Researchers at The University of Manchester's National Graphene Institute have shown that electrons in ultra-clean graphene can be steered with high precision while keeping their spin information intact, a key requirement for future low power electronics and quantum devices.
In a new study published in Physical Review X, the team demonstrates how electrons can travel ballistically, i.e. without experiencing any scattering or resistance, over micrometre distances in graphene at low temperature and maintain spin coherence all the way up to room temperature. By using a technique known as transverse magnetic focusing (TMF), they were able to bend electron trajectories like light rays traversing a lens and show that these curved paths carry a clear spin signature.
Manchester-based Co-author Dr Daniel Burrow said, "What's exciting here is that we can shape the path of electrons in graphene and, at the same time, tune how their spins behave. It's a bit like using a set of lenses and mirrors, but for spin-polarised electrons. This opens a practical way to control spin without needing strong spin–orbit interaction in the material."
Electron paths reveal spin behaviour
The team's graphene device uses ferromagnetic cobalt contacts to inject and detect spin-polarised electrons at the edge of an encapsulated graphene channel. When a small out-of-plane magnetic field is applied, electrons paths curve into so-called cyclotron orbits. If those orbits are the right size, they land directly on the detector contact producing distinct peaks in signal at specific magnetic fields. These TMF peaks provide a direct fingerprint of ballistic electron motion. Three such peaks were resolved in the study.
Crucially, the height and sign of these TMF peaks changed depending on the alignment of the magnetic contacts, showing that the focused signal carried spin information. This confirms that ballistic trajectories, rather than diffusive scattering processes, were responsible for transporting spin across the device.
Control at the flick of a gate voltage
By varying the voltage applied to the back gate, which tunes the density of electrons in graphene, the researchers could modulate the spin signal dramatically. In some conditions, they enhanced the signal relative to standard nonlocal spin-valve measurements. In others, they could reverse its polarity altogether.
This tunability arises from a coupling between the electrons' orbital motion and their spin, which occurs because the ferromagnetic contacts induce local charge-transfer doping as well as proximity-exchange effect at the graphene edge. So, the graphene next to the contact behaves like a magnetic material, and the ballistic movement of electrons from this region into the rest of the non-magnetic graphene channel leads to the spin-dependent electron optics. The result is a transistor-like behaviour for spin, achieved without introducing spin–orbit coupling into the graphene channel.
A route toward practical spin-based devices
The team observed clear ballistic behaviour at low temperature (25 K), with quasi-ballistic transport still present at room temperature. Because the TMF peaks remained sensitive to spin at these higher temperatures, the researchers demonstrate that spin-coherent ballistic transport can survive under conditions suitable for real world devices.
This approach provides a new operational principle for spintronic components: devices that rely on controlling the spin of electrons rather than their charge. The mechanism echoes the idea behind the Datta–Das spin field-effect transistor but achieves spin modulation through electron optics effects rather than spin–orbit interactions.
Co-author Dr Ivan Vera Marun added, "We have shown that electron optics in graphene can do more than guide electrons, it can actively shape their paths in a spin-dependent manner. Being able to control spin in this way, using low-power and scalable materials, moves us closer to practical spin-based technologies and future quantum systems."
