Role of MgO barriers for spin and charge transport in Co/MgO/graphene nonlocal spin-valve devices

Abstract

We investigate spin and charge transport in both single and bilayer graphene non-local spin-valve devices. An inverse dependence of the spin lifetime τs on the carrier mobility µ is observed in devices with large contact resistance area products (RcA > 1 kΩµm 2 ). Furthermore, we observe an increase of τs with increasing RcA values demonstrating that spin transport is limited by spin dephasing underneath the electrodes. In charge transport, we measure a second contact-induced Dirac peak at negative gate voltages in devices with larger RcA values demonstrating different transport properties in contact covered and bare graphene parts. We argue that the existence of the second Dirac peak complicates the analysis of the carrier mobilities and the spin scattering mechanisms.Graphene has drawn strong attention because of measured spin diffusion length of some µm at room temperature. While most spin transport devices only exhibit spin lifetimes up to several hundred picoseconds at room temperature [1][2][3][4][5][6][7][8][9] there are only few reports with spin lifetimes above one nanosecond.10-12 Nevertheless, all experimental values of the spin lifetimes are some orders of magnitude shorter than theoretically predicted 13,14 indicating that in present devices spin transport is limited by extrinsic sources of spin scattering. These include spin-orbit coupling by adatoms, edge effects and ripples.6,10,13,15-18 Additionally, spin scattering may result from the underlying substrate or the spin injection and detection contacts. 14,19,20 The importance of the latter might be indicated by recent electron spin resonance (ESR) experiments on graphene nanoribbons and small flakes that were only weakly coupled to the substrate and had no electrodes.21,22 Interestingly, the measured spin lifetimes of localized spin states are at least 200 ns while the estimated spin lifetimes of conduction electrons are 30 ns, which is larger than any reported values from electrical Hanle spin precession measurements.In this Rapid Communication, we investigate the influence of MgO barriers on spin and charge transport properties by fabricating both single layer (SLG) and bilayer graphene (BLG) non-local spin-valve devices with variable contact resistance area products R c A of the MgO/Co electrodes. We explore the relationship between spin lifetime τ s and charge carrier mobility µ in SLG and find a similar 1/µ dependence as seen in previous spin transport studies on exfoliated bilayer graphene (BLG) devices.12 This dependence is only seen in samples with R c A > 1 kΩµm 2 . In fact, we observe that devices with long τ s additionally exhibit a second Dirac peak in charge transport, which stems from the electrodes. This contact-induced Dirac peak overlaps with the Dirac peak of the bare graphene which complicates the analysis of the carrier mobility and thus complicates a clear assignment of the dominant spin scattering mechanism in graphene. For devices with low R c A we find an overall strong decrease of τ s showing that transparent contacts yie...

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