Crossover from superspin glass to superferromagnet inFexAg100−xnanostructured thin films

Abstract

Fe x Ag 100−x granular thin films, with 20Յ x Յ 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 nm͒ 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 ͑x Ͻ 35͒ to a superferromagnetic ͑x Ͼ 35͒ behavior for the magnetic moments of the Fe nanoparticles.

Superferromagnetism in chain-like Fe@SiO2 nanoparticle ensembles

“…These properties in thermomagnetic ZFC/FC curves are associated with a ferromagnetic behavior. [15][16][17][18][19]28 A similarly shaped ZFC curve, as a result of strong ferromagnetic correlations between particles, has also been observed by Sousa et al 28 in granular CoFe=Al 2 O 3 multilayers and by JimenezVillacorta et al 19 in Fe/Si 3 N 4 multilayers. Hansen et al…”

“…15,16 Bedanta et al 16,17 reported the suppression of dipolar disorder among the superspins in dense CoFe nanoparticle ensembles leading to the long-range ordered SFM state. Similarly, Alonso et al 18 observed the crossover from SSG to SFM behavior in FeAg thin films. At lower concentrations of Fe nanoparticles, a collective freezing of the magnetic moments was observed; whereas at higher concentrations, direct exchange interactions were present between Fe nanoparticles.…”

“…19 The SFM state, characterized by ferromagnetic interparticle correlations, was associated by many authors with high concentration of the nanoparticles and/or with an increase of their size. [12][13][14][15][16][17][18][19] However, the growth of NPs can also cause a shift from mono-domain particles to multidomain ferromagnets, thereby invalidating the simplifying superspin approximation. Therefore, it is of strong interest to modify the magnetic properties of nanoparticles while preserving the mono-domain state.…”

Interparticle interactions mediated superspin glass to superferromagnetic transition in Ni-bacterial cellulose aerogel nanocomposites

“…6. The progressive freezing of magnetic moments of the interacting nanoparticles increases the irreversibility between ZFC and FC [1,32,33]. The magnetic relaxation in such systems is dominated by the strength of dipolar interparticle interactions Ed and they can exhibit a range of behaviors from superspin glasses to superferromagnets with increasing Ed.…”

“…On the other hand, if the composite is made of insulating magnetic nanoparticles, direct exchange does not lead to finite simple electrical conductivity of the composite. If the particles concentration is reduced to avoid physical contact, then different types of interactions that can influence the collective magnetic behavior are; tunneling, RKKY interaction in the case of a composite with non-magnetic metallic medium separating the magnetic nanoparticles and simple dipole-dipole interactions [1][2][3]. Hence it can be seen that collective behavior of single domain magnetic nanoparticles is a strong function of their concentration as well as their spatial arrangement.…”