Magnetic Nanoparticle Aggregation States in Ag 100-x Fe x Cosputtered Granular Films Investigated by Magnetic and Magnet
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S5.4.1
Magnetic Nanoparticle Aggregation States in Ag100-xFex Cosputtered Granular Films Investigated by Magnetic and Magnetotransport Measurements Paolo Allia1, Federica Celegato1, Marco Coisson2, Paola Tiberto2, Franco Vinai2 and Federico Spizzo3 1
Physics Department, Politecnico di Torino, Corso Duca degli Abruzzi 24, I- 1029 Torino, Italy Materials Department, IEN Galileo Ferraris, Strada delle Cacce 91, I-10135 Torino, Italy 3 Physics Department, Università di Ferrara, Via Paradiso 12, I-44100 Ferrara, Italy 2
ABSTRACT Magnetoresistance and.magnetization measurements have been performed on nanogranular, cosputtered Ag100-xFex films (x= 10 to 30) in the 4 K – 300 K temperature interval. The analysis reveals that the films with x ≤ 14 are interacting superparamagnets, characterized by a magnetic correlation length of the order of the electronic mean free path λ. Films with x ≥ 26 behave as concentrated magnets with strong competing interactions among magnetic moments (frustrated ferromagnets) and magnetic correlation length much larger than λ. INTRODUCTION The isotropic giant magnetoresistance of bi-metallic granular systems containing magnetic nanoparticles in a metallic matrix has been extensively studied because of its interest in basic research and applications [1]. Nanogranular bimetallic films have application as comparatively cheap sensing materials [2]. The role and nature of interparticle interaction on magnetic and magnetotransport properties of nanogranular systems are still debated. Cosputtered Ag-Fe films with variable Fe concentration deserve attention because of their high magnetoresistance (MR) [3] associated with good tunability of the relative concentration between ferromagnetic metal and diamagnetic host. As known, the MR effect in granular systems is related to the re-orientation of magnetic moments on the scale of the electron mean free path (λ) [1]. Combined magnetoresistance and magnetisation measurements provide information about magnetic correlation extending to the scale of λ [4] in magnetic nanoparticle systems. When the MR is plotted as a function of the reduced magnetization (m = M/Ms), a parabolic behavior is expected for uncorrelated particle magnetic moments. Deviations from such a law can be primarily ascribed to magnetic correlation [5]. A box-like behavior of the MR vs. m curves, typical of magnetic systems with competing interactions, has been recently observed in some Ag100-xFex sputtered films (x = 26, 30) [6]. In these films, the magnetic scatterers are identified with regions of magnetic coherence, each including many nanoparticles. In any coherence region, the nanoparticle moments are rigidly blocked, one with respect to another, by the overall effect of competing interactions, and coherently react to an applied magnetic field. In this work, magnetisation and magnetoresistance measurements are exploited to study the magnetic correlation among scatterers and its field and temperature dependence in the Ag100-xFex film system (x = 10 to 30).
S5.4.2
EXPERIMENT Ag100-xFex
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