Effect of magnetic impurities on electronic transport of heterogeneous ferromagnets

Electronic transport in granular Co-Ag films is studied between subhelium and room temperatures in fields up to 16 T. A number of unusual properties are found including a strong variation of magnetoresistance at low temperatures and sublinear ${(d}^2{R/dT}^2<0)$ temperature dependence of high field resistance above 50 K. We show that paramagnetic impurities or small magnetic clusters embedded in the normal-metal matrix influence significantly the electronic transport in heterogeneous ferromagnets. The effect is strongly enhanced as compared with paramagnetic impurities in homogeneous normal metals, and is comparable and even superior over the phonon scattering up to room temperatures. This mechanism is argued to be responsible for the peculiar properties of resistance and magnetoresistance of granular ferromagnets.