Iron oxide magnetic nanoparticles (MNPs) have received increasing attention due to their expanding application fields, from catalysis1to biomedicine2, where they are used as magnetic hyperthermia mediators or contrast agents for enhanced magnetic resonance imaging. A careful comprehension of the spin state and dynamics in such structures remains a key-point to understand and manipulate their magnetic behavior. Often the processes involving the surface spins depend not only on the uncompensated atomic structure, typical of the surface geometry, but also on a variation of the electronic state of the surface atoms related to processes like oxidation or contamination. The turn to element sensitive techniques like the x-ray absorption spectroscopy (XAS) and the x-ray magnetic circular dichroism (XMCD) offers to the scientist the capability of combining and exploring these two aspects3.
We have conceived a XMCD measurements procedure at low magnetic field (160 Oe) in hollow and full maghemite NPs in order to disclose the presence of an increased magnetic disorder in the hollow NPs system compared to the full one. The results show that the state of oxidation in the maghemite NPs is preserved in the full as well as in the hollow structures. However part of the spins in the hollow NPs cannot follow the magnetic order forced by the small external field. This reflects an increased magnetic disorder. By comparing total and partial electron yield measurements, it results that the disordered spins are located on the external surface of the maghemite hollow NPs.
1M. Hermanek, R. Zboril, I. Medrik, J. Pechousek, and C. Gregor, J. Am. Chem. Soc. 129, 10929 (2007).
2A. Figuerola, R. Di Corato, L. Manna, and T. Pellegrino, Pharmacol. Res. 62, 126 (2010).
3S. Brice-Profeta, M.-A. Arrio, E. Tronc, N. Menguy, I. Letard, C. Cartier dit Moulin, M. Noguès, C. Chanéac, J.-P. Jolivet, and P. Sainctavit, J. Magn. Magn. Mater. 288, 354 (2005).