Contrasting magnetism in dilute and supersaturated cobalt–fullerene mixture films

Abstract The combination of cobalt with nanocarbons promises hybrid nanostructures that are ideal for the development of memory storage and spin-transfer electronics. Here, we report a dramatic effect of composition on the magnetic properties of the Co x C 60 mixtures, whose nanostructure was organized upon simultaneous deposition and sequential exposure to air. We assert a critical change in the mixture’s organization yielding either the composite nanostructure as array of the Co/CoO core–shell nanoparticles (NPs) in the C 60 -based matrix at a high content of Co (a supersaturated mixture or SSM) or a coexistence of fcc-C 60 and Co a C 60 fulleride when the Co content x is lower than some critical value (an ultradilute mixture or UDM). Magnetization of the SSM composite exhibits a superparamagnetic effect caused by the small Co/CoO NPs. Similar magnetization of the UDM with revealed a stable ferromagnetism and evidenced the formation of a magnetic Co 2 C 60 fulleride. Phase composition in the UDM and SSM films was verified with the XRD and Raman spectra. The UDM and SSM films reveal great difference in content of the remaining oxygen which implies easy diffusion of O 2 molecules within the C 60 -based phases and their splitting at the Co NP surface followed by formation of CoO shells. The results obtained indicate controlled access to a variety of promising Co–C 60 magnetic nanostructures.

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