  Type, degree and evolution of structural order are important aspects for
understanding and controlling the properties of highly spin polarized Heusler
compounds, in particular with respect to the optimal film growth procedure. In
this work, we compare the structural order and the local magnetic properties
revealed by nuclear magnetic resonance (NMR) spectroscopy with the macroscopic
properties of thin Co$_2$FeAl$_{0.5}$Si$_{0.5}$ Heusler films with varying
thickness. A detailed analysis of the measured NMR spectra presented in this
paper enables us to find a very high degree of $L2_1$ type ordering up to 81%
concomitantly with excess Fe of 8 to 13% at the expense of Al and Si. We show,
that the formation of certain types of order do not only depend on the
thermodynamic phase diagrams as in bulk samples, but that the kinetic control
may contribute to the phase formation in thin films. It is an exciting finding
that Co$_2$FeAl$_{0.5}$Si$_{0.5}$ can form an almost ideal $L2_1$ structure in
films though with a considerable amount of Fe-Al/Si off-stoichiometry.
Moreover, the very good quality of the films as demonstrated by our NMR study
suggests that the novel technique of off-axis sputtering technique used to grow
the films sets stage for the optimized performance of
Co$_2$FeAl$_{0.5}$Si$_{0.5}$ in spintronic devices.
