Researchers at Carnegie Mellon University, Boston College, the University of California, Los Angeles, the Air Force Research Laboratory, Kansas State University, Japan's National Institute for Materials Science, and Harvard University have experimentally demonstrated an in-plane anomalous Hall effect (AHE) in a low-dimensional heterostructure, overturning the long-held assumption that the anomalous Hall response can only be driven by a material's out-of-plane magnetization.
The Hall effect, first observed in 1879, has served for well over a century as a core tool for probing a material's electronic properties: applying a magnetic field perpendicular to a conducting material deflects moving charges and produces a measurable voltage, revealing whether current is carried by positive or negative charge carriers and how many are flowing. In magnetic materials, the anomalous Hall effect adds a magnetization-dependent contribution to that signal, but symmetry constraints have generally required that response to scale with the out-of-plane component of magnetization alone. An in-plane version of the effect had been theoretically proposed but never demonstrated, largely because it requires a magnetic material with a very specific, reduced crystalline symmetry that is difficult to realize.
To build a system with the right symmetry, the team started with tantalum iridium telluride (TaIrTe4), a low-symmetry topological semimetal, and thinned it down to just a few atomic layers before interfacing it with a ferromagnetic insulator, chromium germanium telluride (Cr2Ge2Te6, or CGT). Because the two layers sit in close proximity, magnetism from the CGT layer leaks into the nonmagnetic TaIrTe4, imbuing it with magnetic character while preserving its own electronic properties, and leaving only a single mirror plane of symmetry intact in the resulting heterostructure. When the magnetization develops a component within that surviving mirror plane, the remaining symmetry breaks down, producing a Hall response that depends on both the in-plane and out-of-plane components of magnetization simultaneously, rather than on the out-of-plane component alone.
Measurements across multiple devices showed a gate-tunable anomalous Hall response, indicating that the effect can be controlled electrostatically, and a minimal symmetry-constrained theoretical model developed by the team traced the effect to interfacial spin-orbit coupling and exchange interaction arising once the CGT layer becomes ferromagnetic at low temperature. In practical terms, the result means that a single ultrathin device can register both the conventional out-of-plane and the new in-plane anomalous Hall signals at once, allowing magnetic fields to be sensed along multiple axes without needing separate sensors for each direction.
"For a long time, people thought the Hall effect only worked when the magnetic field was applied perpendicular to the plane of the film. We've shown that that's not true — you can also get a response when the field is in-plane," said Simranjeet Singh, associate professor of physics at Carnegie Mellon and one of the study's lead researchers. "Beyond fundamental importance, this discovery can enable novel planar device architectures and sensor types, such as vector magnetometry, via measuring the out-of-plane and in-plane anomalous Hall effect signals in the same device." Jyoti Katoch, associate professor of physics at Carnegie Mellon and an expert in fabricating devices from two-dimensional quantum materials, added that the result "demonstrates the power of building atomically precise heterostructures of emergent two-dimensional quantum materials to obtain on-demand electronic and magnetic properties." Shubhayu Chatterjee, assistant professor of physics at Carnegie Mellon, led the theoretical modeling explaining the mechanism: "We found that the reduced symmetry due to pairing with CGT allows additional spin-orbit coupling at the interface. These spin-orbit coupling terms are crucial for the in-plane anomalous Hall effect to emerge once CGT becomes ferromagnetic at low temperatures."
The team, working in Carnegie Mellon's Lab for Investigating Quantum Materials, Interfaces and Devices (LIQUID), is now exploring other material systems that might exhibit the same unconventional Hall response and studying whether the effect persists at room temperature, a necessary step toward real-world sensing applications in fields such as electronics, transportation, and medical imaging.