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Spin Transport In Graphene-hBN Heterostructures In Inverted Non-local Spin Valve Devices

M. Drögeler, C. Stampfer, T. Schäpers
Published 2017 · Materials Science

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In this thesis a novel fabrication technique for graphene non-local spin valves is presented. This technique allows high charge carrier mobilities of 20,000cm2/(Vs), spin diffusion length of more than 30 μm and spin lifetimes of more than 12ns at room temperature. In contrast to previous studies the spin lifetime is a factor of six larger. Here, the Co/MgO electrodes are fabricated first on a Si/SiO2 substrate and subsequently an exfoliated graphene flake is transferred on top of the electrodes using hexagonal boron nitride (hBN). This may lead to a more homogeneous interface compared to old fabrication methods where electrodes were grown on the inert graphene surface. First, we investigate the homogeneity of the MgO layer by conductive scanning force microscopy prior to the graphene deposition. We find that the layer is homogeneous but exhibits conducting pinholes which lower the spin injection and detection efficiency. Moreover, we investigate the spatial variation of doping and strain of the graphene flake by confocal Raman spectroscopy. We find that some regions of the device are quasi suspended between the electrodes and some are in contact to the substrate which dopes the graphene and induces uniaxial strain in the graphene flake. However, the spin transport properties are not influenced by these variations. On the other hand, we observe that the spin transport properties are deteriorated if small hBN flakes are used as solvents and dissolved polymers can get in contact to the graphene flake. Additionally, we find that the charge carrier density dependence of the spin lifetime behaves differently at low temperatures for single and bilayer graphene. While there is no change for single layer in comparison to room temperature there is an enhanced spin dephasing near the charge neutrality point for bilayer graphene. This observation shines new light on the underlying spin dephasing mechanism in graphene. Additionally, the new fabrication technique was applied to chemically synthesized graphene. Despite some difficulties in the fabrication process it shows similar spin transport properties as exfoliated graphene which is important for technological applications and mass fabrication. Finally, simulations of the device structures demonstrate the effects of sample inhomogeneities on the spin transport properties. We show that regions with an enhanced spin dephasing always influence the measurements even if these regions are only adjacent to the actually measured one. This is important for future electrode design and employed materials.
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