Abstract: FexAg100?x granular thin films, with 20 x 50, have been prepared by the dc-magnetron sputtering deposition technique. With this technique we have been able to obtain samples comprising small Fe nanoparticles 2.5?3 nm embedded in a Ag matrix, remaining their size practically constant with increasing Fe content. Their magnetic behavior has been fully characterized by dc magnetic measurements between 5?350 K. They have revealed a crossover in the collective magnetic behavior of the Fe nanoparticles around a 35 at. %. Below such a concentration, a collective freezing of the magnetic moments is observed at low temperatures, while at high temperatures a transition, mainly mediated by dipolar interactions, to a magnetically disordered state is obtained. Above this concentration, direct exchange interactions overcome the dipolar magnetic interactions and a long-range order tends to prevail in the range of temperatures analyzed. ac magnetic measurements have indicated a crossover from a superspin glass x35 to a superferromagnetic x35 behavior for the magnetic moments of the Fe nanoparticles.

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