Magnetic properties of selected Mn-based transition metal compounds withβ−Mnstructure: Experiments and theory

Two compounds, ${\mathrm{Mn}}_{3}\mathrm{CoSi}$ and ${\mathrm{Mn}}_{3}\mathrm{CoGe}$ have been synthesized and found to crystallize in the ${\mathrm{AlAu}}_{4}$ type structure, an ordered form of the $\ensuremath{\beta}\ensuremath{-}\mathrm{Mn}$ structure. The magnetic structure and properties have been studied by magnetometry and neutron powder diffraction and the theoretical work is based on first principles total energy calculations. Comparison is made with the magnetic properties of the isostructural compounds ${\mathrm{Mn}}_{3}\mathrm{IrGe}$ and ${\mathrm{Mn}}_{3}\mathrm{IrSi}$. The solid solutions ${\mathrm{Mn}}_{3}{\mathrm{Ir}}_{1\ensuremath{-}y}{\mathrm{Co}}_{y}\mathrm{Si}$ $(0\ensuremath{\leqslant}y\ensuremath{\leqslant}1)$ and ${\mathrm{Mn}}_{3}{\mathrm{CoSi}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$ $(0\ensuremath{\leqslant}x\ensuremath{\leqslant}1)$ are also studied. A noncollinear antiferromagnetic structure is experimentally observed for $y=0.20$ as well as for $x=0.50$ and 1.0, similar to that of ${\mathrm{Mn}}_{3}\mathrm{IrSi}$ and ${\mathrm{Mn}}_{3}\mathrm{IrGe}$, with 120\ifmmode^\circ\else\textdegree\fi{} angles between magnetic moments on a triangular network of Mn atoms, and this finding is corroborated by theoretical calculations. For $y=0.80\text{--}1.0$ a transformation to a new type of magnetic structure takes place. The magnetic transition temperature decreases on decreasing unit cell dimension, with good qualitative agreement with the decay of the calculated interatomic exchange energy. Both theory and experiments find the magnitude of the Mn magnetic moment to decrease on decreasing unit cell volume, the same trend is found in calculations for $\ensuremath{\beta}\ensuremath{-}\mathrm{Mn}$.