Dilute magnetic alloys with transition metals as host

Abstract

The theory of dilute magnetic alloys with 4d and 5d transition metals as host is reviewed. The close relationship between the Wolff and Anderson models permits a unified interpretation of the single impurity properties of dilute alloys with noble and transition metals as host. We show that some dilute alloys such as Rh Fe are probably Kondo systems with temperature dependent transport properties characteristic of the Wolff model. Weakly magnetic impurities with local spin-fluctuations (LSF) are treated in the random phase approximation (RPA) and the results are compared with those of a systematic perturbation expansion. The RPA yields the correct power laws for all low temperature properties. However, it strongly overestimates electron interaction effects and therefore yields the wrong coefficients and characteristic temperature T s . In the order in T exact relations can be derived between the susceptibility, specific heat and resistivity for all values of the interaction constant U. Dilute ferromagnets essentially are dealt with in a model due to Doniach and Wohlfarth. In contrast to LSF systems, the resistivity and thermopower for T→0 turn out to be proportional to T 3 2 . The effect of the local environment on the formation of local moments is considered within a generalized Landau approach and from a more microscopic point of view. The present state of the theory of spin glasses is summarized. Although the models considered are rather crude the agreement between theory and experimental data seems to be satisfactory for Kondo, LSF and dilute ferromagnetic systems. In the case of spin glasses, the results of the various theories remain controversial.