Pseudomorphic-to-bulk fcc phase transition of thin Ni films on Pd(100)

We have measured the transformation of pseudomorphic $\mathrm{Ni}$ films on $\mathrm{Pd}\left(100\right)$ into their bulk fcc phase as a function of the film thickness. We made use of x-ray diffraction and x-ray induced photoemission to study the evolution of the $\mathrm{Ni}$ film and its interface with the substrate. The growth of a film with tetragonally strained face centered symmetry (fct) has been observed by out-of-plane x-ray diffraction up to a limit thickness of $10\mathrm{Ni}$ pseudomorphic layers (some of them partially filled and intermixed with the substrate), where a new fcc bulklike phase is formed. After the formation of the bulklike $\mathrm{Ni}$ domains, we observed the pseudomorphic fct domains to disappear preserving the number of layers and their spacing. The phase transition thus proceeds via lateral growth of the bulklike phase within the pseudomorphic one, i.e., the bulklike fcc domains penetrate down to the substrate when formed. This large depth of the walls separating the domains of different phases is also indicated by the increase of the intermixing at the $\text{substrate}–\text{film}$ interface, which starts at the onset of the transition and continues at even larger thickness. The bulklike fcc phase is also slightly strained; its relaxation towards the orthomorphic lattice structure proceeds slowly with the film thickness, being not yet completed at the maximum thickness presently studied of $30\mathrm{\AA }$ ($\sim 17$ layers).

Figure 1

Logarithm of the x-ray reflectivity energy scan (dots), taken at fixed scattering angles of $7°$ and $6°$ for two films of $\sim 22$ and $\sim 30\mathrm{\AA }$, respectively. The two curves have been shifted for the sake of clarity. The film thickness and interface widths are expressed as $\mathrm{Ni}$ fcc layer-equivalent (Leq) after fitting to the experimental data (full lines) with a simple $\mathrm{Ni}∕\mathrm{Pd}$ bulk model. The $\mathrm{Ni}∕\mathrm{Pd}$ and $\mathrm{vacuum}∕\mathrm{Ni}$ interfaces are assumed to have a Gaussian depth profile, whose widths full width at half maximum are also indicated in the figure legend.

Figure 2

$\mathrm{Pd}$ $3d$ photoemission spectra taken at a photon energy of $500\mathrm{eV}$ with an overall energy resolution of $\sim 210\mathrm{meV}$. The spectra are not to scale and they have been amplified and vertically shifted for a better comparison of the shape changes. The bulk component is the shoulder at $335.2\mathrm{eV}$ binding energy of the $\mathrm{Pd}$ $3d$ spectrum taken on the clean $\mathrm{Pd}$ substrate (bottom curve). The surface component yields a core level shift of $-0.35\mathrm{eV}$ to lower binding energy. As $\mathrm{Ni}$ is deposited, a new component, due to $\mathrm{Pd}–\mathrm{Ni}$ alloying, appears at a binding energy higher than the bulk one. The $\mathrm{Pd}$ surface component fully disappears after the deposition of $5\mathrm{\AA }$ of $\mathrm{Ni}$. The interface component is the main component at $16\mathrm{\AA }$ and beyond. The vertical bars mark the binding energies of the bulk and interface components. Each spectrum is labeled by its nominal coverage.

Figure 3

Valence band of the $\mathrm{Pd}\left(100\right)$ sample before and after deposition of $2.5\mathrm{\AA }$ of $\mathrm{Ni}$. The curves have been vertically shifted for the sake of clarity. The top curve represents the difference spectrum of the former ones and puts in evidence the satellite peak at $-5\mathrm{eV}$, which is due to $\mathrm{Pd}–\mathrm{Ni}$ alloying.

Figure 4

Upper panel: intensity of the components of the $\mathrm{Pd}$ $3d_{5∕2}$ photoemission peak (markers) as a function of the coverage of the $\mathrm{Ni}$ films (see text for the calibration of the film coverage). The full line is a fit to the bulk component (closed circles) with an exponential decay. The interface component (dotted line and open circles) follows a different decay law. Lower panel: intensity of the bulk (closed circles) and interface (open circles) $\mathrm{Pd}3d_{5∕2}$ components, after normalization to the attenuation factor yielded by the exponential decay of the $\mathrm{Pd}$ bulk component.

Figure 5

Anisotropy of the polar scans taken for the $\mathrm{Ni}$ $2p_{3∕2}$ photoelectron peak (kinetic energy of $\sim 415\mathrm{eV}$) along the $⟨100⟩$ and $⟨011⟩$ substrate symmetry directions, upper and lower panel, respectively. The curves have been vertically shifted for a better comparison. The vertical bars indicate the angular position of the forward scattering focusing peaks which are characteristic of an fcc symmetry. Deviations from the nominal values, i.e., $45°$ and $54.7°$ from the surface, are mainly due to distortions of the lattice cell in the topmost layers (which changes with the film coverage).

Figure 6

Radial scans of the $\left(\overline{2}\mathrm{0\hspace{0.17em}0}\right)$ XRD peak for a few $\mathrm{Ni}$ films of different coverage (alternating full and dotted lines). The XRD measurements have been taken by scanning the photon energy between $5400$ and $7000\mathrm{eV}$ at fixed scattering geometry. The photon beam impinges the surface at grazing incidence, forming an angle $\theta \sim 35°$ with respect to the $\mathrm{fcc}\left(100\right)$ planes of the direct lattice. The substrate XRD peak is always out of scale, and the corresponding lattice spacing is indicated by the thick vertical line, as well as the lattice spacing of bulk fcc $\mathrm{Ni}$.

Figure 7

Integrated intensity of the XRD $\left(\overline{2}0L\right)$ rods of $\mathrm{Pd}$ for a few $\mathrm{Ni}$ films of different coverage (closed markers). The curves have been vertically shifted for the sake of clarity. Modulations in the low coverage films arise from Bragg interference among the $\mathrm{Ni}$ pseudomorphic layers, which are confined between sharp interfaces at both the top and bottom of the film. Fitting to the data (full lines) yields a structural model as reported in Table .