Growth rate effects in soft CoFe films

In this paper we demonstrate the preparation of CoFe thin films with control of H_c and M_s over a broad range of values. A change in H_c of a factor 10 and a change in M_s by a factor of 2.5 have been achieved by using a special plasma sputtering design called HiTUS (high target utilization sputtering). The change in both H_c and M_s has been achieved by tuning the growth rate. We observed a close relationship between sputtering rate and the final grain size structure of the films. It appears that a low sputtering rate promotes the formation of grains as small as 7 nm, while a fast sputtering rate results in films with mean grain size of 26 nm. This change in the grain size has been observed by a consistent TEM and grain size analysis. However, the saturation magnetization of the two samples is also modified by a factor similar to that in the mean grain size. This suggested that an increase in the saturation magnetization can be achieved by increasing the packing density of samples having small grains. By changing the packing fraction and grain size, the structural, magnetic and electric properties of the films can be substantially altered depending upon the specific application.