A research team, led by Associate Professor Hironori Yamaguchi at Osaka Metropolitan University, has found that the Kondo effect behaves differently depending on spin size. In systems with small spins, it suppresses magnetism, but when spins are larger, it actually promotes magnetic order. This discovery highlights a new quantum boundary with major implications for future materials.
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. 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.
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 thinking. 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.
"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.