SPIN DYNAMICS OF MIXED-VALENT RARE-EARTH COMPOUNDS

A survey is given on the dynamical behaviour of homogeneous mixed-valent compounds as observed from magnetic neutron scattering experiments. Mixed-valent compounds are characterized by the existence of two nearly degenerate electronic configurations for the rare earth ion with integer occupation numbers of the 4f shell. From XPS- and Mössbauer effect measurements a lower (10-15 sec) and an upper (10-9 sec) limit for the lifetime of both configurations (involving charge fluctuations) can be estimated. From the neutron scattering experiments we learn that in these compounds there are also very fast spin fluctuations of the order of 10-13 sec. The characteristic differences in the dynamical behaviour of compounds with a magnetic and a nonmagnetic configuration (Ce and Yb compounds) and a compound with two magnetic configuration (TmSe) will be discussed.