Long-distance spin transport in graphene

Spintronics differs from conventional electronics by exploiting the spin of the electron as well as the charge. It is currently used by computers in order to read the magnetic information stored on hard discs, while in future it could be used for advanced logic and memory circuits. This goal requires long-distance spin transport in non-magnetic materials, which is challenging because magnetically polarized electrons tend to depolarized in transit. Depolarization is limited in carbon-based materials, and long-distance spin transport has been observed in carbon nanotubes [Nature 445 (2007) 410], but nanotubes are challenging to place at specific locations in devices. By contrast it is easy to position flat sheets of graphene, which are like nanotubes that have been slit and unrolled. The resulting two-dimensional network of carbon atoms is nowadays well known for displaying exceptional electronic and mechanical properties, but the distance over which magnetically polarized electrons can travel without depolarization remains uncertain.

To address this issue, we used photoemission electron microscopy (PEEM) with contrast from x-ray magnetic circluar dichroism (XMCD) to verify correct magnetic switching in magnetic electrodes of the complex oxide La0.67Sr0.33MnO3(see image), with highly spin-polarized mobile electrons. We subsequently transferred a five-layer flake of graphene to span two such electrodes, forming highly resistive interfaces. When the electrodes switched between the parallel and antiparallel magnetic configurations, large changes of resistances demonstrated the transit of magnetically polarized electrons, implying a long graphene spin diffusion length of ~100 μm.

Figure: PEEM image of four electrodes, whose magnetizations switch between left (blue) and right (red) when an applied magnetic field is varied. When the electrodes are bridged by graphene (not shown), parallel and antiparallel configurations yield low and high states of resistance, from which one calculates a graphene spin diffusion length of ~100 μm. Electrode widths vary from 1-6 μm. Data from the Nanoscience beamline (I06) at Diamond Light Source.