Researchers from Japan have unveiled a groundbreaking theoretical framework predicting the emergence of non-reciprocal interactions in magnetic metals using light. This discovery, led by Associate Professor Ryo Hanai from the Institute of Science Tokyo, suggests that by irradiating light of a specific frequency onto a magnetic metal, one can induce a torque that causes two magnetic layers to engage in a spontaneous, persistent “chase-and-run” rotation.
The findings, published in the journal Nature Communications, open new avenues in non-equilibrium materials science and propose novel applications in light-controlled quantum materials. This development represents a significant step forward in understanding and manipulating non-reciprocal interactions in solid-state systems.
Understanding Non-Reciprocal Interactions
In equilibrium, physical systems typically adhere to Newton’s third law, where every action has an equal and opposite reaction. However, non-reciprocal interactions, which effectively violate this law, are common in non-equilibrium systems such as biological or active matter. For example, the interaction between inhibitory and excitatory neurons in the brain, predator-prey dynamics, and colloids in optically active media all demonstrate non-reciprocal interactions.
The research team, including Associate Professor Daiki Ootsuki from Okayama University and Assistant Professor Rina Tazai from Kyoto University, has successfully demonstrated a method to induce such interactions in solid-state systems using light. Their work answers a long-standing question about the possibility of implementing non-reciprocal interactions in electronic systems.
Mechanism of Light-Induced Non-Reciprocal Effects
According to Hanai, the study proposes a method to transform ordinary reciprocal spin interactions into non-reciprocal ones through the use of light. A well-known interaction in magnetic metals, the Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction, can acquire a non-reciprocal character when the material is exposed to light at a frequency that selectively opens a decay channel for certain spins, while others remain off-resonant.
The team developed a dissipation-engineering scheme that uses light to selectively activate decay channels in magnetic metals. These metals, which possess localized spins and freely moving conduction electrons, undergo spin-exchange coupling. By activating decay channels, an energy imbalance is created between different spins, resulting in non-reciprocal magnetic interactions.
Implications and Future Applications
The researchers applied their dissipation-engineering scheme to a bilayer ferromagnetic system, predicting a non-equilibrium phase transition known as a non-reciprocal phase transition. This transition was previously introduced by one of the authors in the context of active matter. It involves one magnetic layer attempting to align with the other while the other tends to anti-align when exposed to light, leading to a continuous rotation of magnetization—a “chiral” phase characterized by persistent chase-and-run dynamics.
Hanai notes,
“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. It could be applied to Mott insulating phases of strongly correlated electrons, multi-band superconductivity, and optical phonon-mediated superconductivity.”
This research could potentially lead to the development of new types of spintronic devices and frequency-tunable oscillators, highlighting the applicability of non-reciprocal interactions to solid-state systems and their potential implications for innovative next-generation technologies.
Overall, the study represents a significant advancement in the field of non-equilibrium materials science, offering new insights and tools for the manipulation of quantum materials using light.
