Physical studies of heterometallic rings: an ideal system for studying magnetically-coupled systems

Heterometallic rings of general formula [Cat][M7M'F-8((O2CBu)-Bu-t)(8)] (M = a trivalent metal, M' = a divalent metal, cat = a secondary ammonium cation, caesium or rubidium) contain an octagon of metal centres with each metal-metal edge bridged by a fluoride and two carboxylates. The rings when M = Cr-III give remarkably beautiful EPR and INS spectra, and allow us to examine new techniques. Use of these techniques allows us to study the models used to describe magnetic behaviour in exchange coupled systems. In this tutorial review we discuss the two main approaches to modelling magnetic data - the strong exchange limit and the microscopic Hamiltonian approaches, aiming to explain the major differences between the two approaches. We also describe some more esoteric measurements and theories.