Magnetic properties of selected Mn-based transition metal compounds with $\beta -\mathrm{Mn}$ structure: 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 $\beta -\mathrm{Mn}$ 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 ${\mathrm{Mn}}_3{\mathrm{Ir}}_{1-y}{\mathrm{Co}}_y\mathrm{Si}$ $(0⩽y⩽1)$ and ${\mathrm{Mn}}_3{\mathrm{CoSi}}_{1-x}{\mathrm{Ge}}_x$ $(0⩽x⩽1)$ 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° 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 $\beta -\mathrm{Mn}$.

Figure 1

(Color online) Observed (points), calculated (solid line) and difference (bottom solid line) neutron powder diffraction profiles for ${\mathrm{Mn}}_3\mathrm{CoGe}$ at 295 K. Tick marks indicate the positions of Bragg reflections. The regions excluded from the refinement correspond to peaks from impurity phases (marked with “X”), and a minor peak from the vanadium sample holder, partly overlapping a peak from the crystal structure (marked with “V”).

Figure 2

(Color online) Observed (points), calculated (solid line) and difference (bottom solid line) neutron powder diffraction profiles for ${\mathrm{Mn}}_3\mathrm{CoSi}$ at 295 K. Tick marks indicate the positions of Bragg reflections.

Figure 3

(Color online) The crystal structure of ${\mathrm{Mn}}_3{\mathrm{Ir}}_{1-y}{\mathrm{Co}}_y{\mathrm{Si}}_{1-x}{\mathrm{Ge}}_x$. Near neighbor Mn atoms form a network of corner sharing triangles (shaded). Three Mn atoms outside the unit cell (smaller, with hatched links) are added to complete the network.

Figure 4

(Color online) Temperature dependence of the field cooled (fc) and zero field cooled (zfc) magnetic susceptibility for ${\mathrm{Mn}}_3{\mathrm{Ir}}_{1-y}{\mathrm{Co}}_y\mathrm{Si}:y=1.0$, 0.80, 0.60, 0.40, and 0.20.

Figure 5

Temperature dependence of the field cooled (fc) and zero field cooled (zfc) magnetic susceptibility for ${\mathrm{Mn}}_3{\mathrm{CoSi}}_{1-x}{\mathrm{Ge}}_x:x=1.0$ and $x=0.50$.

Figure 6

(Color online) Orientation of the magnetic moments on three corner sharing triangles of Mn atoms.

Figure 7

(Color online) Observed (points), calculated (solid line) and difference (bottom solid line) neutron powder diffraction profiles for ${\mathrm{Mn}}_3\mathrm{CoGe}$ at 10 K. Tick marks indicate the positions of Bragg reflections for the crystal structure (upper set) and the magnetic structure (lower set). The regions excluded from the refinement correspond to peaks from impurity phases (marked with “X”), and a minor peak from the vanadium sample holder, partly overlapping a peak from the crystal and magnetic structures (marked with “V”).

Figure 8

(Color online) Neutron powder diffraction profiles for (a) ${\mathrm{Mn}}_3\mathrm{CoSi}$ recorded at 295, 120, 75, 10 K, plotted with offset along the intensity axis. (b) ${\mathrm{Mn}}_3{\mathrm{Ir}}_{0.20}{\mathrm{Co}}_{0.80}\mathrm{Si}$ recorded at 295 and 10 K, with the 10 K data offset along the intensity axis (please note that only a fraction of the full peak intensity is shown, highly magnified, in order to more clearly illustrate the small differences in the background curvature).

Figure 9

(Color online) The calculated density of states (DOS) of ${\mathrm{Mn}}_3\mathrm{CoGe}$ in (a) ferromagnetic configuration, (b) noncollinear configuration. The energies are given relative to the Fermi level, which is indicated by a dashed line.

Figure 10

(Color online) Experimental magnetic transition temperature $\left(T_t\right)$, in K, and the calculated exchange energy per Mn atom $\left(J_0\right)$, in meV, plotted as a function of the lattice parameter, in Å.

Figure 11

(Color online) Mn magnetic moments, in ${\mu }_B∕\text{atom}$, as a function of the lattice parameter, in Å.