A Century-Old Physics Effect Just Got a New Dimension

Physicists at Carnegie Mellon University have identified an unexpected form of the Hall effect — a phenomenon discovered in 1879 that is widely used to study the electrical and magnetic properties of materials — challenging a century-old assumption about how it works. The finding, published in the journal Nature Materials, could eventually help build simpler and more flexible magnetic sensors for electronics, transportation and medical imaging.

The Hall effect occurs when a magnetic field applied perpendicular to a material carrying an electric current deflects the moving charges, creating a measurable voltage. For more than a century, scientists believed the effect only worked when the magnetic field was applied perpendicular to the plane of the material. Researchers in Carnegie Mellon's Department of Physics, led by Simranjeet Singh, have now shown that a response also occurs when the magnetic field is in-plane — oriented within the material rather than perpendicular to it.

To demonstrate the in-plane effect, the team, working with Jyoti Katoch, built ultrathin devices from tantalum iridium telluride (TaIrTe4), a material with the crystalline symmetry needed to produce the effect, reduced to only a few atomic layers and paired with a magnetic layer called Cr2Ge2Te6 (CGT). Because the two layers sit extremely close together, magnetism from the magnetic layer gives the nonmagnetic layer magnetic properties while it keeps its distinctive electronic behavior.

Inside the devices, the researchers detected both the conventional Hall signal and the new, unconventional one tied to magnetization within the plane of the material — meaning a single ultrathin device can detect magnetic fields along more than one axis, work that previously required two separate sensors.

Theoretical modeling by Shubhayu Chatterjee found that pairing the two materials reduces their combined symmetry, allowing additional spin-orbit coupling — an interaction between an electron's motion and its quantum spin — at the interface. That coupling lets the in-plane effect emerge once the CGT layer becomes magnetic at low temperatures.