Controlling Spin Relaxation in Hexagonal BN-Encapsulated Graphene with a Transverse Electric Field

Abstract

We experimentally study the electronic spin transport in hBN encapsulated single layer graphene nonlocal spin valves. The use of top and bottom gates allows us to control the carrier density and the electric field independently. The spin relaxation times in our devices range up to 2 ns with spin relaxation lengths exceeding 12 µm even at room temperature. We obtain that the ratio of the spin relaxation time for spins pointing out-of-plane to spins in-plane is τ ⊥ /τ || ≈ 0.75 for zero applied perpendicular electric field. By tuning the electric field this anisotropy changes to ≈0.65 at 0.7 V/nm, in agreement with an electric field tunable in-plane Rashba spin-orbit coupling.PACS numbers: 72.80. Vp, 85.75.Hh Keywords: Graphene, spin transport, Rashba spin-orbit interaction, anisotropic spin relaxation, Hanle precession, electric fieldThe generation, manipulation and detection of spin information has been the target of several studies due to the implications for novel spintronic devices [1, 2]. In the recent years graphene has attracted a lot of attention in spintronics due to its theoretically large intrinsic spin relaxation time and length of the order of τ s ≈ 100 ns and λ s ≈ 100 µm respectively [4, 9]. Although experimental results still fall short of these expectations [3][4][5] 7], graphene has already achieved the longest measured nonlocal spin relaxation length [5, 9] and furthest transport of spin information at room temperature [10]. However, the mechanisms for spin relaxation in graphene are still under heavy debate with various theoretical models proposed [4, 8, 9,[11][12][13].To take advantage of the long spin relaxation times in graphene, e.g. for spin logic devices, one requires easy control of the spin information, for example by an applied electric field. Single layer graphene is an ideal system for this purpose, not only because of its high mobilities and low intrinsic spin-orbit fields (SOF), but also due to the simple relation between the carriers' wavevector, the applied perpendicular electric field and the induced Rashba SOF [4, 7, 9,[15][16][17][18][19]. In bilayer graphene a more complicated behavior is expected when spin-orbit coupling is considered [21].Here we report nonlocal spin transport measurements on single layer graphene in which we address both topics specified above. Our devices consist of a single layer graphene flake on hexagonal Boron Nitride (hBN) of which a central region is encapsulated with another hBN flake and hence protected from the environment. The presence of a top and bottom gate give rise to two independent electric fields that are experienced by the graphene: [22], where tg(bg) ≈ 3.9 is the dielectric constant, d tg(bg) is the dielectric thickness and V 0 tg(bg) the position of the charge neutrality point for the top (bottom) gate. Their difference controls the carrier density in the graphene (n = (E bg − E tg ) 0 /e) and their average gives the effective electric field experienced by the graphene (Ē = (E tg + E bg )/2), which breaks the inversion symmetry in t...

Electronic spin transport in dual-gated bilayer graphene

“…38 We first note that these room temperature mobilities are higher than for similar devices measured on SiO 2 (see refs 9 and 11, and Supplementary Figure S2) and are consistent with previous works using spin valve devices on BN, where a final annealing step is not applied to keep the integrity of the spin-polarized contacts. 8,23,24 It is also important to note that these mobilities are mainly limited by the residues formed during the electrode fabrication process. However, the residue concentration is significantly reduced in the encapsulated junction (n res = 5.8 × 10 11 cm − 2 ) compared with that in the non-encapsulated one (n res = 1.2 × 10 12 cm − 2 ) because the top BN strip in the encapsulated device protects a large fraction of the junction against polymer contamination.…”

“…23,24 To understand the effect of the substrate and polymer residues on spin relaxation, we fabricated three types of bilayer graphene spin valve devices in a single sample. (1) A spin valve on SiO 2 , (2) a spin valve on BN and (3) an encapsulated spin valve between a BN substrate and a pre-patterned BN strip.…”

“…This represents a technological improvement over the approach prevalent in the literature, which is limited in practice to a single encapsulated region. 23,24 Electrical measurements of the devices Measurements are carried out with standard a.c. lock-in techniques at low frequencies using the local four-terminal setup for charge transport measurements and the non-local setup for spin transport measurements (Figure 1a). In the local charge transport measurements, the current flows between electrode 1 and electrode 4 and a local voltage drop is measured between electrode 2 and electrode 3.…”

“…[18][19][20] The demonstration of an order-of-magnitude improvement in the mobility of graphene encapsulated between atomically flat, charge trap free boron nitride crystals 21,22 has triggered the recent spin transport studies in encapsulated single layer and recently bilayer graphene-based spin valves, where a spin-relaxation length of up to~12 μm and~24 μm have been observed, respectively. 23,24 For the case of bilayer graphene, the initial experiments on SiO 2 revealed an inverse scaling between spin and momentum relaxation times, for example, the longest spinrelaxation times were observed in the lowest-mobility devices. 9,11 Within the standard picture of the spin-relaxation mechanism, such scaling was interpreted as the dominance of a Dyakonov-Perel type scattering mechanism 25 in bilayer graphene-based spin valves on SiO 2 .…”

Resonant Scattering by Magnetic Impurities as a Model for Spin Relaxation in Bilayer Graphene

“…Unfortunately, spin relaxation in graphene structures has been a baffling problem [3]. While experiments in both single layer graphene (SLG) [4][5][6][7][8][9][10][11] and bilayer graphene (BLG) [7,8] yield spin lifetimes on the 100-1000 ps time scale (the highest values achieved in graphene/h-BN structures [12,13]), theories based on realistic spin-orbit coupling and transport parameters predict lifetimes on the order of microseconds [14][15][16][17][18][19][20][21][22][23][24].…”