Long-distance spin transport in high-mobility graphene on hexagonal boron nitride

Abstract

We performed spin transport measurements on boron nitride based single layer graphene devices with mobilities up to 40 000 cm 2 V −1 s −1 . We could observe spin transport over lengths up to 20 µm at room temperature, the largest distance measured so far for graphene. Due to enhanced charge carrier diffusion, spin relaxation lengths are measured up to 4.5 µm. The relaxation times are similar to values for lower quality SiO2 based devices, around 200 ps. We find that the relaxation rate is determined in almost equal measures by the Elliott-Yafet and D'Yakonov-Perel mechanisms.The potential of graphene [1] as an emerging material for spintronics has been established, revealing spin relaxation lengths λ of 2 µm at room temperature [2]. Spins relax over a length λ = √ D s τ s , where D s is the spin diffusion constant and τ s the spin relaxation time. One straightforward way to achieve spin transport over larger distances is to enhance D s by fabricating high mobility devices. On the other hand, τ s is theoretically predicted to range up to hundreds of nanoseconds [3]. However, observations made in the recent years by experimentalists [2, 4-13] do not match up to the high expectations set by theory. Measurements typically indicate τ s to be in the hundred picoseconds range and the discrepancy between theory and experiment and the exact relaxation mechanism remain yet unclear. Some works suggest that spin relaxation is dominated by the Elliott-Yafet (EY) mechanism [4,11,14], where τ s is proportional to the momentum relaxation time τ p and spins lose their information during scattering events. Other efforts indicate that the D'Yakonov-Perel (DP) mechanism is stronger [3,15,16], where τ s is inversely proportional to τ p and spins dephase in between scattering events.In identifying the limiting factors on spin transport in graphene, the substrate deserves special attention. For charge transport it has already been shown that the standard silicon oxide substrate reduces the mobility of charge carriers considerably [17]. The SiO 2 substrate is expected to also affect the spin relaxation in graphene through its roughness, trapped charges and surface phonons [16]. One approach to reduce the substrate roughness and screen impurities is to use epitaxial graphene on silicon carbide [12,18]. However, the presence of localized states is believed to affect spin transport in this system [13]. Alternatively, eliminating the influence of the substrate by suspending the graphene flake yields a 3 orders of magnitude increase in mobility [19,20]. Suspended spintronic graphene devices have been studied and a lower bound for τ s of ∼200 ps was found [10]. Determination of the actual value was however not possible since the presence of local supports for the suspended device was found to dominate the extraction of τ s .Atomically flat hexagonal boron nitride (h-BN) was found to be a much better substrate than SiO 2 for high quality graphene electronics [21][22][23], yielding a 2 orders of magnitude increase in mobility. In this manuscri...

Electronic spin transport in dual-gated bilayer graphene

“…Although an annealing process after metallization is a standard procedure to achieve very high mobilities in graphene-based heterostructure devices, 18,19 we omitted this process because of the degradation of the ferromagnetic electrodes during annealing (Supplementary Figure S5). 8 The pre-patterned top BN layer therefore adds a new degree of freedom to the device architecture. It allows one to easily design a series of encapsulated regions with arbitrary lengths, whereas the non-encapsulated regions destined for the contacts can be scaled down to the lower precision limit of the lithography technique.…”

“…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.…”

“…16 One approach suggested for achieving a longer distance spin communication is to increase the spin diffusion constants by fabricating higher mobility devices. 8,17 For charge transport, it has been shown that the carrier mobility of graphene devices on SiO 2 is mainly limited by interfacial charged impurities, surface roughness, and phonons. [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.…”

“…5,6 The initial spin transport studies were mainly performed on single-layer [7][8][9][10] and bilayer exfoliated graphene, 9,11 and large-area graphene [12][13][14][15] deposited on conventional SiO 2 substrates. Although enhanced spin-relaxation times have been reported for bilayer graphene-based devices compared with those based on a single layer, the relatively low spin diffusion constants overall yield a lower spin-relaxation length of only 1-2 μm, 9,11 far below the theoretical predictions.…”

Direct coupling between charge current and spin polarization by extrinsic mechanisms in graphene

“…It has been neglected in previous theoretical treatments based on semiclassical transport equations 1,2 . Regarding the magnitude of the spin relaxation time, which determines the degree of quantum coherence, there is substantial disagreement between theory and experiment [3][4][5] , as well as between different experiments [6][7][8][9] . Nevertheless, it seems that graphene exhibits fairly long spin relaxation times compared to metals, making it a promising material for passive spintronics, i.e.…”