EXPLORING SURFACE EFFECTS IN HOLLOW IRON OXIDE NANOPARTICLES

The large surface-to-volume ratio (R=S/V) in the nanoparticles is proving to be a key factor for the novel physical, chemical, and magnetic properties compared to those of the corresponding bulk material. The possibility to synthesize spherical hollow magnetic nanoparticles allows R to be further enhanced with a consequent increase of the disorder and magnetic frustration, thus opening new perspectives to explore the surface magnetism at the nanoscale. In this context, here we present a comparative study of the morpho-structural and magnetic properties of full and hollow maghemite (γ-Fe2O3) nanoparticles (NPs), characterized by a big surface to volume ratio, of corresponding sizes 5.0(5) nm and 7.4(7) nm. These systems have been thoroughly characterized by means of DC magnetization measurement and in field 57Fe Mössbauer spectrometry. The in-field hyperfine structure analysis suggested the presence of non-collinear structure for hollow NPs originated from the increased surface role due to the hollow morphology. Interestingly, an exchange bias effect was noted in the hollow structure, which is demonstrated by the recorded shifted M(H) hysteresis loop curve after field cooling. Monte Carlo (MC) simulations on ferrimagnetic hollow nanoparticles unambiguously corroborate the critical role of the surface. MC simulation results show that the spins in the external thicker surface and at the interface are strongly exchange coupled. This strong exchange coupling enhances the antiferromagnetic character of the hollow particle leading to the decrease of its Ms. At the interface, this strong spin exchange coupling results in an exchange bias field and in the enhancement of the coercive field, in agreement with the experimental results.