In condensed matter physics, some of the most unusual behaviors appear only when many quantum particles interact as a group. A single quantum spin on its own behaves in relatively simple ways, but when spins influence each other across a material, entirely new effects can emerge. Explaining how these collective interactions arise is a central challenge in modern physics.

One of the most important collective phenomena is the Kondo effect. It describes how localized quantum spins interact with mobile electrons in a material and plays a major role in shaping the behavior of many quantum systems.

Why Studying the Kondo Effect Is So Difficult

In real materials, isolating the core physics of the Kondo effect is not easy. Electrons do not only carry spin. They also move through the material and occupy different orbitals, introducing charge motion and additional degrees of freedom. When all of these effects occur at once, it becomes difficult to separate the spin interactions that drive the Kondo effect from everything else happening in the system.

To address this complexity, physicists have long relied on simplified theoretical models. One of the most influential is the Kondo necklace model, introduced in 1977 by Sebastian Doniach. This model removes electron motion and orbital effects, leaving behind a system made entirely of interacting spins. Although it has been widely viewed as a powerful framework for exploring new quantum states, realizing it experimentally remained an open challenge for nearly fifty years.

Does Spin Size Change Quantum Behavior

A fundamental question has persisted for decades. Does the Kondo effect behave the same way for all spin sizes, or does changing the size of the localized spin alter the outcome? Answering this question is critical for understanding quantum materials more broadly.

A research team led by Associate Professor Hironori Yamaguchi of the Graduate School of Science at Osaka Metropolitan University has now provided an answer. The team created a new type of Kondo necklace using a carefully engineered organic inorganic hybrid material made from organic radicals and nickel ions. This precise design was achieved using RaX-D, a molecular design framework that allows fine control over crystal structure and magnetic interactions.

From Spin One Half to Spin One

The researchers had previously succeeded in building a spin-1/2 Kondo necklace. In their latest work, they extended the system by increasing the localized spin (decollated spin) from 1/2 to 1. Thermodynamic measurements revealed a clear phase transition, showing that the system entered a magnetically ordered state.

Detailed quantum analysis explained the origin of this change. The Kondo coupling creates an effective magnetic interaction between spin-1 moments, which stabilizes long range magnetic order across the material.

Challenging a Longstanding View of Magnetism

For many years, the Kondo effect was thought to mainly suppress magnetism by locking spins into singlets, a maximally entangled state with zero total spin. The new results overturn this traditional picture. When the localized spin exceeds 1/2, the same Kondo interaction no longer weakens magnetism. Instead, it actively promotes magnetic order.

By directly comparing spin-1/2 and spin-1 systems within a clean, spin-only platform, the researchers identified a clear quantum boundary. The Kondo effect always forms local singlets for spin-1/2 moments, but it stabilizes magnetic order for spin-1 and higher.

This work provides the first direct experimental evidence that the role of the Kondo effect fundamentally depends on spin size.

Implications for Quantum Materials and Technology

"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 said. "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."

Showing that the Kondo effect can operate in opposite ways depending on spin size offers a new perspective on quantum matter and establishes a fresh conceptual foundation for designing spin-based quantum devices.

Being able to control whether a Kondo lattice becomes magnetic or non-magnetic is especially important for future quantum technologies. Such control could influence key properties such as entanglement, magnetic noise, and quantum critical behavior. The researchers hope their findings will guide the development of new quantum materials and eventually contribute to emerging technologies, including quantum information devices and quantum computing.