Significant Surface Spin Effects and Exchange Bias in Iron Oxide-Based Hollow Magnetic Nanoparticles

Exchange bias (EB) properties have become especially important in hollow magnetic nanoparticles (MNPs) due to the versatility and reduced size of these materials. In this work, we present the synthesis and study of the EB properties of iron-oxide-based hollow MNPs and their precursors Fe/iron oxide MNPs with core/void/shell structure. The two mechanisms involved in EB generation were investigated: the frozen spins present in the nanograins that form the nanoparticles and the surface spins. The effect of external parameters on the coercivity (H_C), remanence (M_R), exchange bias field (H_EB) and frozen spins, such as cooling field (H_FC) and temperature, was investigated. Both H_C and H_EB present a maximum threshold above which their values begin to decrease with H_FC, showing a new trend of H_EB with H_FC and allowing modulation on demand. The existence of surface spins, present on the outer and inner surfaces, was demonstrated, and an intrinsic EB phenomenon (H_EB = 444 Oe for hollow iron oxide-based MNPs of 13.1 nm) with significant magnetization (M_S~50 emu/g) was obtained. Finally, core/void/shell MNPs of 11.9 nm prior to the formation of the hollow MNPs showed a similar behavior, with non-negligible H_EB, highlighting the importance of surface spins in EB generation.