Effect of mixing the low-valence transition metal atoms $Y$ = Co, Fe, Mn, Cr, V, Ti, or Sc on the properties of quaternary Heusler compounds ${\mathrm{Co}}_{2-x}{Y}_x\mathrm{FeSi}$ ($0\le x\le 1$)

In this paper we report an experimental study of structural, magnetic, and mechanical properties of quaternary Heusler alloys ${\mathrm{Co}}_{2-x}{Y}_x\mathrm{FeSi}$ ($Y$ = Co, Fe, Mn, Cr, V, Ti, or Sc, $0\le x\le 1$) and the experimental findings are supported by ab initio electronic structure calculations. The alloys were synthesized using an arc-melting technique. Single phase microstructures are observed for all alloys substituted with low-valence transition metals $Y$ except Sc. X-ray powder diffraction patterns at room temperature show the presence of Heusler-like face-centered cubic crystal structure in all single phase specimens. The low-temperature saturation magnetic moments, as determined from magnetization measurements, agree fairly well with our theoretical results and also follow the Slater-Pauling rule of thumb for half-metals, a prerequisite for half-metallicity. The alloys are predicted to exhibit half-metallic ferromagnetism by ab initio electronic structure calculations using the $\mathrm{GGA}+U$ approach. All stable compounds are observed to have high Curie temperatures with linear dependence with the valence electrons concentration in the alloys. Relatively high hardness values are also measured, approaching 15.7 GPa for Ti-substituted material, highest among the values reported for Heuslers so far. All these properties strongly suggest the alloys are promising for the spintronic applications at room temperature and above.

Figure 1

Optical micrograph of ${\mathrm{Co}}_{2-x}{Y}_x\mathrm{FeSi}$ ($Y$ = Co, Fe, Mn, Cr, V, or Ti, $x=0.50$) annealed at $900{}^{\circ }\mathrm{C}$ for 7 days followed by slow cooling showing the granular microstructure. The samples were etched for 60 s using the Adler etchant.

Figure 2

SEM micrograph of ${\mathrm{Co}}_{2-x}{Y}_x\mathrm{FeSi}$ ($Y$ = Co, Fe, Mn, Cr, V, or Ti, $x=0.50$) alloys annealed at $900{}^{\circ }\mathrm{C}$ for 7 days followed by slow cooling, showing the granular microstructure.

Figure 3

Experimental XRD patterns of ${\mathrm{Co}}_{2-x}{Y}_x\mathrm{FeSi}$ ($Y$ = Co, Fe, Mn, Cr, V, Ti or Sc, $x=0.50$) alloy series annealed at $900{}^{\circ }\mathrm{C}$ for 7 days investigated at room temperature, here $*$ corresponds to Heusler cubic peaks in Sc-based alloy while others are from secondary phase/unknown impurity phase. The first from the bottom is the simulated XRD pattern for ordered $L{2}_1$ structure of ${\mathrm{Co}}_2\mathrm{FeSi}$. The relative intensity ($y$ axis) is plotted in log scale so that all the peaks can be seen clearly.

Figure 4

Crystal structure in a unit cell of (a) CFS and (b) ${\mathrm{Co}}_{1.50}{Y}_{0.50}\mathrm{FeSi}$ ($Y$ = Fe, Mn, Cr, V, Ti, or Sc) (I) configuration mentioned in Table 1 assuming $L{2}_1$ structure. The structures are shown in their ideal, unrelaxed forms.

Figure 5

Variation of lattice parameter with (a) $Y$ content in ${\mathrm{Co}}_{2-x}{Y}_x\mathrm{FeSi}$ ($Y$ = Co, Fe, Mn, Cr, V, or Ti, $0\le x\le 1$), and (b) atomic number of low-valence transition metal element $Y$ in ${\mathrm{Co}}_{2-x}{Y}_x\mathrm{FeSi}$ ($Y$ = Co, Fe, Mn, Cr, V, or Ti, $x=0.50$) showing linear behavior. The pink data points represent literature values [5, 65].

Figure 6

(a) Isothermal magnetization curves of ${\mathrm{Co}}_{2-x}{Y}_x\mathrm{FeSi}$ ($Y$ = Co, Fe, Mn, Cr, V, or Ti, $x=0.50$) at 5 K. (b) The saturation magnetic moment versus valence electron counts per formula units with element $Y$, both experimental (at 5 and 300 K) and expected from the Slater-Pauling rule for half-metals. The pink data points represent reported literature values [5, 65].

Figure 7

(a) Temperature dependence of magnetization at 100 Oe, and (b) variation of Curie temperature as a function of valence electron counts per formula units with element $Y$ ($x=0.50$). The inset shows the first-order derivative of magnetization as a function of temperature, the minima of which is used to extract $T_c$. The pink data points represents reported literature values [5, 65].

Figure 8

Vickers hardness versus $Y$ content in ${\mathrm{Co}}_{2-x}{Y}_x\mathrm{FeSi}$ ($Y$ = Co, Fe, Mn, Cr, V, Ti, or Sc), all annealed at $900{}^{\circ }\mathrm{C}$ for 7 days (a) for all the alloys $0\le x\le 1$, and (b) for alloys with $x=0.50$. The imprints of the indenter with radial cracks for Ti = 1 [bottom right of (a)] and Ti = 0.50 [top right of (b)] are shown.

Figure 9

Spin polarized total density of states (TDOS) of ${\mathrm{Co}}_{1.50}{Y}_{0.50}\mathrm{FeSi}$ ($Y$ = Co, Fe, Mn, Cr, V, Ti, or Sc) alloys using both GGA (blue) and $\mathrm{GGA}+U$ (red) methods. Number of states in each plot is scaled with respect to 1 f.u.