X-ray diffraction analysis on Fe-Ag nanocrystalline superparamagnetic films

The dependence of electronic conductivity on the electron spin state was first ob-served in multilayer systems made of magnetic (M) and non-magnetic (NM) layers [1]. It was later observed in nanogranular systems, as well, where M nanoparticles are dispersed into a NM matrix [2]. Thanks to this phenomenon, large variations of sample resistivity, rho, can be obtained by applying an external magnetic field, H, and this effect is called giant magnetoresistance (GMR). GMR is defined as (rho(H) - rho(0))/rho(0), where the numerator is affected by the magnetic properties of the sample through the spin dependent scattering whilst the denominator is the effect of the spin independent scattering sources, i.e. rough-ness or crystalline defects. As both terms are related to the structural properties of the nanogranular films, x-ray diffraction is a useful technique to probe the connection between growth conditions and GMR properties. We focused on FexAg100-x nanogranular films, where x is the Fe atomic relative con-centration and ranges from 0 up to 40, deposited on (100) Si substrates using dc-magnetron sputtering in cosputtering configuration and Ar atmosphere. The Fe/Ag phase diagram in-dicates that the two elements are not mutually soluble for any relative concentration but thanks to that out-of-equilibrium deposition technique it is possible to produce a system that at room temperature behaves like a magnetic nanogranular one [3]. Diffraction data show reflections ascribed to a FCC crystalline lattice. When x is 0, the values of the lattice parameter, d, are the same as the Ag bulk, but the ratio of the inten-sities I111/I200 is very large indicating a preferred (111) orientation. With increasing x, d values decrease, the full width at half maximum (FWHM) of the peaks increases while the intensity of the 200 peak progressively decreases. Finally, as a function of x, up to now no clear evidence of a BCC or FCC Fe crystalline phase is found. The shift of FCC peaks angular position with Fe concentration points out that iron and silver could possibly give rise to a solid solution, where Ag lattice undergoes a com-pressive strain due to the intermixing of Fe atoms [4]. The stress of the mixed Fe/Ag FCC lattice therefore possibly increase with x, and this fact could explain the FWHM increase, i.e. the reduction of the average crystalline grains size. On the other hand, for all the con-centrations, magnetization data show the presence of Fe precipitates whose size increases with x and goes from a few nanometers up to tens of nm. However, low temperature data support the presence of a Fe-Ag solid solution. In this work, all these data will be compared and discussed to find out a model of the samples that is in agreement with the presented magnetic and structural pictures of the me-tallic films. Eventually, new measurements performed on miscut Si substrates will be pre-sented to point out the possible presence of FCC or BCC Fe reflections, that could be actu-ally hidden by the 400 Si peak. [1] M. N. Baibich et al, Phys. Rev. Lett. 61 (1988) 2472. [2] A. E. Berkowitz et al., Phys. Rev. Lett. 68 (1992) 3745. [3] J.-Q. Wang, G. Xiao, Phys. Rev. B 49 (1994) 3982. [4] N. Kataoka et al., J. Phys. F, Met. Phys. 15 (1985) 1405