  Atomic diffusion at nanometer length scale may differ significantly from bulk
diffusion, and may sometimes even exhibit counterintuitive behavior. In the
present work, taking Cu/Ni as a model system, a general phenomenon is reported
which results in sharpening of interfaces upon thermal annealing, even in
miscible systems. Anomalous x-ray reflectivity from a Cu/Ni multilayer has been
used to study the evolution of interfaces with thermal annealing. Annealing at
423 K results in sharpening of interfaces by about 38%. This is the temperature
at which no asymmetry exists in the inter-diffusivities of Ni and Cu. Thus, the
effect is very general in nature, and is different from the one reported in the
literature, which requires a large asymmetry in the diffusivities of the two
constituents [Z. Erd\'elyi et al., Science 306, 1913 (2004).]. General nature
of the effect is conclusively demonstrated using isotopic multilayers of
57Fe/naturalFe, in which evolution of isotopic interfaces has been observed
using nuclear resonance reflectivity. It is found that annealing at suitably
low temperature (e.g. 523 K) results in sharpening of the isotopic interfaces.
Since chemically it is a single Fe layer, any effect associated with
concentration dependent diffusivity can be ruled out. The results can be
understood in terms of fast diffusion along short-circuit paths like triple
junctions, which results in an effective sharpening of the interfaces at
relatively low temperatures.

Materials Science