Researchers from Japan have developed a theoretical framework that highlights the emergence of non-reciprocal interactions in magnetic metals when exposed to light. This groundbreaking study, published in the journal Nature Communications on September 18, 2025, suggests that these interactions could lead to innovative applications in light-controlled quantum materials.
The research team, led by Associate Professor Ryo Hanai from the Institute of Science Tokyo, collaborated with Associate Professor Daiki Ootsuki from Okayama University and Assistant Professor Rina Tazai from Kyoto University. Their findings indicate that by irradiating light at a precisely tuned frequency onto a magnetic metal, a torque can be induced, prompting two magnetic layers to engage in a persistent “chase-and-run” rotation.
Understanding Non-Reciprocal Interactions
In equilibrium, systems typically adhere to Newton’s third law, where every action has a corresponding reaction. However, this principle is often violated in non-equilibrium systems, such as biological entities and active matter. Non-reciprocal interactions are prevalent in these systems; for example, the dynamics between predator and prey or the behavior of neurons in the brain exhibit such characteristics.
The researchers aimed to explore whether these non-reciprocal interactions could be implemented in solid-state electronic systems. Their affirmative answer is a significant advancement in materials science. As Hanai explains, “Our study proposes a general way to turn ordinary reciprocal spin interactions into non-reciprocal ones using light.”
The team particularly focused on the well-known Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction found in magnetic metals. By carefully selecting the frequency of the light used, they demonstrated that it can selectively activate decay channels for certain spins while leaving others unaffected. This selective activation leads to an imbalance in energy injection, resulting in non-reciprocal interactions.
Implications for Future Technologies
The researchers introduced a dissipation-engineering scheme that utilizes light to manipulate the behavior of localized spins and conduction electrons in magnetic metals. By applying this scheme to a bilayer ferromagnetic system, they predicted a new non-equilibrium phase transition termed the non-reciprocal phase transition. This transition leads to a unique “chiral” phase defined by continuous magnetization rotation, an effect characterized by persistent dynamics that break the symmetry of action and reaction.
The light intensity required to induce these transitions is found to be achievable with current experimental technologies, making this research not only theoretical but also practically applicable. Hanai emphasizes, “Our work not only provides a new tool for controlling quantum materials with light but also bridges concepts from active matter and condensed matter physics.”
The potential applications of this research extend to the development of new spintronic devices, frequency-tunable oscillators, and insights into Mott insulating phases of strongly correlated electrons and optical phonon-mediated superconductivity. As such, this study opens a new frontier in non-equilibrium materials science and highlights the broader implications of non-reciprocal interactions in solid-state systems.
Overall, this research presents a compelling case for the applicability of non-reciprocal interactions in advancing next-generation technologies, illustrating the intersection of physics and innovative material applications.
