Vacancy induced half-metallicity in half-Heusler semiconductors

First-principles calculations are performed to investigate the effect of vacancies on the electronic structure and magnetic properties of the two prototypical half-Heusler semiconductors NiTiSn and CoTiSb. The spin degeneracy of the host materials is broken for all types of isolated vacancies under consideration, except for Ni-deficient NiTiSn. A half-metallic character is identified in Sn-deficient NiTiSn and Co/Ti/Sb-deficient CoTiSb. We can explain our findings by introducing an extending Slater-Pauling rule for systems with defects. A ferromagnetic ordering of the local moments due to double exchange appears to be likely.

Figure 1

Unit cell of a half-Heusler alloy $XYZ$, comprising four formula units.

Figure 2

Electronic density of states (DOS) as obtained for (a) the pure NiTiSn $2\times 2\times 2$ supercell and for supercells with (b) Ni, (c) Ti, and (d) Sn deficiency. The dark and light lines indicate spin-polarized and nonpolarized calculations, respectively. Majority and minority spin states are labeled $\uparrow$ and $\downarrow$, respectively. Short vertical dashed lines mark the value $E_g$ of the extended Slater-Pauling model.

Figure 3

Electronic density of states (DOS) as obtained for (a) the pure CoTiSb $2\times 2\times 2$ supercell and for supercells with (b) Co, (c) Ti, and (d) Sb deficiency. The dark and light lines indicate spin-polarized and nonpolarized calculations, respectively. Majority and minority spin states are labeled $\uparrow$ and $\downarrow$, respectively. Short vertical dashed lines mark the value $E_g$ of the extended Slater-Pauling model.