Spin-lattice relaxation time (/'1) of Fe 3+ ions has been measured in four borate glasses doped with 0.25, 0.5, 1 and 3 wt.% Fe203 by the modulation method over the temperature range 5-250 K. In the three less concentrated samples, ir was observed that T; -I oc 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 Ti ~ is slowed down by cross-relaxation. In the most concentrated sample, the exchange interaction plays a dominant tole leading to a very different relaxation-time behaviour, described well by the Bloembergen and Wang three-reservoir model.

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