Ion irradiation damage in nanocrystalline FeCo thin films

Abstract
Thin films of the Fe60Co40 alloy produced by electron beam evaporation under ultra high vacuum are irradiated at 77 K with different ions: He, Ne, Ar, Xe. The as-evaporated films are polycrystalline with a very small grain size of about 8 nm and exhibit the structure of nanocrystalline materials formed of a crystalline component inside the grains and a second component in the grain boundaries with a random atomic arrangement. We have continuously measured the radiation damage as a function of the ion dose at 77 K using in situ electrical resistivity measurements. An important resistivity decrease is observed at high doses but only with heavy ions. This particular behaviour is discussed in relation with both the displacement cascade effects and the microstructural state of the films. It is suggested that nanocrystalline materials could be useful to study thermal spike behaviour and specific effects about displacement cascades.