Revealing superstructure ordering in ${\mathrm{Co}}_{1+x}\mathrm{MnSb}$ Heusler alloys and its effect on structural, magnetic, and electronic properties

We have performed a combined experimental and theoretical study of the ${\mathrm{Co}}_{1+x}\mathrm{MnSb}$ system to understand the progressive evolution of the crystal structure and physical properties as a function of Co content. We find that all the arc-melted polycrystalline samples show superstructure ordering similar to CoMnSb. In the CoMnSb superstructure with $Fm\text{−}3m$ symmetry, one set of the $32f$ sites is filled with Co atoms while the other set is vacant. With increasing Co content, although the vacant set of $32f$ sites gets progressively filled with the Co atoms, some of the Co atoms segregate out of the main phase into the grain boundaries. The maximum Co that enters in the ${\mathrm{Co}}_{1+x}\mathrm{MnSb}$ phase is $x=\sim 0.45$. Thus, we find that the theoretically predicted ${\mathrm{Co}}_2\mathrm{MnSb}$ in $L{2}_1$ phase does not stabilize. All the samples are ferromagnetic above room temperature and the trend in the measured magnetic moments with increasing $x$, agrees reasonably well with the density-functional theory calculations done using the structural and compositional parameters obtained from the Rietveld refinement of the synchrotron x-ray diffraction patterns. However, the electronic structure indicates that in spite of the large magnetic moment, none of the alloys are half metallic. Finally, we find that a minor deviation from stoichiometry in CoMnSb, i.e., excess of Co and Sb as compared to Mn, is accommodated in the set of vacant $32f$ sites of the superstructure. This explains the increase in the lattice parameter and the saturation magnetization, as compared to the calculated stoichiometric CoMnSb superstructure. Calculations also predict that this minor deviation from stoichiometry destroys the half metallicity in the CoMnSb superstructure.

Figure 1

Unit cell of (a) CoMnSb in $C{1}_b$ structure, (b) ${\mathrm{Co}}_2\mathrm{MnSb}$ in $L{2}_1$ structure, and (c) CoMnSb in superstructure. (d) Alternate arrangement of ${\mathrm{Co}}_2\mathrm{MnSb}$ and MnSb substructural units in the superstructure. To identify the inequivalent atoms in different sites of the unit cells, the atoms are marked with different colors.

Figure 2

(a) Room-temperature powder XRD patterns of the CMS-1 to CMS-5 samples, annealed at $850{}^{\circ }\mathrm{C}$. The presence of all the diffraction peaks indicates superstructure ordering. If the samples had formed in $C{1}_b$ or $L{2}_1$ structure, only the peaks marked with star (*) would have appeared in the diffraction pattern. The peak positions marked with (#) correspond to the fcc Co phase. To clearly indicate the existence of the fcc Co in the XRD profiles of the samples CMS-3, CMS-4, and CMS-5, the (220) peak is expanded and shown in the inset. (b) The normalized intensity of the (844) peak showing the change of peak shape and 2θ shift as a function of sample composition.

Figure 3

Optical microscopy images (left column), SEM images (center column), and elemental mapping images (right column) of the (a) CMS-1, (b) CMS-2, (c) CMS-3, (d) CMS-4, and (e) CMS-5 samples. In elemental mapping, the red, green, and blue colors represent Co, Mn, and Sb atoms, respectively.

Figure 4

Rietveld refinement of synchrotron XRD patterns for the samples (a), (b) CMS-1, (c) CMS-2, (d) CMS-3, (e) CMS-4, and (f) CMS-5, respectively. In the insets to the figures (a) and (b), the (440) peak is magnified to show the quality of fitting.

Figure 5

$M$ vs $H$ curves at (a) 2 K and (b) 300 K for fields up to 70 kOe.

Figure 6

Comparison of spin-polarized total DOS and partial DOS calculated for different compositions of the Co-Mn-Sb superstructure. The $E_f$ has been set to zero.