Polymorphic crystallization of interface stabilized amorphous Fe-Zr thin films under variable driving force

Abstract

We report about experiments concerning the stability of thin films of ${\mathrm{Fe}}_{100\ensuremath{-}x}{\mathrm{Zr}}_{x}$ in the concentration range $0lxl7 \mathrm{at}.%.$ The films are grown using electron beam evaporation under UHV conditions on Zr base layers at 300 K. On these substrate layers, pure Fe and the Fe-Zr alloy films initially grow in the amorphous phase. At a critical thickness ${d}_{c},$ crystallization of the films is observed at room temperature. The crystallization is monitored quantitatively using the magnetic properties of the Fe-Zr alloys which are paramagnetic at room temperature in the amorphous state but ferromagnetic in the bcc phase. The thickness ${d}_{c}$ increases with increasing Zr concentration from about 2 nm for pure Fe to 30 nm for $x=7 \mathrm{at}.%.$ A model for the transformation of the amorphous layer is presented which includes the variation of the thermodynamic driving force with the Zr concentration and the stabilizing effect of the interface to the Zr substrate layer. The model can account for the concentration dependence of ${d}_{c}$ and yields a reasonable value for the interface energy contributions. Additional contributions to the phase stabilities such as elastic energy and defect contributions will modify the energy balance between driving force and interface stabilization and may therefore influence the transformation. A quantitative treatment shows that contributions from grain boundaries formed during the crystallization have to be considered whereas the elastic energy contributions are less important. This is a consequence of the large driving forces for polymorphous crystallization. The results are not unique to the Fe-Zr system but should also apply to other Fe\char21{}early transition metal or Fe\char21{}rare-earth multilayers.