Effective-field distributions and resistivity minima in amorphous ferromagnets

Abstract

We show that the anomalous low-temperature resistivity minima that have been observed in practically all amorphous metals can be explained by a modified Kondo mechanism as suggested by Tsuei. If some spins exist in zero field, even in a ferromagnet, they will give rise to a Kondo minimum in the resistivity. We have performed a Monte Carlo calculation of the effective fields that are present at each ion in a strong ferromagnet such as ${\mathrm{Fe}}_{0.8}$ ${\mathrm{B}}_{0.2}$, using measured radial distribution functions (RDF's). Including both direct-exchange and Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions, we find no spins which are in zero field. However, if we include superexchange interactions between those next-nearest-neighbor magnetic atoms which are separated by a metalloid atom, there is a long tail in the distributions of fields which goes through zero and is nonzero at relatively large negative fields. This distribution can explain why the resistivity is unaltered in applied magnetic fields of up to 50 kOe. It can also explain why the resistivity minimum can shift to much higher temperatures upon the addition of a second type of magnetic element.