Structural properties of Co${}_2$TiSi films on GaAs(001)

Co${}_2$TiSi films were grown by molecular beam epitaxy on GaAs(001) and analyzed using reflection high-energy electron diffraction, and electron microscopy. In addition, X-ray diffraction was combined with lattice parameter calculations by density functional theory comparing the $\mathit{\text{L}}{2}_1$ and $\mathit{\text{B}}$2 structures and considering the influence of non–stoichiometry. Columnar growth is found and attributed to inhomogeneous epitaxial strain from non-random alloying. In films with thicknesses up to 13~nm these columns may be the origin of perpendicular magnetization with the easy axis perpendicular to the sample surface. We found $\mathit{\text{L}}{2}_1$ and $\mathit{\text{B}}$2 ordered regions, however the [Co]/[Ti]–ratio is changing in dependence of the position in the film. The resulting columnar structure is leading to anisotropic $\mathit{\text{B}}$2–ordering with the best order parallel to the axes of the columns.