Local Magnetic
and Electronic Structure of the Surface Region of Postsynthesis Oxidized
Iron Oxide Nanoparticles for Magnetic Resonance Imaging
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Abstract
Iron
oxide nanoparticles (FeO<sub><i>x</i></sub>-NP) are applied
in medicine as contrast agents in magnetic resonance imaging (MRI)
where they reduce the spin–spin relaxation time (T<sub>2</sub>-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<sub><i>x</i></sub>-NPs (Fe<sub>3</sub>O<sub>4</sub>/γ-Fe<sub>2</sub>O<sub>3</sub>) 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
Fe<sup>2+</sup> and Fe<sup>3+</sup> 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