Atomic relocation in ion-bombarded ultra-thin films analyzed with sub-nm spatial resolution

The displacement of atoms in a magnetic trilayer Fe (10 nm)/Cr (0.7 nm)/Fe (10 nm) system by 30 keV Ga+ ion irradiation was studied by 3D Atom Probe Tomography (APT). From APT, the positions of individual Cr and Fe atoms could be located with sub-nm spatial resolution, both before and after ion bombardment. In the pristine specimen the presence of the 0.7-nm Cr layer was identified and individual lattice planes with a distance of ∼0.15 nm were observed which would correspond to the monolayer spacing of the Fe lattice, in agreement with the growth process. Upon irradiation, the Cr layer broadens to ∼1.2 nm at a fluence of 3 × 1014 Ga+/cm2 and to ∼3.4 nm at 3 × 1015 Ga+/cm2. From this broadening the mean squared relocation distance of (Cr) atoms was derived, 〈r2〉 ∼0.4 nm2. Computer simulations indicate that, at a fluence of 3 × 1014 Ga+/cm2, each Cr atom in the intermediate layer is displaced on average once in the collision processes. The distribution of implanted Ga ions appears to exhibit discontinuities at the Fe/Cr interfaces which might be caused by a demixing of Ga at the Cr layer.