Temperature Driven Reversible Breakdown of Pseudomorphism in Ultrathin $\mathrm{F}\mathrm{e}/{\mathrm{C}\mathrm{u}}_3\mathrm{A}\mathrm{u}$ Films

We observe that ultrathin $\mathrm{F}\mathrm{e}/{\mathrm{C}\mathrm{u}}_3\mathrm{A}\mathrm{u}(001)$ films in the $6–13Å$ thickness range, beyond the thickness of pseudomorphism breakdown at room temperature, exhibit a temperature dependent structural phase transition in the range $T_c\approx 345–380\mathrm{K}$. In the high temperature state the Fe film becomes pseudomorphic, while breakdown of pseudomorphism reversibly occurs as the system is cooled below the transition temperature. The difference between substrate and overlayer thermal expansion coefficient is highlighted as the driving force for the observed transition.

Figure 1

Temperature dependent LEED line scans measured along (110) azimuth for $8Å$ $\mathrm{F}\mathrm{e}/{\mathrm{C}\mathrm{u}}_3\mathrm{A}\mathrm{u}$ (see text for details).

Figure 2

Temperature dependence of HeD and LEED diffraction peak parameters for an annealed $8Å$ $\mathrm{F}\mathrm{e}/{\mathrm{C}\mathrm{u}}_3\mathrm{A}\mathrm{u}$ film: open symbols refers to HeD data, and full symbols to LEED. (a) $I_{\pi }$ (up triangles), $I_{\gamma }$ (down triangles), and $f=I_{\gamma }/(I_{\gamma }+I_{\pi })$ (circles) vs temperature. (b) $k_{\parallel }(\Gamma )$ vs temperature measured by HeD (circles) and LEED (rhombi); $(k_{\parallel }^{\pi }-k_{\parallel }^{\gamma })$ vs temperature (squares). (c) HeD specular (diamonds) and HeD $\Gamma$ peak (squares) intensity vs temperature. Lines are guides to the eye.