Spin-Vibronic Coupling and Magnetic Blocking Temperature in High-Performance Single-Atom Magnet NpO@MgO

Actinide-based single-atom magnet (SAM) represents a promising platform for ultrahigh-density magnetic data storage. However, their performance is fundamentally constrained by low effective barriers (Ueff) and blocking temperature (TB). Traditional design focuses on maximizing static magnetic anisotropy, often overlooking how spin-vibronic coupling governs Ueff and TB. Through integrated multi-level calculations and ab initio spin dynamics simulation, we demonstrate a paradigm shift: spin-vibronic coupling, not spin levels, dictates performance. The apical oxygen coordination transforms Np@MgO (Ueff = 46.3 meV) into the high-performance system NpO@MgO, yielding an intrinsic high barrier (Ueff = 329.4 meV) and a prolonged quantum-tunneling time (τQTM =2.22 s). Crucially, strong spin-vibronic coupling with a low-energy twisting mode at 8.17 meV drives efficient two-phonon Raman relaxation below 32 K, thereby reducing Ueff to merely 8.17 meV. Despite this dynamic bottleneck, NpO@MgO maintains a high TB of 50 K. Our work establishes that suppressing detrimental low-energy vibrational modes, rather than solely optimizing static anisotropy Ueff, is essential for advancing SAM performance, thereby introducing a “spin-phonon landscape engineering” paradigm for future design of molecular nanomagnets.