Temperature dependence for the metastable phase formation in Au50Ni50 thinfilms under ion bombardment

The transformation of crystalline (111) oriented Au50Ni50 thin films produc ed by ion beam mixing of Au/Ni multilayers was studied under krypton ion ir radiation in the temperature range from 4 to 100 K. The volume fraction of the amorphous phase was determined as a function of the ion fluence from in situ resistivity measurements, while ex situ x-ray diffraction was used to analyze the residual crystalline fraction. The experimental results indica te that the total Kr+ fluence necessary for complete amorphization of a met astable Au50Ni50 solid solution exponentially increases with increasing bom bardment temperatures up to a critical value T-c of approximately 100 K, ab ove which an ion induced amorphization is no longer possible. Furthermore, within the amorphization regime, two different mechanisms can be distinguis hed: A direct amorphization process via dense cascades at very low temperat ures, while close to T-c overlapping of sequentially damaged regions is req uired to obtain the amorphous phase. The fact that amorphization is never o bserved under light ion irradiation even at 4 K demonstrates that the idea of a lattice collapse by defect accumulation cannot explain the amorphizati on behavior in this system. This points to the importance of energetic dens e cascades for the amorphization process. Additional experiments on nanocry stalline Au50Ni50 films suggest that the lack of structural relaxation and/ or the reduction of crystalline regrowth at amorphous/crystalline interface s are necessary conditions for the stabilization of the amorphous clusters formed within the cores of dense cascades. Thus, the observed kinetic behav ior is explained by considering the balance between dynamic annealing and a morphous phase formation. The importance of interface processes for crystal line growth is corroborated by studying the ion induced crystallization of an amorphous film. (C) 1999 American Institute of Physics. [S0021-8979(99)0 6911-X].