Speaker
Description
Magnetic nanoparticles have been extensively studied over the past few decades due to their unique magnetic properties, which are strongly influenced by finite size effects and, more prominently, by surface effects resulting from their high surface-to-volume ratio ($R = S/V$). The synthesis of spherical hollow magnetic nanoparticles takes this a step further, significantly enhancing the surface-to-volume ratio [1]. The possibility to synthesize spherical hollow magnetic nanoparticles allows $R$ to be further enhanced with a consequent increase of topological disorder and magnetic frustration, thus opening new perspectives to explore the surface magnetism at the nanoscale. In addition, hollow spherical nanoparticles can be considered as a thin spherical shell, i.e., as one of the simplest $3$D object for studying the effect of curvature at the nanoscale level After a short review about the more exciting results about possibility to prepare different hollow nanostructures, this talk will present a comparative study of the morpho-structural and magnetic properties of full and hollow maghemite ($\gamma$-Fe$_2$O$_3$) nanoparticles, characterized by large surface to volume ratio, of corresponding sizes $\sim 5.0$nm and $\sim 7.5$nm. These systems have been thoroughly characterized by means of DC and AC magnetization measurement and in field $^{57}$Fe Mössbauer spectrometry. The in-field hyperfine structure analysis confirm the presence of a non-collinear magnetic structure in the hollow NPs, originated from the increased surface effects attributed to their hollow morphology. Interestingly, after field cooling, a horizontal shift of the hysteresis loop was observed, revealing the presence of an exchange bias (EB) effect. The observed EB effect was systematically analyzed in relation to temperature and cooling field parameters. Complementing these experimental results, Monte Carlo (MC) simulations of assemblies of ferrimagnetic hollow nanoparticles revealed that strong exchange coupling between spins in the external thicker surface and at the interface enhances the antiferromagnetic behavior of the hollow nanoparticle, resulting in a decrease of their saturation magnetization (Ms).
Acknowledgements
This work was partially supported by Project funded under the National Recovery and Resilience Plan (NRRP), Mission 4 Component 2 Investment 1.3 - Call for tender No. 1561 of 11.10.2022 of Ministero dell’Università e della Ricerca (MUR); funded by the European Union – NextGenerationEU • Award Number: Project code PE0000021, Concession Decree No. 1561 of 11.10.2022 adopted by Ministero dell’Università e della Ricerca (MUR), CUP D33C22001330002 -Project title “Network 4 Energy Sustainable Transition – NEST
References
[1] F. Sayed et al., “Surface Effects in Ultrathin Iron Oxide Hollow Nanoparticles: Exploring Magnetic Disorder at the Nanoscale,” The Journal of Physical Chemistry C, vol. 122, no. 13. American Chemical Society (ACS), pp. 7516–7524, Mar. 26, 2018. https://doi.org/10.1021/acs.jpcc.8b00300
