Some of our most essential everyday objects, such as computers, medical equipment, stereos, generators, and many others, are powered by magnets. We know what happens as computers become more powerful, but what could be possible if magnets became more versatile? What if you could change the physical property that defines their usefulness? What innovation could that catalyze?
That’s the question that MIT Plasma Science and Fusion Center (PSFC) researchers Hang Chi, Yunbo Ou, Jagadeesh Moodera, and their coauthors explore in a recent, open-access paper:Strain-tuned Berry curvature in quasi-two-dimensional chromium telluride.”
To understand the scale of the authors’ discovery, a quick trip back in time is necessary: In 1879, a 23-year-old graduate student named Edwin Hall discovered that when he placed a magnet at right angles to a strip of metal carrying a current, one side of the strip would have a greater charge than the other. The magnetic field deflected the current’s electrons toward the edges of the metal, a phenomenon he named the Hall effect in his honor.
In Hall’s day, the classical system of physics was the only kind, and forces like gravity and magnetism acted on matter in predictable and unchanging ways: just as dropping an apple would cause it to fall, so making a “T” out of a strip of electrified metal and a magnet would cause the Hall effect, period. But that wasn’t really the case; we now know that quantum mechanics plays a role, too.
Imagine classical physics as a map of Arizona, and quantum mechanics as a car trip through the desert. The map provides a macro view and generalized information about the area, but it can’t prepare the driver for all the random events he might encounter, such as an armadillo crossing the road. Quantum spaces, like the journey the driver embarks on, are subject to a different set of local traffic rules. So, while the Hall effect is induced by an applied magnetic field in the classical frame, in the quantum case the Hall effect can occur even without an external field, at which point it becomes an anomalous Hall effect.
When traveling in the quantum world, a person is equipped with the knowledge of the so-called “Berry phase,” named after the British physicist Michael Berry. It serves as a GPS recorder for the car: it is as if the driver has recorded his entire journey from start to finish, and by analyzing the GPS history, one can better map out the peaks and troughs, or the “curvature” of space. This “Berry curvature” of the quantum landscape can naturally shift electrons to one side, creating a Hall effect without a magnetic field, just as hills and valleys dictate the path of a car.
Although many have observed the anomalous Hall effect in magnetic materials, no one had been able to manipulate it by squeezing and/or stretching it—until the authors of this paper developed a method to demonstrate the change in the anomalous Hall effect and Berry curvature in an unusual magnet.
First, they took half-millimeter-thick bases made of aluminum oxide or strontium titanate, which are crystals, and on top of them grew an incredibly gaunt layer of chromium telluride, a magnetic compound. By themselves, these materials wouldn’t do much; but when combined, the magnetism of the film and the interface it formed with the bases on which it was grown caused the layers to stretch or compress.
To further understand how these materials interact, the researchers teamed up with Oak Ridge National Laboratory’s (ORNL) Spallation Neutron Source to conduct neutron scattering experiments—essentially blasting the material with shots of particles and seeing what bounced off—to learn more about the film’s chemical and magnetic properties. Neutrons were an ideal tool to study because they are magnetic but have no electrical charge. The neutron experiments allowed the scientists to build a profile that revealed how the chemical elements and magnetic behaviors changed at different levels as they probed deeper into the material.
The researchers observed an unusual Hall effect and Berry curvature in response to the degree of compression or stretching of the base after the film was applied, which was later confirmed using data modeling and simulation.
Although this breakthrough occurred at the smallest molecular level, the researchers’ discovery has significant, real-world implications. For example, challenging drives store data in minuscule magnetic regions, and if they were built from “stress-tunable” materials, such as film, they could store additional data in regions that were stretched differently. In robotics, stress-tunable materials could be used as sensors that could provide precise feedback on the movements and positioning of robots. Such materials would be particularly useful for “soft robots,” which operate pliable, elastic components that better mimic biological organisms. Or a magnetic device that changes its behavior when bent or flexed could be used to detect minuscule changes in the environment or to create extremely sensitive health monitoring equipment.
In addition to Chi, Ou, and Moodery, who is also in the MIT Physics Department, others involved in the MIT work include postdoc Alexandre C. Foucher and professor Frances Ross in the Department of Materials Science and Engineering.
The other co-authors are: Tim B. Eldred and Wenpei Gao of North Carolina State University; Sohee Kwon, Yuhang Liu, and Roger K. Lake of the University of California, Riverside; Joseph Murray, Michael Dreyer, and Robert E. Butera of the Laboratory for Physical Sciences; Haile A. Ambaye, Valerie Lauter, and Jong K. Keum of ORNL; Alice T. Greenberg, Yuhang Liu, Mahesh R. Neupane, George J. de Coster, Owen A. Vail, Patrick J. Taylor, Patrick A. Folkes, and Charles Rong of the Army Research Lab; Gen Yin of Georgetown University; and Don Heiman of Northeastern University.
This research was supported in part by the U.S. Army Research Office, the U.S. National Science Foundation (NSF), the U.S. Office of Naval Research, the U.S. Air Force Office of Scientific Research, and the MIT-IBM Watson AI Research Lab. Access to facilities was provided by the MIT Materials Research Laboratory, MRSEC, MIT.nano, SNS, and the Center for Nanophase Materials Sciences, the Department of Energy Office of Science User Facilities operated by ORNL, and the Advanced Cyberinfrastructure Coordination Ecosystem: Services and Support supported by NSF.