Magnetic Transitions in Dilute Solutions of Iron in Gold and Copper

Abstract

The existence of magnetic transitions in alloys of Fe in Au and Cu has been shown by using the Mössbauer effect. There is a significant difference in the internal-field distribution (and hence alignment of the atomic spins) between the Cu and the Au alloys. In the former a continuous distribution exists, whereas, in the latter a unique (or very nearly unique) internal field occurs in the dilute alloys. The results for the Cu alloys are consistent with an indirect interaction of the Ruderman-Kittel-Yosida type of spins localized at the iron atoms. The internal-field distribution appears to develop a minimum at zero field and a rather broad maximum which shifts gradually to higher fields with decreasing temperature. The nearly unique internal field in the dilute Au-Fe alloys seems to exclude an explanation on the basis of a Ruderman-Kittel-Yosida exchange interaction. A spiral static spin density wave as the mechanism for antiferromagnetically ordering the localized spins (or ferromagnetic order for the case in which the spin density wave vector is zero) seems possible. More concentrated Au-Fe (>16 at. % Fe) alloys show a much more rapid increase in the magnetic transition temperature than the more dilute alloys. Their behavior is ferromagnetic and can be described quite well with a nearest-neighbor interaction using the average coordination number method suggested by Sato, Arrott, and Kikuchi. The appropriate nearest-neighbor exchange interaction energy is J≃2.9×10−2 eV.