  Ion-irradiation-induced modifications of a periodic Pt/C multilayer system
containing Fe impurity have been analyzed by transmission electron microscopy
(TEM). The multilayer stack with 16 Pt/C layer pairs (period 4.23 nm) was
fabricated on a glass substrate. A 2 MeV Au$^{2+}$ ion beam was rastered on the
sample to obtain uniformly irradiated strips with fluences from
1$\times10^{14}$ to 1$\times10^{15}$ $ions/cm^2$. Ion-irradiation has been
found to cause preferential migration of Fe towards Pt layers [Nucl. Instr.
Methods Phys. Res. B212 (2003) 530]. Cross-sectional transmission electron
microscopy (XTEM) shows considerable atomic redistribution for irradiation at
the highest ion fluence (1$\times10^{15}$ $ions/cm^2$). Individual entities in
this structure is like a cluster. Periodic multilayers have periodicity only in
the direction normal to the multilayer surface. However, Fourier transform of
the XTEM images of the sample irradiated at the highest-fluence shows new
off-normal Fourier components of superlattice periodicities arising due to ion
irradiation. With a proper understanding of this phenomenon it may be possible
to fabricate three dimensional periodic structures of nanoclusters.
