Photoinduced non-reciprocal interactions in magnetic metals

A theoretical framework predicts the emergence of non-reciprocal interactions that effectively violate Newton’s third law in solids using light, report researchers from Japan. They demonstrate that by irradiating light of a carefully tuned frequency onto a magnetic metal, one can induce a torque that drives two magnetic layers into a spontaneous, persistent “chase-and-run” rotation. This work opens a new frontier in non-equilibrium materials science and suggests novel applications in light-controlled quantum materials.

In equilibrium, physical systems obey the law of action and reaction as per the free energy minimization principle. However, in non-equilibrium systems such as biological or active matter—interactions that effectively violate this law—the so-called non-reciprocal interactions are common. For instance, the brain comprises inhibitory and excitatory neurons that interact non-reciprocally; the interaction between predator and prey is asymmetric, and colloids immersed in an optically active media demonstrate non-reciprocal interactions as well. A natural question arises: Can one implement such non-reciprocal interaction in solid-state electronic systems?

A research team led by Associate Professor Ryo Hanai from the Department of Physics at Institute of Science Tokyo (Science Tokyo), Japan, in collaboration with Associate Professor Daiki Ootsuki from Okayama University, Japan, and Assistant Professor Rina Tazai from Kyoto University, Japan, answered this question affirmatively by proposing a theoretical method to induce non-reciprocal interactions in solid-state systems using light. Their recent findings were published online in Volume 16 of the journal Nature Communications on September 18, 2025.

“Our study proposes a general way to turn ordinary reciprocal spin interactions into non-reciprocal ones using light,” explains Hanai. “As a concrete example, we show that a well-known interaction in magnetic metals—the Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction—can acquire a non-reciprocal character when the material is irradiated with light at a frequency that selectively opens a decay channel for certain spins, while leaving others off-resonant.”

Driven by the ubiquity of active and non-reciprocal phenomena in nature, the team developed a dissipation-engineering scheme that uses light to selectively activate decay channels in magnetic metals. These magnetic metals possess localized spins and freely moving conduction electrons, leading to spin-exchange coupling. Activating decay channels creates an imbalance in energy injection between different spins, resulting in non-reciprocal magnetic interactions.

By applying the dissipation-engineering scheme to a bilayer ferromagnetic system, the researchers predicted a non-equilibrium phase transition called a non-reciprocal phase transition that was previously introduced by one of the authors (Fruchart*, Hanai*, et al., Nature, 2021) in the context of active matter, where one magnetic layer attempts to align with the other, while the other tends to anti-align, when irradiated with light. This leads to a spontaneous and continuous rotation of magnetization—a “chiral” phase characterized by persistent chase-and-run dynamics. This novel “chiral” phase transition is unique to broken action-reaction symmetry. The researchers also found that the required light intensity for inducing non-reciprocal phase transitions was estimated to be within reach of current experimental capabilities.

“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 and could be applied to Mott insulating phases of strongly correlated electrons, multi-band superconductivity, and optical phonon-mediated superconductivity,” concludes Hanai. Furthermore, this work could potentially enable the development of new types of spintronic devices and frequency-tunable oscillators.

Overall, this research sheds light on the applicability of non-reciprocal interactions to solid-state systems and their potential implications for innovative next-generation technologies.

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About Institute of Science Tokyo (Science Tokyo)
Institute of Science Tokyo (Science Tokyo) was established on October 1, 2024, following the merger between Tokyo Medical and Dental University (TMDU) and Tokyo Institute of Technology (Tokyo Tech), with the mission of “Advancing science and human wellbeing to create value for and with society.”