From University of Chicago Pritzker School of Molecular Engineering: New Material for Optically-Controlled Magnetic Memory Discovered

By Sarah C.P. Williams, The University of Chicago , Pritzker School of Molecular Engineering

Researchers at the University of Chicago Pritzker School of Molecular Engineering (PME) have made unexpected progress toward developing a new optical memory that can quickly and energy-efficiently store and access computational data.

Researchers in Yang Lab at UChicago Pritzker School of Molecular Engineering have made unexpected progress toward developing new optical memory that can quickly and energy-efficiently store and access computational data.
(Illustration by Peter Allen)

While studying a complex material composed of manganese, bismuth and tellurium (MnBi2Te4), the researchers realized that the material’s magnetic properties changed quickly and easily in response to light. This means that a laser could be used to encode information within the magnetic states of MnBi2Te4.

“This really underscores how fundamental science can enable new ways of thinking about engineering applications very directly,” said Shuolong Yang, assistant professor, molecular engineering and senior author of the new work. “We started with the motivation to understand the molecular details of this material and ended up realizing it had previously undiscovered properties that make it very useful.”

In a paper published in Science Advances, Yang and colleagues showed how the electrons in MnBi2Te4 compete between two opposing states – a topological state useful for encoding quantum information and a light-sensitive state useful for optical storage.

Solving topological puzzle
In the past, MnBi2Te4 has been studied for its promise as a magnetic topological insulator (MTI), a material that behaves like an insulator on its interior but conducts electricity on its outer surfaces. For an ideal MTI in the 2D limit, a quantum phenomenon emerges in which an electric current flows in a 2Dstream along its edges. These so-called ‘electron freeways’ have the potential to encode and carry quantum data.

While scientists have predicted that MnBi2Te4 should be able to host such an electron freeway, the material has been hard to work with experimentally.

“Our initial goal was to understand why it has been so hard to get these topological properties in MnBi2Te4,” said Yang. “Why is the predicted physics not there?”

To answer that question, Yang’s group turned to spectroscopy methods that let them visualize the behavior of the electrons within MnBi2Te4 in real time on ultrafast time scales. They used time- and angle-resolved photoemission spectroscopy developed in the Yang lab, and collaborated with Xiao-Xiao Zhang’s group at the University of Florida to perform time-resolved magneto-optical Kerr effect (MOKE) measurements, which allows the observation of magnetism.

“This combination of techniques gave us direct information on not only how electrons were moving, but how their properties were coupled to light,” explained Yang.

Two opposing states
When the researchers analyzed their spectroscopy results, it was clear why MnBi2Te4 was not acting as a good topological material. There was a quasi-2D electronic state, which was competing with the topological state for electrons.

“There is a completely different type of surface electrons that replace the original topological surface electrons,” said Yang. “But it turns out that this quasi-2D state actually has a different, very useful property.”

The 2nd electronic state had a tight coupling between magnetism and external photons of light – not useful for sensitive quantum data but the exact requirements for an efficient optical memory.

To further explore this potential application of MnBi2Te4, Yang’s group is now planning experiments in which they use a laser to manipulate the material’s properties. They believe that an optical memory using MnBi2Te4 could be orders of magnitude more efficient than today’s typical electronic memory devices.

Yang also pointed out that a better understanding of the balance between the 2 electron states on the surface of MnBi2Te4 could boost its ability to act as an MTI and be useful in quantum data storage.

“Perhaps we could learn to tune the balance between the original, theoretically predicted state and this new quasi-2D electronic state,” he said. “This might be possible by controlling our synthesis conditions.”

Funding: This work was supported by the U.S. Department of Energy and the National Science Foundation.

Article: Distinguishing surface and bulk electromagnetism via their dynamics in an intrinsic magnetic topological insulator

Science Advances has published an article written by Khanh Duy Nguyen, Woojoo Lee, Pritzker School of Molecular Engineering, The University of Chicago, Chicago, IL 60637, USA, Jianchen Dang, Tongyao Wu, Department of Physics, University of Florida, Gainesville, FL 32611, USA, Gabriele Berruto, Chenhui Yan, Chi Ian Jess Ip, Haoran Lin, Qiang Gao,Pritzker School of Molecular Engineering, The University of Chicago, Chicago, IL 60637, USA, Seng Huat Lee, Department of Physics, Pennsylvania State University, University Park, PA 16802, USA.Binghai Yan, Department of Condensed Matter Physics, Weizmann Institute of Science, Rehovot 7610001, Israel, Chaoxing Liu, Zhiqiang Mao, Department of Physics, Pennsylvania State University, University Park, PA 16802, USA, Xiao-Xiao Zhang, Department of Physics, University of Florida, Gainesville, FL 32611, USA, and Shuolong Yang, Pritzker School of Molecular Engineering, The University of Chicago, Chicago, IL 60637, USA.

Abstract: “The indirect exchange interaction between local magnetic moments via surface electrons has been long predicted to bolster the surface ferromagnetism in magnetic topological insulators (MTIs), which facilitates the quantum anomalous Hall effect. This unconventional effect is critical to determining the operating temperatures of future topotronic devices. However, the experimental confirmation of this mechanism remains elusive, especially in intrinsic MTIs. Here, we combine time-resolved photoemission spectroscopy with time-resolved magneto-optical Kerr effect measurements to elucidate the unique electromagnetism at the surface of an intrinsic MTI MnBi₂Te₄. Theoretical modeling based on 2D Ruderman-Kittel-Kasuya-Yosida interactions captures the initial quenching of a surface-rooted exchange gap within a factor of two but overestimates the bulk demagnetization by one order of magnitude. This mechanism directly explains the sizable gap in the quasi-2D electronic state and the nonzero residual magnetization in even-layer MnBi₂Te₄. Furthermore, it leads to efficient light-induced demagnetization comparable to state-of-the-art magnetophotonic crystals, promising an effective manipulation of magnetism and topological orders for future topotronics.“