Magnetization and magnetoresistance in Fe-ion-implanted Cu and Ag thin films

Diluted granular films of Cu-Fe and Ag-Fe (iron content less than or equal to 2%) were produced using super(57)Fe ion implantation on Cu(Ag) films previously grown by laser ablation. Conversion electron Mossbauer spectroscopy shows that the implanted Fe forms either very small clusters (up to a few atoms) or large iron alpha -phase particles. These structural characteristics directly reflect on the magnetization, which exhibits ferromagnetic-like behaviour at room temperature (due to large clusters) superimposed by a significant paramagnetic contribution at low temperatures due to the small clusters. We observe deviations from strict superparamagnetic behaviour due to non-negligible local anisotropy effects at low temperatures and low fields. The Kondo effect is particularly enhanced in the Cu-Fe films which have higher concentration of isolated Fe atoms and small sizes clusters. The magnetoresistivity Delta rho / rho of our films is dominated (for 0 less than or equal to mu sub(0)H less than or equal to 15 T) by a linear term in H, attributed to GMR-like effect from spin-dependent scattering when an electron passes between adjacent large and small clusters. At low fields we observe instead Delta rho / rho proportional to H super(2), due to the usual GMR effect between large clusters, during the alignment of their easy axes. The relevant physical differences (structural, magnetic and magnetoresistive) observed in our ion-implanted diluted Fe films, with respect to the concentrated granular films, are critically analysed.