Researchers at Carnegie Mellon University have discovered an unusual magnetic response that challenges long-standing assumptions about the Hall effect, a fundamental phenomenon used to understand how materials conduct electricity and respond to magnetic fields.
The study expands scientists’ understanding of the Hall effect in natural materials and could lead to simpler, more flexible magnetic sensors for electronics, transportation, medical imaging, and other advanced technologies.
Rethinking the 100-year-old Hall effect
Scientists have relied on the Hall effect for more than a century. In 1879, Edwin Hall discovered that when a magnetic field is applied perpendicular to a material carrying an electric current, the moving charges are pushed to one side. This creates a measurable voltage across the material.
The resulting Hall signal provides important information about a material, including whether its current is carried by positive or negative charges, the number of charge carriers, and how easily those carriers move. Today, Hall-effect sensors are used in technologies ranging from automobiles and industrial equipment to computer keyboards.
Researchers in the Laboratory of Quantum Materials, Interfaces, and Devices (LIQUID) at Carnegie Mellon University’s Department of Physics have now demonstrated another form of the Hall effect: the in-plane anomalous Hall effect.
“For a long time, people thought that the Hall effect only worked when the magnetic field was applied perpendicular to the plane of the film. We showed that that is not true. We can get a response even when the magnetic field is in-plane,” said Simranjeet Singh, associate professor of physics.
The finding demonstrates that a magnetization-related Hall response can occur in multiple directions. This gives physicists a new method for studying multidimensional magnetic and topological structures in condensed matter.
“Beyond its fundamental importance, this discovery may enable new planar device architectures and sensor types, such as vector magnetometry, by measuring out-of-plane and in-plane anomalous Hall effect signals within the same device,” Singh said.
Turning theoretical predictions into experiments
Scientists had previously predicted the in-plane anomalous Hall effect through theoretical models, but no experiment had successfully demonstrated it before this study.
“People have proposed it and ideas have been floating around, but it’s very difficult to make magnetic materials with the right symmetry to make it happen,” Singh said. “What we did was find a material with the right symmetry and make it magnetic.”
Building the nanometer-scale devices required for the experiment involved specialized expertise in two-dimensional quantum materials. Singh collaborated with Jyoti Katoch, an associate professor of physics who specializes in fabricating devices from these materials.
The research team, including postdoctoral fellows I-Hsuan Kao and Ravi Kumar, began with tantalum iridium telluride (TaIrTe4). The material’s crystal structure has the symmetry needed to support a multidimensional Hall response. Researchers reduced TaIrTe4 to just a few atomic layers and placed it next to a magnetic material called Cr2Ge2Te6 (CGT).
Because the two layers are positioned close together, the magnetic behavior of CGT influences TaIrTe4, which is normally nonmagnetic. This process gives TaIrTe4 magnetic properties while preserving its underlying electronic characteristics.
“This truly demonstrates the ability of emerging two-dimensional quantum materials to construct atomically precise heterostructures to obtain on-demand electronic and magnetic properties,” Katoch said.
One device can detect multiple magnetic directions
Inside the atomically thin device, the researchers observed both the conventional Hall signal and a second, unconventional signal associated with magnetization within the plane of the material.
This discovery could have important practical applications. A single ultra-thin device may be able to detect magnetic fields along multiple axes.
“We have expanded the range of potential applications for these materials,” Singh said. “Multidimensional magnetic sensing is now possible with just one sensor. Previously, we had to install two sensors to measure magnetic fields in two directions.”
The study points toward magnetic sensing systems that can measure fields in multiple directions without requiring separate sensors for each axis. Such technology could support more compact and efficient sensors for vehicles, electronic devices, robotics, and medical imaging systems.
Explaining the unusual Hall response
Alongside the experiments, Shubayu Chatterjee, assistant professor of physics, used theoretical modeling to investigate why the in-plane anomalous Hall effect appears and how the symmetry of the combined materials enables it.
“We find that the reduced symmetry due to pairing with the CGT further enables spin-orbit coupling at the interface. These spin-orbit coupling terms are important for the appearance of the in-plane anomalous Hall effect after the CGT becomes ferromagnetic at low temperatures. Certain features of the observed anomalous Hall effect signal are consistent with an intrinsic origin, but detailed characterization of several layers of TaIrTe4 is required to pinpoint the exact mechanism,” Chatterjee said.
The LIQUID research team is now investigating other material combinations that could produce the same unconventional Hall response. The researchers are also testing how these devices perform at room temperature, an essential step toward developing practical in-plane anomalous Hall effect sensors and next-generation magnetic technologies.
Source: www.sciencedaily.com