  A detailed study of the spin dynamics of the S-state ions Gd3+ and Eu2+ in
the filled skutterudites La1-xRxPt4Ge12 (R= Gd, Eu) is reported. The spin
dynamics is investigated directly by means of Gd3+ and Eu2+ electron spin
resonance (ESR), performed at X-band (~ 9.4 GHz) and Q-band (~ 34 GHz)
frequencies in the temperature intervals 8<T<300 K and 1.5<T<300 K
respectively. The ESR parameters provide direct evidence for the vibrational
behavior of the Gd3+ ions but not for the Eu2+ ions. These results are
interpreted in the light of recent discussions about the spin relaxation in
cage systems. In particular, the Gd3+ spin relaxation in La0.9Gd0.1Pt4Ge12
provide evidence for the existence of an extra phonon mode with an Einstein
temperature (thetaE) ~ 24 K in this system. The work suggests that the
so-called "rattling" modes have a general and important role that should be
taken into account in the study of spin dynamics in cage systems.
