Size and surface effects in the magnetic properties of maghemite and magnetite coated nanoparticles.

An increasing number of promising applications for future technology is arising from size constraints in nanoparticles (NPs) and from the chemical manipulation of their surfaces. In this work, we analyse the finite-size and surface effects on polyacrylic acid-coated Fe(3)O(4) NPs and oleic acid-coated gamma-Fe(2)O(3) NPs by studying their magnetization curves at different temperatures. The measured thermal dependence of the saturation magnetization is no longer explained by the typical T(3/2) Bloch law, yielding higher values than those expected for its exponent. When incorporated in polymeric matrixes to form magnetic transparent nanocomposites, the oleic acid-coated gamma-Fe(2)O(3) NPs also deviate from Bloch's law, but following the opposite trend observed in free coated NPs.