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

Abstract

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 (HC), remanence (MR), exchange bias field (HEB) and frozen spins, such as cooling field (HFC) and temperature, was investigated. Both HC and HEB present a maximum threshold above which their values begin to decrease with HFC, showing a new trend of HEB with HFC and allowing modulation on demand. The existence of surface spins, present on the outer and inner surfaces, was demonstrated, and an intrinsic EB phenomenon (HEB = 444 Oe for hollow iron oxide-based MNPs of 13.1 nm) with significant magnetization (MS~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 HEB, highlighting the importance of surface spins in EB generation.

Diversity of Iron Oxides: Mechanisms of Formation, Physical Properties and Applications

“…In this case, the MSP technique can give additional information on the crystal structure, including magnetite to maghemite partial transition or the superparamagnetic state of NPs [38]. MSP can be applied to characterize samples composed of homogenously sized iron oxide NPs above the blocking temperature in a superparamagnetic regime [368]. Mössbauer measurements were performed to investigate detailed iron mineralogy compositions in magnetic fractions of fly ashes [405].…”

“…The use of ferritin protein as a carrier of iron oxide NPs renders it more suitable for cancer diagnosis as an effective T 2 contrast agent with an expected reduced toxicity due to the prevention of NP interaction with the environment [367]. Two-phase iron oxide NPs (e.g., magnetite/maghemite core-shell structures) are promising for applications implying an intrinsic exchange bias effect [368].…”

Thermal Decomposition of Siderite and Characterization of the Decomposition Products under O2 and CO2 Atmospheres

“…The δ values of this doublet are in the range 0.31-0.43 mm/s and the ∆ values are in the range 0.88-1.07 mm/s. Such hyperfine parameters can be related to iron-oxide nanoparticles (NPS) [40][41][42]. To be sure that this doublet is related to the NPS, the S1 sample annealed at 400 • C was measured at the liquid nitrogen (LN) temperature.…”