1In the system under consideration, the thicknesses of the Pt layer and Ni layer are 23 Å and 22 Å, respectively. The thickness of the C layer was varied from 29.7 Å (at the bottom-most layer) to 35.7 Å (at the top-most layer), since the system is a depth-graded multilayer. Despite the location of antinodes at the interfaces, there will be some contribution to the signal from the non-interfacial regions of the layers.

2With the penetration depth of 2 MeV Au2+ ions being of the order of micrometers, ions pass through the ML sample and become embedded deep in the substrate; the variation owing to the ion impact factor is negligible.

3Bulk Ni f.c.c. lattice has 12 neighbors while interfacial Ni has nine neighbors. Three missing neighbors correspond to dangling bonds for the {111} plane. In the case of the {110} and {100} planes, interfacial Ni has eight neighbors and the four missing neighbors correspond to dangling bonds. Since the layers are polycrystalline in the sample, we have to consider an average over these three planes.

4The intensity variation (Fig. 1[link]) in the various layers is normalized with respect to the intensity in (i) the C layer for low angle of the Bragg peak and (ii) the Pt layer for high angle of the Bragg peak.

5There are no Ni atoms at the C/Pt interface.

6Total NNi-C = 0.88, out of which 0.37 is interfacial. This implies that 0.51 is in the C layer. Now, each Ni atom in the C layer has two C coordination (amorphous carbon). 2 × (total number of Ni atoms) contribute to NNi-C = 0.51. Therefore the percentage of Ni atoms diffused into the C layer = 0.51/2 = 26%.

7The integrated intensities for each interface and layer were calculated. The total NNi-Ni has contributions from the Ni + Pt mixed layer plus the C layer plus the Ni/C interface: [NNi-Ni]Total = ([N_{\rm C\,layer}\textstyle\int I_{\rm C}] + [N_{\rm Ni+Pt\,mixed\,layer}\textstyle\int I_{\rm Ni+Pt}] + [N_{\rm Ni/C}\textstyle\int I_{\rm Ni/C}]) / ([\textstyle\int I_{\rm C}] + [\textstyle\int I_{\rm Ni+Pt}] + [\textstyle\int I_{\rm Ni/C}]).

8NNi-Ni = 12(1 - x) + 9x; NNi-Pt = 3x.

9A Pt-core Ni-shell would imply that surface Pt atoms see an average of three Ni nearest neighbors, assuming the surface to be (111); so each Ni atom will see 1/3 = 0.3 Pt atoms.

10An ideal sharp interface implies that there is no diffusion between the layers beyond the interface.

11The following equations were used to deduce the degree of mixing between the Ni and Pt layers: NNi-Ni = [[12(N_A-\Delta{N})\textstyle\int{I_{\rm Ni}}/\!\textstyle\int{I_0}] + [(8.33\textstyle\int{I_{\rm Ni/Pt}}/\!\textstyle\int{I_0})N_B]]/[[(\textstyle\int{I_{\rm Ni}}/\textstyle\int{I_{\rm 0}})N_A] + [(\textstyle\int{I_{\rm Ni/Pt}}/\textstyle\int{I_{\rm 0}})N_B]], NNi-Pt = [[12\Delta{N}\textstyle\int{I_{\rm Pt}}/\textstyle\int{I_{\rm 0}}] + [(1/12)\Delta{N}\textstyle\int{I_{\rm Ni}}/\textstyle\int{I_{\rm 0}}] + [3.67(\textstyle\int{I_{\rm Ni/Pt}}/\textstyle\int{I_{\rm 0}})N_B]]/[[(\textstyle\int{I_{\rm Pt}}/\textstyle\int{I_{\rm 0}})\Delta{N}] + [(\textstyle\int{I_{\rm Ni}}/\textstyle\int{I_{\rm 0}})N_A] + [(\textstyle\int{I_{\rm Ni/Pt}}/\textstyle\int{I_{\rm 0}})N_B]], where [Delta]N is the number of Ni atoms exchanged, NA is the number of Ni atoms in the Ni layer, NB is the number of Ni atoms at the Ni/Pt interface.

12Note that the total Ni vacancy, owing to diffusion, = 26% (Ni [rightwards arrow] C) + 22% (Ni [rightwards arrow] Pt) = 48%, is consistent with the total Pt [rightwards arrow] Ni diffusion = 54%.