Electronic spin transport in dual-gated bilayer graphene

The elimination of extrinsic sources of spin relaxation is key to realizing the exceptional intrinsic spin transport performance of graphene. Toward this, we study charge and spin transport in bilayer graphene-based spin valve devices fabricated in a new device architecture that allows us to make a comparative study by separately investigating the roles of the substrate and polymer residues on spin relaxation. First, the comparison between spin valves fabricated on SiO2 and BN substrates suggests that substrate-related charged impurities, phonons and roughness do not limit the spin transport in current devices. Next, the observation of a fivefold enhancement in the spin-relaxation time of the encapsulated device highlights the significance of polymer residues on spin relaxation. We observe a spin-relaxation length of ~10 μm in the encapsulated bilayer, with a charge mobility of 24 000 cm2 Vs−1. The carrier density dependence on the spin-relaxation time has two distinct regimes; n<4 × 1012 cm−2, where the spin-relaxation time decreases monotonically as the carrier concentration increases, and n ⩾ 4 × 1012 cm−2, where the spin-relaxation time exhibits a sudden increase. The sudden increase in the spin-relaxation time with no corresponding signature in the charge transport suggests the presence of a magnetic resonance close to the charge neutrality point. We also demonstrate, for the first time, spin transport across bipolar p–n junctions in our dual-gated device architecture that fully integrates a sequence of encapsulated regions in its design. At low temperatures, strong suppression of the spin signal was observed while a transport gap was induced, which is interpreted as a novel manifestation of the impedance mismatch within the spin channel. An analysis of charge and spin transport in bilayer graphene-based spin valve devices reveals strategies to improve their performance. Graphene has great potential for spintronics, but is limited by imperfections in the material that arise from the supporting substrate or the fabrication process lead to spin relaxation. Barbaros Özyilmaz from the National University of Singapore and colleagues have demonstrated a design that maintains the information carried by the electrons for longer and improves transport through the material for spintronics applications. They created their device by sandwiching bilayer graphene between atomically flat boron nitride layers. This structure allowed them to determine the limiting spin relaxation sources and mechanism. The researchers improved the spin lifetime by a factor of five in these spin valves and consider that further enhancement could be realized in fully encapsulated devices. Here, authors report spin transport in dual-gated, high-mobility bilayer graphene spin valves. Their comparative study suggests that substrate and contacts are not the key limiting sources for spin relaxation, but rather it pinpoints the role of polymer residues in current devices. Spin transport in bilayer graphene is gate tunable and this allows authors to demonstrate the evidence of magnetic moments which act as spin hot spots. By taking the advantage of their novel device architecture, they demonstrate the complete suppression of the spin signal while a transport gap was induced in these spin valve devices.