Magnetism: General Introduction

One of the distinctive properties of transition-metal compounds is the presence of unpaired electrons in the partially filled d-shell which give rise to magnetic properties. The metal ions are therefore paramagnetic, which means that they are attracted by an applied magnetic field. The paramagnetism has its origins in both the spin and orbital moments of the unpaired electrons. In general, the passage from isolated ions to coordination compounds is associated with the quenching of the orbital moment, and the paramagnetic properties can be described to a first approximation by considering the orbital contribution as a perturbation on the spin term. It is the presence of the unquenched orbital contribution that makes the measurement of the paramagnetic properties an important tool for the structural characterization of transition-metal coordination compounds. In fact magnetic measurements have long been used in order to obtain structural information. Magnetochemistry (Carlin, R. L. Magnetochemistry ; Springer-Verlag: Berlin, 1986; O’Connor, C. J. Magnetochemistry – Theory and Experimentation ; Wiley: New York, 1972) was extensively developed in the 1950s in conjunction with the first impact of quantum mechanics in transition-metal chemistry as a fundamental tool for the characterization of coordination compounds. In the eighties there was a gradual shift from simple paramagnetic properties to all kinds of cooperative magnetic phenomena, with the investigation of complex zero-, one-, two-, and three-dimensional molecule-based magnetic materials (de Jongh, L. J.; Miedema, A. R. Adv. Phys. 1974 , 23 , 1–260; Kahn, O. Molecular Magnetism ; VCH: Weinheim, 1993; Gatteschi, D.; Kahn, O.; Miller, J. S.; Palacio, F., Eds. Magnetic Molecular Materials ; NATO ASI; Kluwer: Dordrecht, The Netherlands, 1991). This shift of interest was accompanied by a progressive increase in the complexity of the magnetic techniques used to investigate transition-metal compounds. In the early stages the measurement of the effective magnetic moment at room temperature was enough, now it is almost routine to measure down to the liquid helium region and below, in both d.c. and a.c. mode. Sophisticated techniques are also used for the investigation of single crystals.