Green light for graphene-based spintronics

Long-distance spin transport observed via PEEM study

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.
 
Exploiting spintronics to its full potential depends on being able to control how far magnetised electrons can travel through a circuit without losing their orientation. Maintaining this so-called ‘spin polarisation’ allows magnetic information to be communicated and manipulated, in contrast with the exclusive focus on charge in conventional electronics.
 
In a quest to investigate the potential of graphene in spintronics, a team of researchers using Diamond’s Nanoscience beamline have been able to demonstrate that electrons can retain their spin-polarised state in transit through a non-magnetic material for a distance (∼100 µm) that is several times further than previously recorded. The data, published in Physical Review Letters, were achieved by using the graphene to bridge magnetic electrodes in which the mobile electrons are highly spin polarised.

Long-distance spin transport is demonstrated
Spintronics requires spin-polarised electrons to maintain their magnetic orientation as current traverses through non-magnetic materials that lie between the magnetised electrodes. Finding a robust channel for this ‘spin transport’ over long distances will show manufacturers that a key step has been taken towards applications.

Polarised electrons tend to get depolarised in transit across many non-magnetic materials, such as metals and semiconductors. Carbon-based materials have been seen to limit depolarisation, as seen in the research team’s previous work on carbon nanotubes1. The carbon nanotubes research was published in Nature in 2007, and co-authored by Professor Albert Fert, who went on to win the Nobel Prize for Physics later that year for the discovery of ‘Giant Magnetoresistance’, which gave birth to modern disc-drive read heads and thus spintronics.

Nanotubes however present a practical problem in their application as they are difficult to place at specific locations in spintronic 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 polarised electrons can travel without depolarisation has remained uncertain.

 

Novel devices to test spin transport
The research team, from the University of Cambridge, Aalto University in Finland, Parma University in Italy, and Diamond Light Source, developed a device whose performance was verified on the Nanoscience beamline (I06). The device consisted of four magnetised electrodes of the complex oxide La0.67Sr0.33MnO3, bridged by a five‑layer flake of graphene, forming two highly resistive interfaces. Of the 20 devices, made at the University of Cambridge, between the Department of Materials Science and the Cambridge Graphene Centre, only one showed the desired results, with the rest showing no result.

Professor Neil Mathur, Principal Investigator in the study, has been working with the I06 team on spintronics and magnetoresistance over the past seven years. Of the devices, he said: “Creating them was very tricky because the two materials – the electrodes and the graphene – are structurally and chemically very different. To combine them and create a good interface is rather unlikely, but after a lot of hard work we ended up with a device that demonstrated long spin transport.

"The principle behind the calculation is that you have the magnetisation of two electrodes parallel and antiparallel, you get high and low states of resistance, and hence you can calculate the spin diffusion length.”

Figure 1: Resistance RBC and magnetoresistance MRBC on decreasing (blue) and increasing (red) magnetic field. Dashed lines indicate RBC = 24.8 MΩ and RBC + DRBC = 25.6 MΩ for parallel and antiparallel electrode magnetisations, respectively. The antiparallel configuration is indicated by grey shading in 12.5 mT < |μ0H| < 34.5 mT.

 

Using the Nanoscience beamline (I06)
The evidence for the long spin diffusion length hinged on the high resistance seen when sweeping an applied magnetic field (Fig. 1). The large changes of resistances demonstrate the successful transit of magnetically polarised electrons, implying a long graphene spin diffusion length of ∼100 µm.
 
This is where I06 came into its own, as Prof Mathur explained: “We used PhotoEmission Electron Microscopy (PEEM) with contrast from X-ray Magnetic Circular Dichroism (XMCD) to verify correct magnetic switching in the magnetic electrodes, with highly spin-polarised mobile electrons.
 
“The contribution from Diamond was absolutely critical. I06 can create a magnetic map that allows us to see the electrode magnetisations in their parallel and antiparallel states (Fig. 2). Without this, we would not be able to confirm that the high-resistance state arose due to the intended reason.”
 
Figure 2: PEEM image of four electrodes (1 – 4), whose magnetisations 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 the researchers calculated a graphene spin diffusion length of ∼100 µm. Electrode widths vary from 1-6 µm.
 
Next phase of the journey
This experiment has given evidence of a long spin-diffusion length, and the next step is for the team to improve the device fabrication procedure, and achieve improved performance.
 
To find out more about the I06 beamline, or to discuss potential applications, please contact Principal Beamline Scientist Professor Sarnjeet Dhesi: dhesi@diamond.ac.uk
 
 

Reference:

  1. Hueso EH et al. Transformation of spin information into large electrical signals using carbon nanotubes. Nature 445, 410–413 (2007).

Related publication:

Yan W et al. Long Spin Diffusion Length in Few-Layer Graphene Flakes. Physical Review Letters  117, 147201 (2016). DOI: 10.1103/PhysRevLett.117.147201

 

Photo credit (homepage promo): 'Green for Go!' by Alan Perryman, adpated under CC BY-NC-SA 2.0