Semi-Magic Fe₁₅^-: A Magnetic Superatom with Unique Stability and Anisotropic Strong Ferromagnetism

Miniaturizing ferromagnetic materials is key for spintronics and high-density data storage but is hindered by superparamagnetic effects and stability challenges at the subnanoscale. Here we prepared pure Fe_n^- clusters and examined their reactions with common gases. The reactions of these clusters exhibit dramatic size dependence, interestingly with n = 13-18 standing out and constantly highlighting the prominent inertness of Fe₁₅^- during sufficient gas-collision conditions. In conjunction with DFT calculations, we reveal that Fe₁₅^- adopts a D_6d-symmetric structure and a high-spin ground state. Its exceptional stability and ferromagnetism stem from a "semi-magic" number of valence electrons of superatomic features, 1S²|2S²|1P⁶|2P⁶||1D⁵|3S¹. This semi-magic Fe₁₅^- cluster shows a magnetic moment up to 50 μ_B and magnetic anisotropy energy up to 1.8 meV. Our findings underscore the critical role of spin accommodation in stabilizing magnetic superatoms, providing potential for designing magnetic subnanoparticles with anisotropic ferromagnetism, applicable in high-density magnetic storage media and spintronics.