Effects of interparticle interactions in magneticFe/Si3N4granular systems

Abstract

An experimental evidence of the progressive modification in the magnetic behavior of granular Fe/ Si 3 N 4 samples due to interaction effects between particles is reported. Microstructural features and local structure were determined by x-ray absorption spectroscopy and transmission electron microscopy to select granular samples with predetermined cluster size. Fe/ Si 3 N 4 systems have been characterized by ac-and dcmagnetization measurements to study the gradual evolution of magnetic properties of granular systems, where three different behaviors have been observed. As-deposited samples with Fe thickness layers of 2.5 nm, present a modified superparamagnetic behavior, due to very weak interactions between very small Fe clusters separated by a nonmagnetic FeN phase. An evolution of the average blocking temperature at intermediate fields ͑T B ϳ H 3/2 ͒ is observed, similar to noninteracting systems, but first signatures of a frozen spin state at low temperatures appear. Annealed samples exhibit a noticeable modification from the multilayer character to a random three-dimensional organization of Fe clusters embedded in a Si 3 N 4 matrix. After annealing, samples with initial Fe layer thickness of 0.7 nm provide iron cluster in the range of 1.3 nm and exhibit a superspinglass state, with a de Almeida-Thouless evolution of the energy barriers ͑T B ϳ H 2/3 ͒ that is explained in terms of increasing interparticle interactions. Moreover, annealed samples, with initial layer thickness of 1.3 nm, supply iron cluster of near 3 nm that present stronger interactions and yield a superferromagnetic state, likely provided by residual ultrasmall particles between the blocked clusters.

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…”

“…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. Jim enez-Villacorta et al 19 reported evidence of modification of the magnetic behavior of NPs evoked by the interaction effects between Fe clusters in granular Fe/Si 3 N 4 particles. The sample with very small Fe cluster dimensions showed superparamagnetic features due to weak dipolar interactions; while in the sample with larger Fe clusters, stronger interactions led to superferromagnetism.…”

“…The sample with very small Fe cluster dimensions showed superparamagnetic features due to weak dipolar interactions; while in the sample with larger Fe clusters, stronger interactions led to superferromagnetism. 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.…”

X-ray magnetic circular dichroism study of the blocking process in nanostructured iron-iron oxide core-shell systems

“…19 On the other hand, some interpretations have considered that the blocking and reorientation processes exhibited in magnetothermal curves are associated with the superspin reorientation, 20,21,22 and that even the de Almeida-Thouless evolution of the energy barriers can be considered to be an effect of interparticle and/or core-shell interactions. 23,24 In this paper, we present a detailed study, using the microscopic information gained from x-ray magnetic circular dichroism (XMCD), on the magnetic reorientation processes in a transition-metal core-oxide shell system that exhibits spin-glass-like features at low temperatures. It has been reported that sputtering deposition of iron on cooled substrates at temperatures far below room temperature induces a granular structure and thus a granular magnetic behavior in these materials.…”

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

“…An ultimate state in the magnetic transitions is the formation of superferromagnetic entities at high concentration of nanoparticles. The formation of superferromagnetic chain like entities has been observed both in ordered as well as disordered arrangement of magnetic particles in a matrix through a systematic increase of concentration/density [11][12][13][14][15][16][17][18][19] . In all the studies reported so far the major emphasis has been to control the absolute size and size distribution of the nanoparticles such that they remain in a single state of magnetic order and probe the static and dynamic behavior of this state.…”