Ion irradiation effects on bcc Fe/Tb multilayers

Fe/Tb multilayers with crystallized Fe layers were irradiated with Kr, Xe, Pb, and U ions at various fluences. Damaging processes, investigated by ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ Mössbauer spectrometry at room temperature, give evidence for two thresholds, one for Fe-Tb mixing (${\mathrm{T}}_1$∼25 keV/nm) and one relative to the creation of defects in the bcc Fe layers (${\mathrm{T}}_2$∼45 keV/nm). If the electronic stopping power (dE/dx${)}_{\mathrm{e}}$ value of ions is less than ${\mathrm{T}}_1$, only a demixing of Fe and Tb atoms can occur at the interfaces, producing both a thickening of the pure bcc Fe layer and a sharpening of the interfaces whatever the ion fluence is. Between ${\mathrm{T}}_1$ and ${\mathrm{T}}_2$, 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 ${\mathrm{T}}_2$ value, a third phenomenon appears in addition to the two previous ones: the initial bcc 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.