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The k dependence of the conduction-electron-rare-earth-moment exchange interaction is directly measured for the first time and found to be opposite in sense for Gd and Yb impurities in Au. Atomic exchange dominates for Au(Gd), while a phase-shift analysis indicates that covalent mixing of conduction p waves accounts for the observed k dependence in Au(Yb).PACS numbers: 71.70.GmThe nature of the conduction-electron-localmoment exchange interaction in metals is a fundamental problem which has stimulated widespread interest. It is known empirically 1 that the exchange integral J which is observed in metals is the sum of two contributions with distinctly different origins: J=J SLt +J C m° ^at is the atomic ("direct" or "on-site") contribution, 2 " 4 which would occur also in a nonmetallic host containing magnetic impurities. If the conduction states ^ and the local states

0]. «/ cm is the covalent-mixing contribution 3 " 7 which is attributed to spin polarization of the conduction electrons in the neighborhood of a magnetic impurity. It arises from hybridization of the local magnetic states with the conduction-electron states, creating net conduction-electron spin opposite to that of the impurity. This is equivalent to an antiferromagnetic exchange coupling and leads to spin compensation of the local moment and a variety of Kondo anomalies in various properties.Theoretical models for both the atomic and covalent-mixing contributions are usually simplified by assumption of a single exchange constant for coupling between all conduction-electron states and the impurity. In this paper we present direct experimental evidence for significant T£ de-pendence of the exchange coupling in both the atomic and covalent mixing limits. We find that the strength of the interaction varies in opposite directions for the two cases, and in the covalentmixing case correlates with the p character of the conduction states. We also report the first measurements of the £ dependence of the up-and down-spin scattering rates for conduction electrons from rare-earth impurities and show how these data can be interpreted by the 5d virtualbound-state model.As prototypes of atomic and covalent mixing exchange we have chosen dilute alloys of Au(Gd) and Au(Yb), respectively. Gd has a very stable moment in all metallic hosts and always displays atomiclike ferromagnetic couplingo 8 " 10 Conversely, Yb is strongly affected by hybridization as evidenced by its variable valence (trivalent in Au but divalent in Ag) 11 and net antiferromagnetic exchange coupling, leading to Kondo anomalies at T K~0o 01 Ko 12 Such clear examples of atomic and covalent-mixing exchange are impossible to find among transition-metal solutes in noble-metal hosts o Our results were obtained from an analysis of the amplitude 5 harmonic content, and spin-splitting zeros of de Haas-van Al...