Surface Effects in Ultrathin Iron Oxide Hollow Nanoparticles: Exploring Magnetic Disorder at the Nanoscale

A detailed study of the structural and magnetic properties of polycrystalline hollow γ-Fe2O3 nanoparticles of ∼9.4 nm size was performed. High-resolution transmission electron microscopy images confirmed the crystalline structure and the presence of a ultrathin shell thickness of ∼1.4 nm, implying a very high surface/volume ratio. These hollow nanoparticles were investigated using zero-field and in-field 57Fe Mössbauer spectrometry. The zero-field hyperfine structure suggests some topological disorder, whereas the in-field one shows the presence of a comp magnetic structure that can be fairly described as two opposite pseudosperomagnetic sublattices attributed to octahedral and tetrahedral iron sites. Such an unusual feature is consistent with the presence of noncollinear spin structure originated from the increased surface due to the hollow morphology. Such a complex local spin structure evidenced from Mössbauer experiments was correlated with exchange bias coupling showing at low temperature by magnetization measurements. Monte Carlo simulations on a ferrimagnetic hollow nanoparticle unambiguously corroborate the critical role of the surface anisotropy on the noncollinearity of spin structure in our samples.