Ferromagnetism in stacked bilayers of Pd/C₆₀

We provide experimental evidence for the existence of ferromagnetism in bilayers of Pd/C₆₀ which is supported by theoretical calculations based on density functional theory (DFT). The observed ferromagnetism is surprising as C₆₀ and Pd films are both non-ferromagnetic in the non-interacting limit. Magnetization (M) versus applied field (H) data acquired at different temperatures (T) show magnetic hysteresis with typical coercive fields (H_c) on the order of 50 Oe. From the temperature-dependent magnetization M(T) we extract a Curie temperature (^TC≥550K) using Bloch-like power law extrapolations to high temperatures. Using DFT calculations we investigated all plausible scenarios for the interaction between the C ₆₀ molecules and the Pd slabs, Pd single atoms and Pd clusters. DFT shows that while the C₆₀ molecules are nonmagnetic, Pd films have a degenerate ground state that, subject to a weak perturbation, can become ferromagnetic. Calculations also show that the interaction of C₆₀ molecules with excess Pd atoms and with sharp edges of a Pd slab is the most likely configuration that render the system ferromagnetic. Interestingly, the calculated charge transfer (0.016 e per surface Pd atom, 0.064 e per Pd for intimate contact region) between C₆₀ and Pd does not appear to play an important role.