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January 17th 2018

Magnetic sensing devices commonly use anisotropic magnetoreistance (AMR), an effect first decribed by Lord Thomson in 1857. "Thomson observed that the resistivity of ferromagnetic materials depends on the relative direction between the electrical current and the magnetization", says Carmine Ortix. "The performance of AMR sensor devices depends highly on the relative change between minimum and maximum resistivity. For transition metals and their alloys, generally used in AMR sensors, this relative change is only a few percent."

Special geometry
Together with Ching-Hao Chang from the Leibniz Institute for Solid State and Materials Research Dresden Ortix has theoretically predicted a gigantic AMR effect in a special nanostructure. "We investigated a so-called carbon nanoscroll, which is basically a rolled up layer of graphene", he says. "Immersed in relatively weak magnetic fields, a carbon nanoscroll shows an unprecedented AMR effect of about 80 percent."

This extremely large AMR effect is due to the peculiar 'spiral-like' tubular geometry of carbon nanoscrolls. "The geometry allows for the natural formation of snake orbits: unusual electron trajectories of charge carriers curving back and forth during their motion", Ortix explains. "The number of snake-orbits changes with the direction of an externally applied magnetic field. As such, the anisotropic magnetoresistance is geometric and does not require either magnetism or the so-called spin-orbit interaction, which are necessary ingredients in the conventional AMR effect."

Nanosensing devices
Ortix thinks the experimental observation of this predicted effect could be happening very soon due to recent advances in the fabrication of high-quality carbon nanoscrolls. He says: "This may pave the way towards miniaturized nanoscale devices exploiting the anisotropic magnetoresistance effect, for magnetic recording or other applications."

Contact information
The work carried out by Carmine Ortix is part of his VIDI grant 'Shaping nanomaterials for future electronics' funded by the NWO.
Carmine Ortix, +31 30 253 57 22.

C.-H. Chang, C. Ortix 'Theoretical prediction of a giant anisotropic magnetoresistance in carbon nanoscrolls', Nano Letters (Articles ASAP) DOI: 10.1021/acs.nanolett.7b00426.