Heteroepitaxy route to substrate limited epitaxial Co2TiSn thin films by sputter beam epitaxy

We report a substrate-limited, high-quality thin film growth of Co2TiSn, requiring the use of a substrate-limited TiN buffer layer as well as a Nb capping layer to achieve an x-ray diffraction rocking curve full width at halfmaximum of 12 arcseconds. This value represents more than an order of magnitude improvement over previous results for TiN, and more than two orders of magnitude better than any previous Co2TiSn thin film reported. Our surface analysis of the TiN film grown on a-plane sapphire reveals a distinctive step-edge-guided nucleation and epitaxial layer growth. Additionally, our first-principles calculations on Co2TiSn in its L21 and XA phases examine the limitations of various exchange correlation functionals and the impact of Hubbard U correction on accurately representing the magnetic properties and density of states of Co2TiSn. The observed lower-than-ideal magnetic moment in the disordered (XA) phase corresponds well with the experimentally observed reduced magnetization of the Co2TiSn film, as no evidence of phase segregation or secondary phase is detected. This work serves as both a roadmap for making high-quality Heusler alloy thin films and a stepping stone for the evaluation of the intrinsic atomic ordering in this and other Heusler systems by providing a high-quality crystal with energetics during formation as close to the thermodynamic limit as possible.