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, investigated 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) and one relative to the creation of defects in the bcc Fe layers (T-2 similar to 45 keV/nm). If the electronic stopping power (dE/dx)(e) value of ions is less than TI, only a demixing of Fe and Tb atoms can occur at the interfaces, producing both a thickening of the pure bce Fe layer and a sharpening of the interfaces whatever the ion fluence is. Between 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 progressively the layered structure. At high-ion fluences, the samples exhibit the magnetic properties of the corresponding amorphous Fe-Tb alloys. When the energy deposited by the ions exceeds the T-2 value, a third phenomenon appears in addition to the two previous ones: the initial bce Fe layers are transformed into ''pure'' disordered Fe layers. A qualitative explanation of the evolution versus the ion fluences is proposed using the thermal spike model.