We report the magnetic properties of size-selected Fe and FeAu nanoparticles (NPs) supported on SiO2/Si(001). These microscopic properties are dominated by two key parameters: finite size effects and surface effects.

In order to systematically study size effects, we use the method of inverse micelle encapsulation (PS-P2VP diblock - cop ol ymers) for the growth of the nanoparticles. This approach results in Fe (and FeAu) NPs with a high homogeneity in size, shape and inter-particle distance. Following this method, the NP size can be tuned by changing the length of the polymer head (P2VP) while the interparticle distance can be modified by changing the length of the polymer tail (PS). Investigating these NPs in-situ (ultrahigh vacuum) after the removal of the encapsulating ligands by means of a variable temperature magneto-optical Kerr effect set-up enables the determination of: (i) the temperature dependen ce of the coercivity and remanence of the NPs, (ii) the blocking temperature for the ferromagnetic-superparamagnetic transition, and (iii) the direction of magnetization in anisotropic nanostructures . This new insight constitutes a step forward towards unrave l ling the influence of the NP geometrical structure ( size and shape ) on its magnetic properties. Additionally, by varying the interparticle distance we can probe the role played by particle-particle interactions in the collective magnetic behavior.