Exchange Interactions I: Mechanisms

Abstract

A most important phenomenon in molecular magnetism is the exchange interaction between magnetic centers. Its relevance as well as the terms and concepts required to its rationalization were stated long ago by physicists working in the quantum-mechanical theory of magnetism (Heisenberg, Dirac, van Vleck, Anderson, Zener, and many others). Depending on the extent of delocalization of the magnetic moments and on the metallic/non-metallic properties of the solid four kinds of exchange coupling were usually distinguished in the physical literature namely direct exchange, superexchange, indirect exchange and itinerant exchange [1]. The relations of these types of couplings are depicted in Figure 1. Direct exchange and superexchange pertain to insulators. In these two cases the magnetic moments are strongly localized on the magnetic centers and interact directly through the space or through diamagnetic groups. Indirect exchange is the coupling between the localized magnetic moments of magnetic metals via the conduction electrons, while itinerant exchange deals with the coupling between conduction electrons in metals. These two couplings have been used to explain the magnetism of rare earth metals and d-transition metals. To all these exchange couplings we should add the double exchange which involves the coupling of two localized magnetic moments through an itinerant electron. As the dashed circles in Figure 1 indicate, the boundaries of these types of exchange are very diffuse. In fact, this classification is only conventional and depends on each particular model and approximation. In a rigorous sense there is no absolute distinction about them.