Correlation between structural and giant magnetoresistance properties of fe-Ag nanogranular films

For a random ensemble of monodispersed superparamagnetic particles, the giant magnetoresistance (GMR) display a quadratic dependence on the reduced magnetization, m. γ can be seen as a global indication of how effective is the magnetic structure in producing GMR. The GMR intensity, I, and γ are usually related with each other, and change with the magnetic nanoparticle density. As this density increases, both I and γ are expected to display a maximum as the percolation threshold is approached. However, the two maxima are not always overlapped, as I is proportional to nanoparticle density, as well. Therefore, it is of interest to single out the two maxima, and to highlight the samples features that are more effective in favouring the GMR phenomenon. We focused on FexAg1-x nanogranular films, where x is the Fe atomic relative concentration measured by Rutherford Backscattering Spectrometry, 0 < x < 0.5. The samples were deposited on Si substrates using dc-magnetron sputtering in cosputtering configuration, to promote a fine dispersion of Fe nanoparticles within the samples. I is maximum at x = 0.32, while γ is maximum at x = 0.26. X-ray diffraction and Mössbauer measurements were performed to investigate the structural properties of the samples. We found that the granular films contain two different components, whose relative contribution changes with x: (A) a Fe-Ag solid solution and (B) bcc Fe clusters. In detail, for x < 0.20, just A is observed; for 0.20 < x < 0.32 A is still detected and a small contribution from B appears; finally, for x > 0.32 there is a Fe-Ag saturated solid solution and a large B contribution. Magnetic measurements detect the presence of magnetic nanoparticles for x < 0.20, as well, and reveal that, for 0 < x < 0.50 their average size increases with x. Field-cooled (FC) and zero-field-cooled (ZFC) magnetization data at low field were collected as a function of x, as well. For x < 0.18, the typical lambda shape is observed but at low temperature a minimum is detected in the FC curve; therefore, these samples possibly behave like a cluster-glass system, where frustrated interactions between magnetic particles may be transmitted by the surrounding Fe-Ag solid solution. The GMR behaviour of the system changes with the relative weight of the A and B components. The GMR effect is maximum when the bcc Fe clusters are embedded in a saturated Fe-Ag solid solution and the size of the Fe clusters is comparable with the size of the Fe-Ag solid solution nanocrystalline grains. Differently, the GMR efficiency is maximum when Fe clusters are embedded in a not saturated Fe-Ag solid solution. The maximum GMR effect is detected when the sample is made of a large number of clusters, whose size is about a few nanometres, and of a disordered matrix including very fine (made of a few atoms) Fe clusters.