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.