Matthias and co-workers, in a series of electron spin resonance (ESR) and nuclear magnetic resonance (NMR) experiments on non-magnetic metals – metals with no permanent magnetic moment – observed surprising evidence for long-lived local spin packets in the ESR lineshape (M1960). These data indicated the persistence of local magnetic moments. The magnetic moment was quickly traced to the presence of small amounts of magnetic impurities. While various systems were studied, such as Mn, Fe, and other iron group impurities in host materials such as Cu, Ag, and Au, the common ingredient shared by all the impurity ions is that they possessed one or more vacant inner-shell orbitals. In addition, the experiments demonstrated that varying the kind and amount of the magnetic impurities did not always result in the formation of local magnetic moments in non-magnetic metals. This finding added to the intrigue and established the question of the formation of local magnetic moments as central to understanding magnetism and transport in solids. In this chapter, we describe the origin of local moments, focusing primarily on Anderson's model (A1961), the model that rose to the fore as the standard microscopic view of local magnetic moment formation in metals.

Local moments: phenomenology

An impurity in a non-magnetic metal can give rise to a local moment if an electronic state on the impurity is singly occupied, at least on the time scale of the experiment. Friedel (F1958) was the first to introduce a phenomenological model to explain the onset of local moments.