Crossover from superspin glass to superferromagnet inFexAg100−xnanostructured thin films(20≤x≤50)

Abstract

${\text{Fe}}_{x}{\text{Ag}}_{100\ensuremath{-}x}$ granular thin films, with $20\ensuremath{\le}x\ensuremath{\le}50$, 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 $35\text{ }\text{at}\text{.}\text{ }\mathrm{%}$. 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 $(xl35)$ to a superferromagnetic $(xg35)$ behavior for the magnetic moments of the Fe nanoparticles.