A 147-Year-Old Rule Said the Magnet Has to Point Sideways. Carnegie Mellon Just Laid It Flat.

In 1879, Edwin Hall ran a current through a thin sheet of gold, held a magnet perpendicular to it, and watched a voltage appear across the sheet. The moving charges were being shoved to one side. The effect became one of the most-used measurements in physics, and it came with a rule that nobody had much reason to question for the next century and a half: the magnetic field has to point out of the plane of the material. Point it sideways, along the plane, and the sideways push vanishes.

A team at Carnegie Mellon University reported in Nature Materials on Thursday that this is not true. Working in the Lab for Investigating Quantum Materials, Interfaces and Devices, the group measured an in-plane anomalous Hall effect — a Hall voltage produced with the magnetic field lying flat inside the material. The phenomenon had been predicted in theory. It had not been pinned down in a real device. "You can also get a response when the field is in-plane," said Simranjeet Singh, the associate professor of physics who led the work with associate professor Jyoti Katoch.

Getting there required stacking the right two materials at atomic thickness. The researchers built devices from layers of tantalum iridium telluride paired with chromium germanium telluride, a magnetic material, and measured the transverse voltage as they rotated the field into the plane. The signal tracked the magnetization rather than disappearing, which is the whole point: the Hall response becomes magnetization-dependent in more than one direction. Postdoctoral researchers I-Hsuan Kao and Ravi Kumar carried out the measurements, with assistant professor Shubhayu Chatterjee on the theory side. The paper carries the DOI 10.1038/s41563-026-02611-9.

The practical payoff the group is chasing is sensing. Today, reading a magnetic field along three axes generally means three sensors, each oriented differently, with the alignment and calibration overhead that implies. A material whose Hall response depends on in-plane magnetization changes that arithmetic. "You can do multidimensional magnetic sensing with one sensor only," Singh said. That matters anywhere magnetic sensors are packed into tight spaces and tight budgets — consumer electronics, automotive and aerospace position sensing, and medical imaging.

The deeper interest is what the result says about the electronic structure of the materials themselves. The anomalous Hall effect is a window onto Berry curvature, the geometric property of electron wavefunctions that governs how charges drift through a crystal, and an in-plane version opens a direction of that window that experimentalists could not previously look through. For a measurement that has been a standard laboratory tool since the 19th century, being handed a new axis is not a small revision.