Spin-dependent scattering of the conduction electrons in nanogranular Fe-Ag films

The magnetoresistance (MR) is the change of resistance of a conductor when it is placed in an external magnetic field. It is a research argument from a lot of time, both for the information of structural character and compositional deduced from it, but also thanks to the possibility to use such effect for the preparation of sensors of magnetic field. The new phenomenon of GMR (giant magnetoresistance) is notable variations of resistance in correspondence with small applied magnetic fields that al¬lows to produce devices displaying a great versatility. Otherwise from the ordinary and anisotropy magnetoresistance, rising from the Lorentz force and from the spin-orbit coupling, respectively, the origin of GMR is a scattering of the conduction elec¬trons which cross section is a function of the electronic spin. To have scattering of the conduction electrons is necessary the contact between materials with two different band structure. The first observation of GMR was made in magnetic multilayer, where layers of ferromagnetic and non-magnetic metals are stacked on each other, subsequently the GMR was observed in granular magnetic materials, composed of nanosized superparamagnetic clusters embedded in a nonmagnetic matrix. The activity of this Ph-D has been devoted to the growth and the characterization of granular thin film of Fe-Ag. The materials used for the preparation of the samples are iron and silver because the two metals are immiscible and the GMR effect associated to this couple of metals it is usually very elevated. The deposition technique used to make the films is a particular methodology that does not create an alloy, a low efficacy system for GMR observation, but allows a fine dispersion of magnetic particles in the non magnetic matrix. This is the ideal structure arrangement that allows a great MR effect. The research regards the magnetic and magnetoresis-tive properties of the nanogranular films, comparing the results with structural inves¬tigation, to understand the best system structure (Fe concentration and structural arrangement) that produces the GMR effect. For the Fe-Ag nanogranular films, the maximum GMR effect is the best arrangement between a structure that displays the maximum efficiency and a structure with a raised concentration of scattering centres, that is a raised magnetic material concentration. The maximum effect is when the sample is formed of a big number of cluster with dimension around a few nanometres and a disordered matrix. Both the Fe cluster and the little aggregates dispersed in the matrix participate in the GMR effect because there is a big volume/surface ratio that increase the GMR effect and decrease the dipolar interactions. In summary to have the maximum effect, the raised amount of magnetic material present in the system has a best arrangement (like at the concen¬tration of the maximum GMR efficiency) in the non-magnetic matrix, and the distance between the magnetic aggregates is of the order of the conduction electrons mean free path.