Coupled Electronic and Structural Dynamics Enable Spin Switching in Endohedral Fullerenes

The pursuit of information storage with ultrahigh density has positioned single-atom magnets as key components for future spintronic applications. However, the magnetic stability of isolated atoms is often compromised by the environmental decoherence. Although large fullerene cages can stabilize single-atom magnetism, their high structural rigidity limits the efficient spin-state regulation. In this Letter, we employed first-principles calculations to systematically investigate the endohedral fullerene series M@C_2n (M = Fe, Cr, Co, Mn; 2n = 20-40) and identified several systems exhibiting pronounced spin isomerism. We further focused on Co@C₃₆, which shows the strongest thermodynamic competition between the high-spin and low-spin states, and investigated the magnetic and transport properties dependent on the spin state. In addition, pulsed laser excitation simulations suggest a feasible route for controlled spin-state interconversion. These findings establish Co@C₃₆ as an atomically precise magnetic switch, providing new insights into controlled spin engineering for nanoscale logic devices.