Room-temperature magnetic order on zigzag edges of narrow graphene nanoribbons

Abstract

Magnetic order emerging in otherwise non-magnetic materials as carbon is a paradigmatic example of a novel type of s-p electron magnetism predicted to be of exceptional hightemperature stability 1 . It has been demonstrated that atomic scale structural defects of graphene can host unpaired spins 2,3 . However, it is still unclear under which conditions longrange 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 only from one sublattice of the bipartite graphene lattice -can give rise to a robust magnetic order. We employ a nanofabrication technique 4 based on Scanning Tunneling Microscopy to define graphene nanoribbons with nanometer precision and well-defined crystallographic edge orientations.While armchair ribbons display quantum confinement gap, zigzag ribbons narrower than 7 nm reveal a bandgap of about 0.2 -0.3 eV, which can be identified as a signature of 2 interaction induced spin ordering along their edges. Moreover, a semiconductor to metal transition is revealed upon increasing the ribbon width, indicating the switching of the magnetic coupling between opposite ribbon edges from antiferromagnetic to ferromagnetic configuration. We found that the magnetic order on graphene edges of controlled zigzag orientation can be stable even at room temperature, raising hope for graphene-based spintronic devices operating under ambient conditions.The intrinsic magnetism of graphite has a long and controversial history 1 . The origin of the measured magnetic signal is generally attributed to atomic scale structural defects locally breaking the sub-lattice balance of the bipartite hexagonal lattice 5,6 . However, the unambiguous identification of the structural sources of the measured magnetic signal has proven challenging as they are buried inside the bulk of the material. The isolation of single graphene layers 7 opens new prospects in this direction 8,9 as their atomic structure is fully accessible for imaging and controlled modification. In particular, graphene edges of specific (zigzag) crystallographic orientation comprising carbon atoms from only one sub-lattice of the bipartite hexagonal lattice are predicted to host magnetic order 10 , in striking contrast to armchair edges incorporating an equal number of carbon atoms from both sublattices.The strong influence of edge orientation on the electronic structure of graphene nanoribbons had However, the random orientation of the edges and the influence of a possible strong edge-substrate hybridization 21 did not allow full access to the nature of edge-magnetism in graphene. Though the 4 magnetic order is expected to persist to some extent on zigzag segments of randomly oriented graphene edges, the mixing of different edge types are expected to substantially weaken the effect 19,22 . Therefore, the lack of experimental control over the edge orientation seems one of the main ...

Width-Tuned Magnetic Order Oscillation on Zigzag Edges of Honeycomb Nanoribbons

“…7 The experimental indication of one AFCE-FMCE phase transition was recently reported in scanning tunnelling microscopy measurements which reveals an electronic bandgap of about 0.2−0.3 eV for the ribbons narrower than 7 nm but gapless bands for the ribbons wider than 8 nm. 17 Such a semiconducting to metallic phase transition was again accurately reproduced in the mean-field theory of the Hubbard model which found the driving force to be the AFCE-FMCE transition. 17 This discovery stimulates the search for more novel effects in zigzag-edged nanoribbons and for the proper understanding of these effects.…”

Spatial structure of correlations around a quantum impurity at the edge of a two-dimensional topological insulator

“…However, theory has predicted that electronic repulsion causes a hybridization between edge states on opposite sides of a ZNRB, inducing edge ferromagnetism [43][44][45][46][47][48] . These effects depend on both the magnitude of the interactions and the width W of the ribbon, and decay very rapidly 49 as W −2 .…”

Edge states in graphene-like systems

“…Graphene ribbons are interesting by their own sake, and there has been enormous progress in the fabrication of ribbons with smooth edges [53][54][55][56][57] , so that inter-edge coupling is also an interesting topic [30][31][32]35,37,38,40,42,51,57,58 .…”