Spin-Polarized Luminescence Modulated by Magnetic Coupling in Glass-Embedded Eu ²⁺ -Doped Lead-Free Perovskite Nanocrystals

Optically active spin-based solids are vital for spintronics, yet materials with robust, room-temperature spin polarization remain elusive. Eu 2+ ions, with their high-spin S = 7/2 ground state, are ideal candidates, but their emission spin polarization is typically quenched at elevated temperatures, as thermal fluctuations easily overwhelm the finite Zeeman splitting energy. Here, we demonstrate high-efficiency, room-temperature spin-polarized luminescence from trace-doped CsCaCl 3:Eu 2+ perovskite nanocrystals, stabilized via in situ glass crystallization. The system exhibits a high degree of circular polarization of ≈45% at cryogenic temperatures, arising from the spin-coupled 4f 6 5d 1 state. We further elucidated the underlying magnetic interactions, revealing a concentration-driven crossover from ferromagnetic to antiferromagnetic coupling, definitively confirmed by the sign reversal of the Weiss constant. Crucially, the strong exchange interaction intrinsic to this specific spin-bound configuration functions as a locking mechanism that preserves the Zeeman splitting against thermal fluctuations, enabling distinct circularly polarized emission even at room temperature. We leverage this to demonstrate reversible, room-temperature emission magnetic encoding, identifying Eu 2+ -doped perovskites as a platform for ambient-condition spintronic and quantum technologies.