Magneto-optical effects in relation to excitation energies of 4f electrons in pure and doped rare earth compounds

Abstract

A significant contribution to the large magneto-optic effects observed in a variety of pure and doped (alloyed) rare earth (RE) compounds has been ascribed to the excitation of 4f electrons. Necessary condition for a detailed understanding of the mechanism of this contribution is a knowledge of the excitation spectrum of the 4f levels into a continuum in pure crystals, and its dependence on the local atomic environment in doped (alloyed) materials. A number of pure and doped RE compounds have been investigated by high resolution X-ray photoemission spetroscopy. For the pure materials the spectra can be reasonably well described by the method of fractional parentage assuming a unique binding energy E B(4f n ) of the 4f levels, but for doped systems a distribution of E B(4f n ) over an energy range Δ E must be assumed. E B(4f n ) depends on the valence of the substituents, and in a nonlinear way their number of nearest neighbors (nn)Δ E can be of the order of 0.5 eV. In pure systems and for a given RE cation we found the experimental width of the 4f-lineshape to be correlated with the number of nn anions. This is viewed as due to the relative position of localized and extended states. In contrast, for the various RE ions within a series of isostructural compounds involving the same anion the linewidth is found to be correlated with the number of holes (15- n) in the 4f shell after photoionization.