  The magnetic behavior of $Fe_{3-x}O_4$ nanoparticles synthesized either by
high-temperature decomposition of an organic iron precursor or low-temperature
co-precipitation in aqueous conditions, is compared. Transmission electron
microscopy, X-ray absorption spectroscopy, X-ray magnetic circular dichroism
and magnetization measurements show that nanoparticles synthesized by thermal
decomposition display high crystal quality and bulk-like magnetic and
electronic properties, while nanoparticles synthesized by co-precipitation show
much poorer crystallinity and particle-like phenomenology, including reduced
magnetization, high closure fields and shifted hysteresis loops. The key role
of the crystal quality is thus suggested since particle-like behavior for
particles larger than about 5 nm is only observed when they are structurally
defective. These conclusions are supported by Monte Carlo simulations. It is
also shown that thermal decomposition is capable of producing nanoparticles
that, after further stabilization in physiological conditions, are suitable for
biomedical applications such as magnetic resonance imaging or bio-distribution
studies.
