Effects of interparticle interactions in magneticFe/Si3N4granular systems

Abstract

An experimental evidence of the progressive modification in the magnetic behavior of granular $\text{Fe}/{\text{Si}}_{3}{\text{N}}_{4}$ 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. $\text{Fe}/{\text{Si}}_{3}{\text{N}}_{4}$ systems have been characterized by ac- and dc-magnetization 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 $({T}_{\text{B}}\ensuremath{\sim}{H}^{3/2})$ 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 ${\text{Si}}_{3}{\text{N}}_{4}$ 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 superspin-glass state, with a de Almeida-Thouless evolution of the energy barriers $({T}_{\text{B}}\ensuremath{\sim}{H}^{2/3})$ 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.