Evidence of interface exchange magnetism in self-assembled cobalt-fullerene nanocomposites exposed to air

We report on the establishing of an exclusive magnetic effect in air-exposed Co_xC₆₀ nanocomposites (x > 2) created through self-assembling in the depositing mixture. In order to verify the influence of ambient air on the Co_xC₆₀ mixture film, we have studied in detail the film magnetization at rather low temperatures, which provides their ferromagnetic behavior. Tracing the possible exchange bias effect, we distinguished a clear vertical shift of the hysteresis loops recorded for the air-exposed Co_xC₆₀ films in the field cooling (FC) regime. The detected vertical shift of the FC loops is caused by an uncompensated magnetic moment M _u induced by exchange coupling of the Co spins at the Co/CoO interface. This interface arises due to the oxidation of small Co clusters distributed in a C₆₀-based matrix of self-assembled composite films, which occurs during air exposure. The core-shell structure of the Co/CoO magnetic clusters (about 2-3 nm in size) consisting of a ε-Co core and fcc-CoO shell was confirmed by means of transmission electron microscopy. Established interface magnetism testifies to a composite nanostructure in the Co_xC₆₀ mixture film with x > 2 and explains the influence of air exposure on the film structure. The discovered magnetic effect implies a new application potential for cobalt-fullerene composites in sensors and catalysis.