Impacts of surface spins and inter-particle interactions on the magnetism of hollow γ-Fe₂O₃ nanoparticles

A comparative study of the static and dynamic magnetic properties of polycrystalline hollow γ-Fe₂O₃ nanoparticles with two distinctly different sizes of 10.3±1.3nm and 14.8±0.5nm has been performed. High-resolution TEM images confirmed the crystalline structure and the presence of the shell thickness of 2.17±0.28nm and 3.25±0.24nm for the 10nm and 15nm particles, respectively. Quantitative fits of the frequency dependent ac susceptibility to the Vogel-Fulcher model, τ= τ_oexp[E_a/k(T -T_o)], show stronger inter-particle interactions in the 15nm nanoparticles than in the 10nm nanoparticles. A systematic analysis of the room-temperature magnetic loops using the modified Langevin function indicates a stronger effect of disordered surface spins in the 10nm hollow particles as compared to the 15nm hollow particles. Our study suggests that while the effect of disordered surface spins dominates the magnetic behavior of the 10nm hollow particles, both the disordered surface spins and inter-particle interactions contribute to the magnetism of the 15nm hollow particles.