Effects of Composition on the Ordered Phase Formation in Mn-Al and Mn-Ge Alloy Thin Films Grown on Cr(001) Single-Crystal Underlayers

Mn _x Al _100-x and Mn _y Ge _100−y alloy epitaxial films of 20 nm thickness are prepared on Cr(001) single-crystal underlayers at 300 °C by varying the Mn content, $x$ or $y$ , from 0 to 100 at. %. The formation of crystallographic phase is systematically investigated as a function of composition by reflection high-energy electron and X-ray diffractions. The phases in Mn _x Al _100−x and Mn _y Ge _100−y films are determined to vary in the orders of $A 1=> L 1_{0} + A 12=> L 1_{0} => A 2=> A12$ and $A 4=> tP => cP => D 0_{22} => A12$ with increasing the Mn content, respectively. Metastable fct-based ordered $L1$ ₀ phase is formed in the Mn _x Al _100−x films with $x $ = 50– $65. L1$ ₀ single phase is obtained at an Mn-rich composition of $x $ = 58, whereas bcc-based disordered $A12$ and $A2$ phases are, respectively, involved at the $L1$ ₀ stoichiometric composition of $x $ = 50 and at a more Mn-rich composition of $x $ = 65. Employment of a slight Mn-rich composition stabilizes $L1$ ₀ phase. On the other hand, metastable fct-based ordered $D0$ ₂₂ phase is formed in the Mn _y Ge _100−y films with $y$ = 60– $75. D0$ ₂₂ single phase is obtained in the range from the stoichiometric composition of $y$ = 75 to a slightly Mn-poor composition of $y $ = 68. The $c$ -axis orientation is normal to the film surface for both $L1$ ₀ and $D0$ ₂₂ crystals. The present study has clarified the crystal structure and growth of Mn _x Al _100−x and Mn _y Ge _100−y films for the respective full compositional ranges and has shown that control of the composition is important to obtain $L1$ ₀ and $D0$ ₂₂ ordered Mn-based alloy films with perpendicular magnetic anisotropies.