Raman Scattering Studies of Optical Magnons and of High-Energy Magnetic Excitons

Abstract

Since the pioneering work of FLEURY et al. /1/, Raman spectroscopy has revealed itself as a powerful tool for the study of magnetic excitations in magnetic insulators. However, there have been only a few studies concerning ferromagnets for the following reasons: first, such compounds are less common than antiferromagnets; second, two-magnon scattering, which allows one to evaluate the exchange interactions, shows a negligibly small intensity in ferromagnets, in contrast to the case of antiferromagnets; third, the magnon frequency at zero wave vector, which is essentially related to anisotropy, is generally too small to allow one-magnon Raman spectroscopy. On the other hand, it is well known /2/ that not only magnons but, more generally, magnetic excitons connected with transitions from the ground state to an excited single-ion level when the magnetic exchange is taken into account, propagate in such crystals and can be optically detected: up to now the extension of Raman studies to magnetic excitons other than magnons has been mainly restricted to measurements involving excitons related to an orbitally degenerate single ion crystal-field ground state /3,4/. Since inter-term transitions within a d configuration are generally allowed for Raman scattering while, in centro-symmetric crystals, they are forbidden for absorption or fluorescence, at least in an electric dipole process, Raman spectroscopy is expected to be a powerful method to derive information about the dⁿ excitonic manifold /5/ provided that a large enough Raman shift spectral range is accessible: such is the case when using an ultraviolet argon laser excitation, which allows one to measure Raman shifts up to 18000 cm⁻¹, a significant fraction of the 3dⁿ configuration.