Spin-half paramagnetism in graphene induced by point defects

Nair, R. R.; Sepioni, M.; Tsai, I-Ling; Lehtinen, Ossi; Keinonen, J.; Krasheninnikov, Arkady V.; Thomson, Thomas; Geǐm, A. K.; Grigorieva, I. V.

doi:10.1038/nphys2183

Cited by 795 publications

(808 citation statements)

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“…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 paramagnetic moments [36]. The most natural candidates for resonant magnetic scatterers appear to be polymer residues from different fabrication steps of graphene devices.…”

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confidence: 99%

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

et al. 2015

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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...

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confidence: 99%

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

et al. 2015

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“…This exponent reflects again the anomalous nature of the sp 3 local moment in infinite graphene, in marked contrast with the behavior of the same chemical functionalization in a gapped graphene structure. Interestingly, χ (T ) has been measured [38] for defective graphene obtaining a Curie law dependence, probably because the samples used are in fact nanoflakes with small confinement gaps that permit the existence of in-gap states with quantized spins.…”

Section: Finite-temperature Susceptibilitymentioning

confidence: 99%

Anomalous magnetism in hydrogenated graphene

et al. 2017

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We revisit the problem of local moment formation in graphene due to chemisorption of individual atomic hydrogen or other analogous sp 3 covalent functionalizations. We describe graphene with the single-orbital Hubbard model, so that the H chemisorption is equivalent to a vacancy in the honeycomb lattice. To circumvent artifacts related to periodic unit cells, we use either huge simulation cells of up to 8 × 10 5 sites, or an embedding scheme that allows the modeling of a single vacancy in an otherwise pristine infinite honeycomb lattice. We find three results that stress the anomalous nature of the magnetic moment (m) in this system. First, in the noninteracting (U = 0) zero-temperature (T = 0) case, the m(B) is a continuous smooth curve with divergent susceptibility, different from the stepwise constant function found for single unpaired spins in a gapped system. Second, for U = 0 and T > 0, the linear susceptibility follows a power law ∝ T −α with an exponent of α = 0.77 different from the conventional Curie law. For U > 0, in the mean-field approximation, the integrated moment is smaller than m = 1μ B , in contrast with results using periodic unit cells. These three results highlight the fact that the magnetic response of the local moment induced by sp 3 functionalizations in graphene is different from that of local moments in gapped systems, for which the magnetic moment is quantized and follows a Curie law, and from Pauli paramagnetism in conductors, for which linear susceptibility can be defined at T = 0.

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“…Recently, it has been reported that the magnetic interactions between two substitutional Co atoms are ferromagnetic for the same sublattice sites, while they are antiferromagnetic or spin compensated for the different sublattice sites [16]. H-or F-functionalized graphene exhibits similar magnetic interactions [9][10][11]. However, Fe-X 4 defects do not intrinsically exhibit such a sublattice dependency because the defects perturb both the sublattices of graphene symmetrically.…”

Section: Very Recently It Is Reported Thatmentioning

confidence: 99%

“…Intrinsic and extrinsic defects in graphene, e.g., vacancies [7,8], H or F sp 3 -functionalization [9][10][11], and transition metals [12][13][14][15][16], can be a source of magnetism by generating dangling bond (DB) states. Particularly, it is theoretically proposed that H-or Ffunctionalized graphene could exhibit local magnetism only when the graphene sub-lattice symmetry is broken [9][10][11].…”

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confidence: 99%