Light Triggers Magnetism in Nonmagnetic Semiconductor... The Secret: "Two Atomic Layers" [Reading Science]

GIST and POSTECH Confirm Induction of Temporary Magnetism with Linearly Polarized Light
Coupling Between Two Atomic Vibrations Is Key... Findings Point to Next-Generation Spintronics Research

Shining light on a semiconductor with no magnetic properties produced temporary magnetism. The phenomenon, which was not observed when the semiconductor consisted of a single atomic layer, appeared when two layers were stacked. A South Korean research team found that the key lies in the coupling of two atomic vibrations. The study points to the potential of next-generation information-processing technologies that rapidly control a material's magnetism with light.

Gwangju Institute of Science and Technology (GIST) said on October 12 that a joint research team led by Professor Shin Dongbin of its Department of Physics and Photon Science and Professor Kim Dongeon of the Department of Physics at Pohang University of Science and Technology (POSTECH) had induced temporary magnetism in a nonmagnetic two-dimensional semiconductor by shining light on it and identified the underlying mechanism. The findings were published in the international journal Nature Communications on October 5.

The principle and experimental results of inducing transient magnetism by shining light on a bilayer molybdenum diselenide (MoSe2) semiconductor. The left shows a schematic of the magnetism-induction concept; the upper right shows atomic vibrations; and the lower right shows the magnetic signal observed under linearly polarized light. Courtesy of the research team.

Magnetism Absent in One Layer Emerges in Two

The material used by the research team was a two-dimensional semiconductor made of molybdenum and selenium atoms called molybdenum diselenide (MoSe2). The team shone light on semiconductors consisting of one and two atomic layers, respectively, and compared the resulting changes.

The experiments used linearly polarized light, in which the electric field oscillates in one direction. Previous research has focused mainly on controlling electron spin using circularly polarized light, in which the direction of the electric field rotates. Spin is a property that makes an electron act like a tiny magnet, and technologies that use its direction to process information are attracting attention.

At room temperature, the single-layer semiconductor showed no magnetic signal. In contrast, the two-layer semiconductor produced a clear rotation of the light's polarization, a signal associated with the presence of magnetism. The findings confirmed the possibility of inducing magnetism using linearly polarized light alone, without an external magnetic field or circularly polarized light.

The research team traced the cause to the motion of the atoms. The breathing mode between layers, in which the two layers move closer together and farther apart, couples with the selenium atoms' stretching vibrations within each layer, creating complex atomic motion. The team concluded that this process produces an effect equivalent to applying a magnetic field to the electron spins, thereby inducing temporary magnetism.

The team detected an interlayer breathing mode at 0.9 terahertz (THz) and selenium stretching vibrations at 5.1 THz. The observation of the two theoretically predicted vibrations in the experiments supported the proposed mechanism for inducing magnetism.

Lasting Tens of Picoseconds at Room Temperature, with Potential for Ultrafast Control

The research team analyzed changes in the magnetic signal by varying the light intensity and energy, as well as the semiconductor's temperature. The magnetic signal grew stronger as the light intensity increased and was strongest when the light energy was around 1.55 electron volts (eV). This matched the energy at which electrons and holes bind inside the semiconductor.

The team also observed differences depending on temperature. As the temperature decreased, the magnetic signal weakened and disappeared more quickly. At room temperature, however, stronger atomic vibrations caused the signal to persist for tens of picoseconds after the light was shone on the material. One picosecond is one trillionth of a second.

GIST-POSTECH joint research team. (From left) Seo Mingyo, Ph.D. student in the Department of Physics at Pohang University of Science and Technology (POSTECH); Kim Hanbyeol, student in the integrated master's and doctoral program in the Department of Physics and Photonics at GIST; Professor Shin Dongbin; and Kim Dongeon, professor in the Department of Physics at POSTECH.

The study is significant because it shows that light and atomic vibrations can produce temporary magnetism even in materials that are not magnetic. However, it did not demonstrate stable control of magnetism or information storage in an actual semiconductor device. Further research is needed to develop the findings into spintronics, which uses spin to process information, and ultrafast information-processing technologies.

"This study confirms that atomic vibrations induced by light and interactions between layers play an important role in inducing magnetism," said GIST Professor Shin Dongbin. "It is particularly significant in showing that atomic vibrations can act as an intermediary linking light with the spins of matter."

"We confirmed in an actual two-layer semiconductor the magnetic induction phenomenon predicted theoretically and were able to experimentally examine the relationship between atomic vibrations and the magnetic signal," said POSTECH Professor Kim Dongeon. "We hope these findings will help expand research into controlling the magnetism of materials using light and atomic vibrations."

This content was produced with the assistance of AI translation services.