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

We report about experiments concerning the stability of thin films of Fe100 -xZrx in the concentration range 0<x<7 at. %. The films are grown using ele ctron 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 qua ntitatively using the magnetic properties of the Fe-Zr alloys which are par amagnetic at room temperature in the amorphous state but ferromagnetic in t he bcc phase. The thickness d(c) increases with increasing Zr concentration from about 2 nm for pure Fe to 30 nm for x = 7 at. %. A model for the tran sformation of the amorphous layer is presented which includes the variation of the thermodynamic driving force with the Zr concentration and the stabi lizing effect of the interface to the Zr substrate layer. The model can acc ount for the concentration dependence of d(c) and yields a reasonable value for the interface energy contributions. Additional contributions to the ph ase stabilities such as elastic energy and defect contributions will modify the energy balance between driving force and interface stabilization and m ay therefore influence the transformation. A quantitative treatment shows t hat contributions from grain boundaries formed during the crystallization h ave to be considered whereas the elastic energy contributions are less impo rtant. This is a consequence of the large driving forces for polymorphous c rystallization. The results are not unique to the Fe-Zr system but should a lso apply to other Fe-early transition metal or Fe-rare-earth multilayers. [S0163-1829(99)00921-2].