Highly crystalline superparamagnetic iron oxide nanoparticles (SPION) in a silica matrix

We report on magnetic properties of iron oxide nanoparticles in a silica matrix synthesized by the sol-gel method. The sample is characterized by using X-ray powder diffractometer (XRPD), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX) and superconducting quantum interference device (SQUID) magnetometer. TEM reveals iron oxide nanoparticles (maghemite/magnetite) of small sizes of about 4 nm, narrow size distribution and no particle agglomeration. The SQUID measurements show low blocking temperature T-B = 6 K and superparamagnetic behavior above 30 K. Obtained saturation magnetization M-S = 61.1 emu/g is very high, among the highest values for iron oxides of particle size below 5 nm. The field-cooled hysteresis measurements do not show displacement of the hysteresis loop thus indicating an absence of exchange bias, whereas AC susceptibility reveals non-interacting nanoparticles. The values of K-eff and K-S (effective and surface anisotropy constants) obtained in this work are smaller than those reported in the literature for systems where shell's disorder spin structure (surface effects) is observed. These results point to highly crystalline iron oxide nanoparticles with a low amount of internal defects and small surface disorder shell thickness which is uncommon for nanoparticles of this size. Superparamagnetic iron oxide nanoparticles (SPION) with such properties are convenient for the biomedical applications in targeted diagnostics and drug delivery. (C) 2012 Elsevier B. V. All rights reserved.