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