A team at Osaka Metropolitan University has shown that the Kondo effect, a key quantum phenomenon, behaves oppositely depending on spin size. For small spins, it suppresses magnetism, but for larger ones, it promotes magnetic order. This finding challenges long-held views and could advance quantum materials.
In the realm of condensed matter physics, collective interactions among quantum spins can lead to unexpected behaviors. The Kondo effect, which describes how localized spins interact with mobile electrons, has long been central to understanding quantum systems. Traditionally viewed as a suppressor of magnetism, this effect now reveals a surprising duality.
A research group led by Associate Professor Hironori Yamaguchi from Osaka Metropolitan University's Graduate School of Science engineered a Kondo necklace model using an organic-inorganic hybrid material of organic radicals and nickel ions. This setup, enabled by the RaX-D molecular design framework, allowed precise control over crystal structure and magnetic interactions.
Building on prior work with spin-1/2 systems, the team increased the localized spin to 1. Thermodynamic measurements indicated a phase transition into a magnetically ordered state. Quantum analysis revealed that the Kondo coupling generates effective magnetic interactions between spin-1 moments, stabilizing long-range order.
This overturns the classic perspective where the Kondo effect forms nonmagnetic singlets for spin-1/2, locking spins into zero total spin states. For spins exceeding 1/2, it instead fosters magnetism. The study marks the first experimental confirmation of this spin-size dependency in a clean, spin-only platform.
The Kondo necklace concept dates back to 1977, proposed by Sebastian Doniach, but experimental realization eluded scientists for decades due to complications from electron motion and orbitals in real materials.
"The discovery of a quantum principle dependent on spin size in the Kondo effect opens up a whole new area of research in quantum materials," Yamaguchi stated. "The ability to switch quantum states between nonmagnetic and magnetic regimes by controlling the spin size represents a powerful design strategy for next-generation quantum materials."
Such control could shape properties like entanglement and magnetic noise, paving the way for spin-based quantum devices and computing technologies. The findings appear in Communications Materials (2026, volume 7, issue 1).