ION IRRADIATION EFFECTS ON BCC FE TB MULTILAYERS/

Fe/Tb multilayers with crystallized Fe layers were irradiated with Kr, Xe, Ph, and U ions at various fluences. Damaging processes, investiga ted by Fe-57 Mossbauer spectrometry at room temperature, give evidence for two thresholds, one for Fe-Tb mixing (T-1 similar to 25 keV/nm) a nd one relative to the creation of defects in the bcc Fe layers (T-2 s imilar to 45 keV/nm). If the electronic stopping power (dE/dx)(e) valu e of ions is less than TI, only a demixing of Fe and Tb atoms can occu r at the interfaces, producing both a thickening of the pure bce Fe la yer and a sharpening of the interfaces whatever the ion fluence is. Be tween T-1 and T-2, the Fe-Tb demixing is still observed at the lowest fluences, but then the mixing of Fe and Tb layers destroys progressive ly the layered structure. At high-ion fluences, the samples exhibit th e magnetic properties of the corresponding amorphous Fe-Tb alloys. Whe n the energy deposited by the ions exceeds the T-2 value, a third phen omenon appears in addition to the two previous ones: the initial bce F e layers are transformed into ''pure'' disordered Fe layers. A qualita tive explanation of the evolution versus the ion fluences is proposed using the thermal spike model.