Concentration dependence of interclusters interaction role in sputtered Fe-Ag nanogranular samples

An ideal nanogranular sample is an ensemble of independent nanosized magnetic (M) particles dispersed in a non magnetic (NM) matrix. The magnetic particles can be seen as large magnetic moments; in case their dynamics is affected just by thermal energy contributions, the whole system is referred to as a superparamagnet (SP) [1]. Real systems, due to growth conditions, often differ from this picture and interparticle magnetic interactions, e.g. dipolar or exchange ones, are one of the main sources of deviation from the ideal SP behaviour. If M and NM metallic species are used, the samples display a spin-dependent electronic resistivity that remarkably decreases if an external magnetic field, H, is applied, i.e. they show the so called giant magnetoresistance (GMR) [1]. GMR is ascribable to the magnetic ordering effect induced by H, so the higher the degree of disorder at zero field the larger the GMR effect. Interactions induce correlations among the magnetic moments, in particular when H is small, viz. an higher degree of order. As a consequence, the overall resistivity change is reduced. It’s therefore important to study interactions effects when H ≈ 0. In this work, we have studied dc-cosputtered nanogranular FexAg100-x thin films with a volume Fe concentration, x, varying from 10 up to 30 as measured by Rutherford Backscattering Spectrometry. At room temperature, for x < 20 a SP behaviour is observed. A recently devel-oped model [2], based on the simultaneous investigation of magnetic and GMR data, has pointed out that magnetic interactions affect samples dynamics for all concentrations. The correlation length, λ, is always larger than particles average distance and increases with tem-perature and x [2]. These systems are therefore suitable to study interparticle interactions and their effect on low-field magnetic configuration. The investigation was performed with sus-ceptibility measurements in field-cooled (FC) and zero-field-cooled (ZFC) configuration, re-laxation and Mössbauer measurements; X-Ray diffraction data were collected, as well. When x < 18, FC and ZFC data display the typical lambda shape but, for temperatures lower than the blocking temperature, FC signal displays an unexpected maximum at about 40 K. This effect is less and less pronounced as x increases and it vanishes starting from x = 18. The comparison between ZFC/FC curves and magnetic relaxation data confirms that interparticle interactions have a remarkable influence on low-field dynamics and this finding is supported by low-temperature Mössbauer measurements. However, the kind of interactions seems to change with x. Indeed, for low Fe concentration the samples possibly behave like a cluster-glass system, where frustrated interactions produce the FC maximum. Whilst approaching x ≈ 18, the interactions turn to dipolar and for higher concentrations the samples approach a re-entrant ferromagnetic behaviour. Eventually, X-Ray diffraction data suggest that the whole transition is related to the effects induced on samples structure/morphology by the change in iron concentration. [1] A. E. Berkowitz et al, Phys. Rev. Lett. 68 (1992) 3745 [2] P. Allia, M. Coisson, F. Spizzo, P. Tiberto, and F. Vinai, Phys. Rev. B 73 (2006) 054409