Evidence for short-range ferromagnetic order in amorphous (Gd,Y)(x)Si1-x alloys

We present electron paramagnetic resonance (EPR) and magnetotransport measurements for amorphous RExSi1-x alloys [rare earth (RE) metals are Gd, Y; 0.1 < x < 0.2], over a wide range of temperatures and magnetic fields. To explain the experimental results, we use a model in which the structural disorder in the system causes the appearance of regions with a higher electron concentration around nanoscale defects enriched with rare-earth ions (droplets). It is shown that the observed EPR signal can be formed only in the double-bottleneck regime. Then, the temperature dependences for the line position and the linewidth are obtained. We also analyze recent measurements of the temperature- and magnetic-field dependence of the electrical conductivity and Hall effect, and show that the spin polarization of the electron states in the droplets favors a giant negative magnetoresistance and enhances the tendency towards the metal-insulator transition. The ferromagnetic transition temperature and the number of Gd ions inside the nanoscale defect enveloped by the droplet are evaluated from a comparison between the theory and the experimental data. We suggest that amorphous RExSi1-x alloys are magnetic materials with unique properties of both strongly inhomogeneous compounds and nanocomposites.