  We have studied the magnetic and power absorption properties of a series of
magnetic nanoparticles (MNPs) of Fe3O4 with average sizes <d> ranging from 3 to
26 nm. Heating experiments as a function of particle size revealed a strong
increase in the specific power absorption (SPA) values for particles with <d> =
25-30 nm. On the other side saturation magnetization MS values of these MNPs
remain essentially constant for particles with <d> above 10 nm, suggesting that
the absorption mechanism is not determined by MS. The largest SPA value
obtained was 130 W/g, corresponding to a bimodal particle distribution with
average size values of 17 and 26 nm.
