Transient E.S.R. absorption in Eu0.4Sr0.6S, in the paramagnetic phase, near the spin-glass temperature : the exchange reservoir

2014 In the insulating spin-glass material EU0.4Sr0.6S, the transient E.S.R. absorption following a strong r.f. pulse has been studied at X band for T ~ Tg. The signal was found to depend strongly upon the experimental conditions (energy of the r.f. pulse, value of the driving magnetic field). At 4.2 K and with a weak perturbing r.f. energy (1 03BCJ), the recovery was exponential (03C4 ~ 40 ms). By coupling continuous saturation experiments with pulse experiments made at different values of the driving field, we have shown that the observed signal corresponds to a transient shift of the absorption line, reflecting a rise of the spin temperature. This signal is finally the dynamical manifestation of a behaviour already met in steady E.S.R. experiments. The experimental results are discussed within the Bloembergen-Wang exchange reservoir model; it is found that, due to the dipolar interactions, the r.f. energy absorbed by the Zeeman reservoir is rapidly (10-10 s) transferred to the exchange reservoir. The energy is then given up to the phonons through the modulation of the exchange coupling by the phonons (direct process). The heat capacity of the spins mainly comes from the exchange energy, and overwhelms that of the phonons up to 15 K; this leads to a phonon-bottleneck : the phonons rapidly reach the spin temperature at the end of the pulse, then the temperature of the whole spin-phonon system slowly relaxes towards the bath temperature. J. Physique 46 (1985) 965-978 JUIN 1985,