experiments on few layer graphene also find a variety of behaviors, with some groups reporting EY-like behavior 28 and other groups reporting DP-like behavior 14,29 . Therefore, it becomes increasingly important to develop systematic experimental methods to isolate microscopic mechanisms that could contribute to spin relaxation.…”

Nanosecond Spin Lifetimes in Single- and Few-Layer Graphene–hBN Heterostructures at Room Temperature

“…Typical non-local spin-valve devices on Si/SiO 2 substrates with charge carrier mobilities of several thousand cm 2 /Vs exhibit spin lifetimes below 1 ns. [1][2][3][4][5][6][7][8][9][10] In contrast, room temperature spin lifetimes above 1 ns have only been observed for epitaxial graphene on SiC 11 and for bilayer graphene devices (BLG) with low carrier mobility of 300 cm 2 /Vs (Ref. 2) or after post-processing of as-fabricated devices either by hydrogenation 12 or by oxygen treatment 13 which both, however, result in a decrease of the mobility.…”

Contact-induced spin relaxation in Hanle spin precession measurements

“…Here, the long spin relaxation lengths of several μm measured at room temperature are already promising 5 but still stay behind the theoretical prospects based on the high mobilities combined with weak spin orbit coupling and low hyperfine interactions. 6 While some research aims to understand the spin relaxation mechanism in graphene [7][8][9][10][11][12] and to understand the influence of the direct environment of the graphene transport channel, [13][14][15][16][17][18] the conductivity mismatch can play an important role in the origin of spin relaxation in the measured devices. To prevent this mismatch, high resistive barriers between the contacts and the graphene channel are included.…”