Physicists find magnetic sensors can work in a direction long thought impossible

Physicists at Carnegie Mellon University have shown that a foundational effect in physics, used in billions of sensors inside cars, phones and hard drives, can be triggered in a way researchers had long assumed was impossible. In a paper published Thursday in Nature Materials, the team reports that an atomically thin engineered material produces a measurable Hall effect signal when a magnetic field is applied parallel to its surface, rather than perpendicular to it, as textbooks have held since the effect was first described in 1879.

The finding does not overturn the underlying physics of magnetism so much as widen it. Researchers built a heterostructure — a stack of different atomically thin materials — by combining tantalum iridium telluride, a material with an unusual crystal symmetry, with a layered magnetic insulator called chromium germanium telluride. Reduced to a few atomic layers, the tantalum iridium telluride is not naturally magnetic, but placed against the magnetic layer it absorbs some of that material's magnetic character through what physicists call a proximity effect, while keeping its own electronic properties intact. In that combined structure, the team measured both the conventional Hall signal, generated by a perpendicular field, and a distinct in-plane signal generated by a field running parallel to the material, in the same device at the same time.

An assumption that held for 147 years

The Hall effect is named for Edwin Hall, who discovered it in 1879 as a 24-year-old graduate student at Johns Hopkins University: when an electric current flows through a conductor sitting in a magnetic field perpendicular to it, a voltage builds up across the material at a right angle to the current. That voltage is proportional to the strength of the field, which is what makes the effect useful for measurement — it is the working principle behind most commercial magnetic-field sensors, from the position sensors in car engines and anti-lock brakes to certain components in medical scanners. For nearly a century and a half, the effect was understood to require a magnetic field oriented perpendicular to the plane of the material; a field applied in-plane was not expected to produce a comparable signal.

"For a long time, people thought the Hall effect only worked when the magnetic field was applied perpendicular to the plane of the film," said Simranjeet Singh, an associate professor of physics at Carnegie Mellon and a senior author of the study. "We've shown that that's not true."

Building the material one atomic layer at a time

Jyoti Katoch, an associate professor of physics who led the material fabrication, thinned the tantalum iridium telluride down before stacking it against the chromium germanium telluride layer. "This truly demonstrates the power of building atomically precise heterostructures of emergent two-dimensional quantum materials," Katoch said. Shubhayu Chatterjee, an assistant professor of physics who led the theoretical modeling, said the pairing changes the symmetry of the system in a way that opens a new channel for the effect: "We found that the reduced symmetry due to pairing with [chromium germanium telluride] allows additional spin-orbit coupling at the interface," he said, referring to the interaction between an electron's motion and its magnetic spin that is thought to drive the in-plane response. Postdoctoral researchers I-Hsuan Kao and Ravi Kumar also worked on the project, according to the university, which houses the group in its physics department on the Pittsburgh campus.

"You can do multidimensional magnetic sensing with one sensor only. Before, you needed to put two sensors to measure the magnetic field in two directions."

That line, from Singh, points to the practical draw of the finding. A single sensor able to register both perpendicular and in-plane fields could, in principle, measure a magnetic field's full orientation without the extra hardware, wiring and calibration that come with combining two separately oriented sensors. Singh and his colleagues describe the work as a step toward vector magnetometry — sensing the direction as well as the strength of a magnetic field — built into one ultrathin chip rather than assembled from multiple parts.

Sensors, and the limits of a lab result

The people most likely to eventually feel the effects of the discovery are engineers who design magnetic sensors for electronics, automotive systems and medical imaging equipment, industries that already lean on Hall-effect devices by the billions. But the CMU result, for now, is a laboratory demonstration rather than a working product. The measurements were carried out at low temperatures typical of experiments on quantum materials, and the researchers say a central next step is determining whether the effect persists at room temperature, which they describe as an important step toward any real-world application. They also plan to test whether other material combinations beyond tantalum iridium telluride and chromium germanium telluride show the same in-plane response, work that will help establish how general the phenomenon is rather than a quirk of one particular pairing.

The study has cleared peer review at Nature Materials, a step that lends it credibility within the physics community, though independent replication by other laboratories — standard practice before a new effect is considered fully established — has not yet been reported. Carnegie Mellon's physics department, where the work was carried out in the Laboratory for Investigating Quantum Materials, Interfaces and Devices, said the group intends to keep pursuing both the fundamental question of what else might be hiding in the symmetry of engineered quantum materials and the applied question of how soon a discovery made in a cryostat might show up in a sensor on a factory floor.