  Some of the synthesis methods and physical properties of iron-oxide based
magnetic nanoparticles such as Fe3-xO4 and CoxFe3-xO4 are reviewed because of
their interest in health, environmental applications, and ultra-high-density
magnetic recording. Unlike high crystalline quality nanoparticles larger than a
few nanometers that show bulk-like magnetic and electronic properties,
nanostructures with increasing structural defects yield a progressive worsening
of their general performance due to frozen magnetic disorder and local breaking
of their crystalline symmetry. Thus, it is shown that single-crystal,
monophasic nanoparticles do not exhibit significant surface or finite-size
effects, such as spin canting, reduced saturation magnetization, high closure
magnetic fields, hysteresis-loop shift or dead magnetic layer features which
are mostly associated with crystallographic defective systems. Besides, the key
role of the nanoparticle coating, surface anisotropy, and inter-particle
interactions are discussed. Finally, the results of some single particle
techniques -- magnetic force microscopy, X-ray photoemission electron
microscopy, and electron magnetic chiral dichroism -- that allow studying
individual nanoparticles down to sub-nanometer resolution with element, valence
and magnetic selectivity, are presented. All in all, the intimate, fundamental
correlation of the nanostructure (crystalline, chemical, magnetic) to the
physical properties of the nanoparticles is ascertained.
