Quantum spin state stabilized by coupling with classical spins

We introduce a model compound featuring a spin-1/2 frustrated square lattice partially coupled by spin-5/2. A significant magnetization plateau exceeding 60 T could be observed, indicating a quantum state formed by S=1/2 spins in the square lattice. The remaining S=5/2 spins exhibited paramagnetic behavior in the low-field regions. The numerical analysis confirmed that the observed quantum state is a many-body entangled state based on the dominant antiferromagnetic interactions and is strongly stabilized by coupling with spin-5/2. The stabilization of this quantum state can be attributed to a compensation effect similar to magnetic field-induced superconductivity, which serves as a strategy to control the stability of quantum spin states in magnetic fields.