A decades-old puzzle in quantum physics has yielded to modern experimental evidence, revealing that the behavior of the Kondo effect – a key interaction governing magnetism and conductivity in materials – is dictated by the size of a particle’s spin. Researchers have demonstrated that this effect can either suppress or enhance magnetism, depending on this fundamental property. The findings, published in the journal Nature, reshape understanding of magnetic order at the quantum level and offer new avenues for designing advanced quantum materials.
At the smallest scales, materials exhibit behaviors that defy classical intuition. The Kondo effect, first described in the 1960s, explains how magnetic impurities within a metal interact with surrounding electrons. Traditionally, this interaction was understood to “screen” the impurity’s magnetic moment, effectively neutralizing it and leading to a non-magnetic state. This concept has been central to condensed matter physics for over half a century.
However, real materials are complex, with electrons moving and occupying different orbitals, making it difficult to isolate the pure spin interactions at play. Scientists have often relied on simplified theoretical models, such as the Kondo necklace model proposed in 1977 by Sebastian Doniach, to understand the underlying physics. This model focuses solely on spins and their interactions, but remained largely theoretical until recently. A key question remained: does the Kondo effect always suppress magnetism, or does its behavior change with increasing spin size?
The breakthrough came with the development of a new molecular design framework, known as RaX-D, which allowed researchers to create a highly controlled quantum material. This approach enabled the construction of a system closely matching the Kondo necklace model, with precise control over spin interactions. Previous work had created a version of the system with spin-1/2 units; the new study increased the localized spin to spin-1.
Thermodynamic measurements revealed a surprising phase transition as the temperature dropped. Instead of becoming non-magnetic, the material entered an ordered magnetic state, with spins aligning in a stable, alternating pattern known as Néel order. Quantum analysis explained that the Kondo coupling between spin-1/2 and spin-1 units did not cancel magnetism. Instead, it created an effective magnetic interaction between the spin-1 moments, spreading across the material and locking the spins into long-range order.
“This discovery reveals a quantum principle that depends directly on spin size,” said Yamaguchi, a researcher involved in the study. “The ability to switch between non-magnetic and magnetic states by controlling spin opens powerful new possibilities.”
The research provides the first direct experimental evidence that the Kondo effect’s role fundamentally changes with spin size. It highlights the importance of clean, well-controlled systems for uncovering basic quantum rules. By minimizing complicating factors like charge motion, the researchers were able to expose the core physics at play.
The findings suggest that existing theories may need revision when applied to systems with larger spins. Understanding and controlling magnetism at the quantum level has practical implications for quantum technologies, potentially improving quantum sensors, memory systems, and computing hardware. The ability to design materials that switch between magnetic and non-magnetic states could be particularly valuable.
The research team is continuing to explore materials with even larger spins, seeking to uncover new quantum phases and further refine understanding of the Kondo effect’s role in complex materials.
