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

Abstract

We propose that the observed spin relaxation in bilayer graphene is due to resonant scattering by magnetic impurities. We analyze a resonant scattering model due to adatoms on both dimer and nondimer sites, finding that only the former give narrow resonances at the charge neutrality point. Opposite to singlelayer graphene, the measured spin-relaxation rate in the graphene bilayer increases with carrier density. Although it has been commonly argued that a different mechanism must be at play for the two structures, our model explains this behavior rather naturally in terms of different broadening scales for the same underlying resonant processes. Not only do our results-using robust and first-principles inspired parameters-agree with experiment, they also predict an experimentally testable sharp decrease of the spin-relaxation rate at high carrier densities. DOI: 10.1103/PhysRevLett.115.196601 PACS numbers: 72.80.Vp, 72.25.Rb Understanding spin relaxation is essential for designing spintronics devices [1,2]. 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].While the magnitudes of the spin-relaxation rates of SLG and BLG are similar, the dependence of the rates on the electron density is opposite in the two systems. In SLG the spin-relaxation rate decreases with increasing the carrier density [5][6][7][8], in BLG the spin-relaxation rate increases [7,8]. Since the diffusivity in the investigated samples decreases with increasing the electron density, it has been a common practice to assign two different mechanisms to both structures: the Elliott-Yafet mechanism [25,26] to SLG [5,6,9,10] and Dyakonov-Perel mechanism [27] to BLG [7][8][9].The main problem with that assignment is quantitative. Spin-orbit coupling in graphene [28] is too weak to yield such a small spin-relaxation time. An explicit first-principles calculation [23] predicts that one would need 0.1% of adatoms to give a 100 ps spin lifetime. Recently a new mechanism for SLG was proposed [29] (see also Ref.[30]), based on resonant scattering off local magnetic moments. It gives the observed spin-relaxation times with as little as 1 ppm of local magnetic moments and also agrees with the experimental behavior for SLG of decreasing the spinrelaxation rate with increasing electron density. Where do these local moments come from? It was theoretically predicted that adatoms such as hydrogen [31,32], but also chemisorbed organic molecules [33] can be responsible. Experimentally it was demonstrated that hydrogen adatoms indeed induce local moments [34,35], but even untreated graphene flakes were shown to exhibit 20 ppm spin 1=2 para...

Electronic spin transport in dual-gated bilayer graphene

“…For instance, this source could be magnetic scatterers that have recently been predicted to increase the spin flipping process without having a significant influence on the charge transport. 27,28 To check the role of the magnetic impurities on spin relaxation, we now investigate the carrier concentration dependence of the local device resistance and Prior to the carrier concentration-dependent spin transport measurements, we first performed charge characterizations of both encapsulated and non-encapsulated bilayer graphene junctions at room temperature. Our devices are weakly electron doped, which is possibly because of a charge-transfer process from the MgO layer.…”

“…Such dependencies in both low and high carrier concentration regimes appear to agree with the recent theoretical work where the spin relaxation in graphene is proposed to be dominated by resonant scatterers because of the presence of a low concentration of magnetic impurities, such as polymer residues. 27 In that theory, the inverse dependence of the spin relaxation on the carrier concentration in bilayer and single-layer graphene was attributed to the different scales of energy fluctuations in these two systems because of their different density of states. Away from the puddle regime, the sudden increase in spin-relaxation time at~4-5 × 10 12 cm − 2 , with no corresponding signature in the charge transport, is expected for bilayer graphene because of the scattering from these resonant magnetic scatterers.…”

“…For the testing of this theory, bilayer graphene is a more ideal platform because a non-monotonic correlation is predicted for the energy dependence of the spin-relaxation time in the bilayer graphene case that can be experimentally verified. 27 Ultimately, a similar mobility enhancement in bilayer graphene may be a successful route to enhance the spin-relaxation lengths, although this is strongly dependent on the scattering source and the mechanism of spin relaxation. 27,[29][30][31] …”

“…27 Ultimately, a similar mobility enhancement in bilayer graphene may be a successful route to enhance the spin-relaxation lengths, although this is strongly dependent on the scattering source and the mechanism of spin relaxation. 27,[29][30][31] …”

Grain boundary-induced variability of charge transport in hydrogenated polycrystalline graphene

“…This can have a considerable impact on electronic transport, as revealed by measurements of a metal-insulator transition with increasing hydrogen density 9 . Recent experimental work has also shown that hydrogen induces a localized magnetic resonance 10 , which could have important implications for graphene spintronics 11,12 . While mechanical exfoliation tends to yield the highest-quality graphene samples in the laboratory, chemical vapor deposition (CVD) is the most efficient method to produce graphene on an industrial scale.…”