Abstract
The possibility that non-magnetic materials such as carbon could exhibit a novel type of s–p electron magnetism has attracted much attention over the years, not least because such magnetic order is predicted to be stable at high temperatures1. It has been demonstrated that atomic-scale structural defects of graphene can host unpaired spins2,3, but it remains unclear under what conditions long-range magnetic order can emerge from such defect-bound magnetic moments. Here we propose that, in contrast to random defect distributions, atomic-scale engineering of graphene edges with specific crystallographic orientation—comprising edge atoms from only one sub-lattice of the bipartite graphene lattice—can give rise to a robust magnetic order. We use a nanofabrication technique4 based on scanning tunnelling microscopy to define graphene nanoribbons with nanometre precision and well-defined crystallographic edge orientations. Although so-called ‘armchair’ ribbons display quantum confinement gaps, ribbons with the ‘zigzag’ edge structure that are narrower than 7 nanometres exhibit an electronic bandgap of about 0.2–0.3 electronvolts, which can be identified as a signature of interaction-induced spin ordering along their edges. Moreover, upon increasing the ribbon width, a semiconductor-to-metal transition is revealed, indicating the switching of the magnetic coupling between opposite ribbon edges from the antiferromagnetic to the ferromagnetic configuration. We found that the magnetic order on graphene edges of controlled zigzag orientation can be stable even at room temperature, raising hopes of graphene-based spintronic devices operating under ambient conditions.
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Acknowledgements
The experimental work was conducted within the framework of the Korea Hungary Joint Laboratory for Nanosciences through the Korean Research Council of Fundamental Science and Technology and the “Lendület” programme of the Hungarian Academy of Sciences. L.T. acknowledges OTKA grant K108753 and the Bolyai Fellowship. L.P.B. acknowledges OTKA grant K101599. C.H. is supported in part by the Nano·Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2012M3A7B4049888). I.H. was supported by the European Union and the State of Hungary, co-financed by the European Social Fund in the framework of the TÁMOP-4.2.4.A/ 2-11/1-2012-0001 National Excellence Program and OTKA grant K100908. I.H. acknowledges discussions with K. Itai. L.T and P.V. acknowledge discussions with Y.-S. Kim.
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L.T. and C.H conceived and designed the experiments. G.Z.M. performed the lithography and STM experiments. I.H. and P.V. provided the theoretical results. X.J. and C.H. performed the graphene growth experiments. Z.O. and P.N.-I. carried out preliminary experiments. L.T., P.V., I.H., G.Z.M. and L.P.B. analysed the data. L.T. wrote the paper. All the authors discussed the results and commented on the manuscript.
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Extended data figures and tables
Extended Data Figure 1 The structure of graphene edges defined by STM lithography.
a, STM image (5 mV, 2 nA) of 15-nm-long edge segments cut by STM lithography, revealing edges that are close to atomically smooth (<5 Å edge roughness) free of detectable reconstructions, contaminations or curvature. b, The increased local density of states on zigzag edges observed under specific imaging conditions (200 mV, 2 nA) can be attributed to the presence of edge states that rule out the possibility of sp3-type edge terminations (for example, di-hydrogenated edges), because no edge states are expected to occur for such edge configurations.
Extended Data Figure 2 Scanning tunnelling spectroscopy of graphene nanoribbons on Au (111).
Tunnelling I–V characteristics acquired on a 5-nm-wide armchair ribbon (a) displaying nonlinear I–V spectra corresponding to a gap of about 250 meV (b). Outside the ribbon a close-to-linear characteristic of the unpatterned graphene is revealed (c). The blue triangle and yellow square mark the positions of the corresponding tunnelling spectra. The insets show the schematics of STM lithography (a), and the differential tunnelling conductance (dI/dV) obtained as numerical derivatives of the measured I–V curves (b and c). The ∼70-mV shift of the Dirac point (curve minimum) from the Fermi level (zero bias) observed on graphene (inset to c) is due to the doping from the Au(111) substrate and the ambient atmosphere.
Extended Data Figure 3 The effect of edge irregularities on edge magnetism.
Calculated spin density distribution in the unit cell of a 3.3-nm-wide zigzag ribbon with a high density of atomic-scale defects, revealing the substantial decrease of the emerging spin polarization (to about a third of that of defect-free zigzag edges). The experimental width dependence can be fitted for defective ribbon edges by using higher values of the on-site repulsion parameter of U = 4.32 eV.
Extended Data Figure 4 Individual tunnelling I–V spectra.
Tunnelling I–V characteristics recorded on various ribbons (the spectra have been shifted along the vertical axis for clarity). Each individual spectrum was recorded as the average of ten voltage sweeps between −500 mV and +500 mV. The metallic (close to linear) or semiconducting (strongly nonlinear) nature of the ribbons is clearly apparent from the individual tunnelling I–V characteristics.
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Magda, G., Jin, X., Hagymási, I. et al. Room-temperature magnetic order on zigzag edges of narrow graphene nanoribbons. Nature 514, 608–611 (2014). https://doi.org/10.1038/nature13831
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DOI: https://doi.org/10.1038/nature13831
