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Optical spectra of CoxC60 mixture films (x  <  10) were studied in order to obtain insights into electronic interactions in an organometallic system that is attractive for spin-transfer electronics. The optical absorption (OA) spectra were described in terms of the Lorentz oscillators model, allowing the quantification of the spectral variations driven by Co concentration x. The variations are found to be most pronounced for 0  <  x  <  2–4, suggesting the effects of the electronic interactions in the Co2C60 fulleride formed in the mixture. The Co–C60 electronic interactions evidenced by the OA edge shift and by the OA peak detected at approximately 1 eV give rise to the Jahn–Teller (JT) splitting of the t1u electronic levels in C60 occupied by the electrons transferred from Co. The JT effect was confirmed by observation of the downshifted T1u(4) infrared-active mode modified by electron–phonon coupling at the t1u levels.

We report on the establishing of an exclusive magnetic effect in air-exposed CoC nanocomposites (x > 2) created through self-assembling in the depositing mixture. In order to verify the influence of ambient air on the CoC 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 CoC films in the field cooling (FC) regime. The detected vertical shift of the FC loops is caused by an uncompensated magnetic moment M 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 C-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 CoC 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.

Driving by interplay between plasmonic and magnetic effects in organic composite semiconductors is a challenging task with a huge potential for practical applications. Here, we present evidence of a quantum plasmon excited in the self-assembled Co C nanocomposite films with x > 15 (interval of the Co cluster coalescence) and analyse it using the optical absorption (OA) spectra. In the case of Co C film with x = 16 (LF sample), the quantum plasmon generated by the Co/CoO clusters is found as the 1.5 eV-centred OA peak. This finding is supported by the establishment of four specific C-related OA lines detected at the photon energies E > 2.5 eV. Increase of the Co content up to x = 29 (HF sample) leads to pronounced enhancement of OA intensity in the energy range of E > 2.5 eV and to plasmonic peak downshift of 0.2 eV with respect to the peak position in the LF spectrum. Four pairs of the OA peaks evaluated in the HF spectrum at E > 2.5 eV reflect splitting of the C-related lines, suggesting great change in the microscopic conditions with increasing x. Analysis of the film nanostructure and the plasmon-induced conditions allows us to propose a Rashba-like spin splitting effect that suggests valuable sources for spin polarization.