CMU Physicists Take Hall Effect in a New Direction
Their discovery overturns a long-held assumption that could simplify next-generation magnetic field sensors.
Heidi Opdyke
- Associate Dean of Marketing and Communications, MCS
- Email opdyke@andrew.cmu.edu
- Phone 412-268-9982
Carnegie Mellon University scientists have uncovered a new phenomenon that challenges a long-standing assumption about how electronic materials respond to magnetic fields. The discovery broadens the fundamental understanding of the Hall effect, a principle widely used to measure the magnetic and electronic properties of materials.
Published in Nature Materials, the research could lay the groundwork for simpler, more versatile magnetic sensing technologies used in electronics, transportation and medical imaging.
The Hall effect has been a key tool for studying material properties for over a century. In 1879, Edwin Hall showed that applying a magnetic field perpendicular to a material deflects moving charges, producing a measurable voltage. By analyzing this signal, scientists can determine whether the electric current is carried by negative or positive charges, how many of those charges are moving through the material, and how easily they flow. Today, the Hall effect is integral to broadly-used sensing technologies found in systems ranging from automobiles to computer keyboards.
In the latest work, researchers in Carnegie Mellon’s Department of Physics, working in the Lab for Investigating Quantum Materials, Interfaces and Devices (LIQUID), identified a new form of the Hall effect.
“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, an associate professor of physics.
The discovery expands the role of the Hall effect as a core tool in physics because it allows for a magnetization-dependent Hall response in more than one direction. This allows researchers to probe and understand multi-dimensional magnetic and topological configurations in condensed matter systems.
“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,” Singh said.
From theory to reality
The idea of an in-plane anomalous Hall effect was theoretically proposed but never experimentally demonstrated — until now.
“People proposed it and ideas were out there, but it’s very difficult to make a magnetic material with the right symmetry to do it,” Singh said. “What we did was we found a material with the right symmetry and we made it magnetic.”
To build the unique nanometer-sized devices used in this research, Singh turned to Jyoti Katoch, an associate professor of physics and an expert on fabricating devices out of two-dimensional quantum materials. The team, including post-doctoral researchers I-Hsuan Kao and Ravi Kumar, started with tantalum iridium telluride (TaIrTe₄), which has the necessary crystalline symmetry to produce a multidimensional Hall effect. They thinned it down to the thickness of a few atomic layers and paired it with a magnetic layer, Cr2Ge2Te6 (CGT).
Because of the layers’ proximity to each other, magnetism leaks into the nonmagnetic layer, imbuing it with magnetic properties while preserving its electronic characteristics.
“This truly demonstrates the power of building atomically precise heterostructures of emergent two-dimensional quantum materials to obtain on-demand electronic and magnetic properties,” Katoch said.
Within these atomically thin devices, the team observed both the conventional Hall signal and an additional, unconventional signal that follows magnetization within the plane. In practical terms, that means a single, ultra-thin device can now detect magnetic fields along multiple axes.
“We have broadened the potential application of these materials,” Singh said. “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.”
Complementing the experimental work, Shubhayu Chatterjee, an assistant professor of physics, carried out theoretical modeling to identify the mechanism behind the effect and to explain how symmetry in the material enables the unusual response.
“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. While certain features of the observed anomalous Hall effect signal are consistent with an intrinsic origin, a detailed characterization of few-layered TaIrTe4 is needed to nail down the precise mechanism,” Chatterjee said.
The LIQUID team is exploring other material systems that may also exhibit the unconventional Hall response. They also are studying the device’s response at room temperature, an important step toward real-world applications.
