In their work, the researchers - who specialize in using scanning probes to conduct magnetic imaging - used an ultrathin graphene "sandwich" to create a tiny magnetic field sensor that can operate over a greater temperature range than previous sensors, while also detecting miniscule changes in magnetic fields that might otherwise get lost within a larger magnetic background. One of the researchers' "go-to" probes, they say, is the superconducting quantum interference device, or SQUID, which works well at low temperatures and in small magnetic fields.
"We wanted to expand the range of parameters that we can explore by using this other type of sensor, which is the Hall-effect sensor," says doctoral student Brian Schaefer, the lead author of a paper on the research. "It can work at any temperature, and we've shown it can work up to high magnetic fields as well. Hall sensors have been used at high magnetic fields before, but they're usually not able to detect small magnetic field changes on top of that magnetic field."
The Hall effect is a well-known phenomenon in condensed matter physics. When a current flows through a sample, it is bent by a magnetic field, creating a voltage across both sides of the sample that is proportional to the magnetic field. Conventional semiconductor Hall-effect sensors are used in a variety of magnetic field sensing applications, from cellphones to robotics to anti-lock brakes.
The researchers decided instead to evaluate graphene - single layers of carbon atoms, arranged in a honeycomb lattice - as a promising material system for high-performing Hall sensors. One challenge, they say, was that graphene devices often fall short of those made from other semiconductors when the graphene sheet is placed directly on a silicon substrate; the graphene sheet "crumples" on the nanoscale, inhibiting its electrical properties.
To address this, the researchers adopted a recently developed technique to unlock graphene's full potential - sandwiching it between sheets of hexagonal