Tunable nanostructure and emergent nanomagnetism in self-assembled iron–fullerene hybrid films exposed to air

We report the formation of self-assembled Fe x O y C 60 hybrid films using co-deposition of iron and C 60 from the vapor and following exposure to air. The nanostructure and phase composition of the created hybrids consisting of Fe-oxide nanoparticles and a fullerene-based matrix can be controlled by the iron concentration x that we confirmed by a thorough structural analysis of the films with rather different two x values. The films with higher x consist of Fe 3 O 4 nanoparticles with a size of about 6 nm, whereas the films with lower x have the γ-Fe 2 O 3 nanoparticles with a size of about 3 nm. Both types of the hybrid films exhibit superparamagnetic behavior revealing high magnetic anisotropy of the Fe-oxide nanoparticle ensembles formed in the hybrid films. Field-cooled magnetization curves measured at low temperature demonstrate the hysteresis loops shifted along both the field and magnetization axes reflecting complex exchange bias effect in the created hybrid systems. The exchange-bias field is significantly larger in the film with lower x , which reflects a dominant role of the particle surface and interface in arrangement of composite spin structure promoting the magnetic effects. The difference in the nanoparticle oxide phases originates from distinct oxidation pathways controlled by the structural state of C 60 molecules at the interface and by the particle size. The obtained results demonstrate an effective route to access the tunable Fe x O y C 60 hybrid systems with potential application in spintronic technologies.