Room temperature magnetic order on zigzag edges of narrow graphene nanoribbons

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 high-temperature stability. It has been demonstrated that atomic scale structural defects of graphene can host unpaired spins. However, it is still unclear under which 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 only from one sub-lattice of the bipartite graphene lattice, can give rise to a robust magnetic order. We employ a nanofabrication technique 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 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

Thermal anomalies and the insulator-metal (I-M) transition in Mn3+/Mn4+ perovskites

We report a comparative study of two ferromagnets-Pr0.7Sr0.1Ca0.2MnO3 and Pr0.85K0.15MnO3-which exhibit the same formal Mn valency, similar T-c and saturated moments at 5 K. While for former sample the magnetic transition is accompanied by the insulator-metal transition at T similar to T-c, the insulator-metal (I-M) transition is not observed in the latter one despite the expectation based on the Goldschmidt's tolerance factor. The first order character of the I-M transition in Pr0.7Sr0.1Ca0.2MnO3 is demonstrated by a discontinuity of the unit cell volume, step of the specific heat and huge enhancement of the thermal conductivity. The lack of these anomalies in Pr0.85K0.15MnO3 suggests that the properties of perovskite manganites are critically influenced by both a strong phonon-electron coupling arising from a Jahn-Teller splitting of e(g) orbitals of the Mn3+ ion and the local lattice effects due to size mismatch of the large cations. (C) 1997 American Institute of Physics