Interactions between ions with orbital angular momentum in insulators

Until recently most interest in interactions between paramagnetic ions has centred upon ions with quenched orbital angular momentum, for which exchange has the isotropic Heisenberg form JS 1 .S 2 and the major anisotropic terms are due to magnetic dipole-dipole interaction. The interactions for ions with unquenched orbital angular momentum are quite different; exchange is anisotropic, and in addition to exchange and magnetic multipole interactions, there are sometimes sizeable anisotropic contributions from electric multipole interactions and virtual phonon exchange. The qualitative theory of these interactions has been developed and is summarized herein, and the problems in making quantitative theories are discussed. A large number of experimental measurements have been made, mainly on rare earth ions, but several on Co 2+ , using electron paramagnetic resonance pair spectra and other techniques. The experimental methods and the data obtained are described. The difficulties of interpreting the mass of data are discussed, and the successful identification of particular mechanisms is critically explained. So far the only definite example of electric multipole interaction is for Ce 3+ ions in LaCl 3 and LaBr 3 . Virtual phonon exchange is highly likely to be the principal nondipolar interaction in UO 2 , rare-earth ethyl sulphates and Pr 3+ in LaCl 3 . Most measured nondipolar interactions are due to exchange.