Local Magnetic and Electronic Structure of the Surface Region of Postsynthesis Oxidized Iron Oxide Nanoparticles for Magnetic Resonance Imaging

Abstract

Iron oxide nanoparticles (FeO_<i>x</i>-NP) are applied in medicine as contrast agents in magnetic resonance imaging (MRI) where they reduce the spin–spin relaxation time (T₂-time) of absorbing tissue. Hence, control of their magnetic properties is essential for these applications. Magnetic properties strongly depend on the particle size and shape as well as the surface functionalization of the iron oxide nanoparticles. Especially, structural and magnetic disorder in the region close to the surface (1–2 nm) lead usually to a reduced magnetization compared to the corresponding bulk material. Therefore, X-ray magnetic circular dichroism (XMCD) in the total electron yield (TEY) mode is used to investigate local magnetic and electronic properties of the surface region of monodisperse, spherical FeO_<i>x</i>-NPs (Fe₃O₄/γ-Fe₂O₃) before and after the postsynthetic treatment in oxygen-rich environment. Charge transfer multiplet calculations of the XMCD spectra are performed to analyze the contributions of Fe²⁺ and Fe³⁺ at different lattice sites, i.e., either in octahedral or tetrahedral environment. The analysis of the XMCD data reveals that both, the magnetization of the nanoparticle surface region as well as their maghemite to magnetite ratio, are strongly increased after tempering in an oxidative environment, which likely causes rearrangement of their crystalline order. The magnitude and the kinetics of these variables depend strongly on the particle size. In addition, after thermal annealing a reduced spin canting is extrapolated from the lower magnetic coercivity, which confirms that a structural rearrangement takes place