TEMPERATURE AND CONCENTRATION DEPENDENCES OF THE SPIN-LATTICE RELAXATION RATE IN 4 BORATE GLASSES DOPED WITH FE2O3

Spin-lattice relaxation time (T-1) of Fe3+ ions has been measured in f our berate glasses doped with 0.25, 0.5, 1 and 3 wt.% Fe2O3 by the mod ulation method over the temperature range 5-250 K. In the three less c oncentrated samples, it was observed that T-1(-1) proportional to T at intermediate temperatures. This is explained in terms of a relaxation mechanism involving TLS (Two Level Systems). At higher temperatures, the temperature dependence of T-1(-1) is slowed down by cross-relaxati on. In the most concentrated sample, the exchange interaction plays a dominant role leading to a very different relaxation-time behaviour, d escribed well by the Bloembergen and Wang three-reservoir model.