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An experimental evidence of the progressive modification in the magnetic behavior of granular Fe/ Si3 N4 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/ Si3 N4 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 (TB ∼ H3⊃/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 Si3 N4 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 (TB ∼ H2⊃/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. © 2010 The American Physical Society.This work has been supported by Spanish MICINN under Contracts No. MAT2009-08786, No. MAT2008-06517-C02-01, and No. CSD2009-00013 and by CAM under Contract No. S2009/MAT-1756. J.S.M. is grateful to MICINN for the financial support within the JdlC program.Peer Reviewe

physical review b

Effects of interparticle interactions in magnetic<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mtext>Fe</mml:mtext><mml:mo>/</mml:mo><mml:msub><mml:mrow><mml:mtext>Si</mml:mtext></mml:mrow><mml:mn>3</mml:mn></mml:msub><mml:msub><mml:mtext>N</mml:mtext><mml:mn>4</mml:mn></mml:msub></mml:mrow></mml:math>granular systems

Effects of interparticle interactions in magneticFe/Si3N4granular systems