Tailoring the electronic structure of half-metallic Heusler alloys

We investigated element-specific magnetic moments and the spin-resolved unoccupied density of states (DOS) of polycrystalline ${\text{Co}}_2\text{Ti}Z$ $\left(Z=\text{Si},\text{Ge},\text{Sn},\text{Sb}\right)$, ${\text{Co}}_2{\text{Mn}}_x{\text{Ti}}_{1-x}\text{Si}$ and ${\text{Co}}_2{\text{MnGa}}_{1-x}{\text{Ge}}_x$ Heusler alloys using circular dichroism in x-ray absorption spectroscopy (XMCD). We find a small $\left(<0.03{\mu }_B\right)$ Ti moment oriented antiparallel and a large $\left(>3{\mu }_B\right)$ Mn moment oriented parallel to the Co moment of approximately $1{\mu }_B$ per atom in the investigated compounds. Orbital magnetic moments are increased for quaternary compounds compared to the corresponding ternary compounds with $x=0$ or $x=1$. The unoccupied spin-resolved partial DOS at the Co atom was extracted from the XMCD data. In the case of ${\text{Co}}_2\text{TiSi}$, ${\text{Co}}_2\text{TiGe}$, and ${\text{Co}}_2\text{TiSn}$, the Co minority DOS reveals a maximum at 0.5 eV above $E_F$ and very low values at $E_F$ in agreement with the expectation for half-metallic ferromagnetism. In contrast, ${\text{Co}}_2\text{TiSb}$ shows a large minority DOS at the Fermi energy like a normal metal. A substitution of Ti by Mn in ${\text{Co}}_2\text{TiSi}$ shifts the minority DOS maximum from 0.5 to 0.9 eV with respect to the Fermi energy. For the series ${\text{Co}}_2{\text{MnGa}}_{1-x}{\text{Ge}}_x$ we observe a gradual shift of the minority DOS maximum from 0.7 eV for $x=1$ to 1.0 eV for $x=0$, indicating half-metallic ferromagnetism for the whole series. Our results, revealing the distribution of magnetic moments and the relative position of the Fermi energy as a function of the number of valence electrons, confirm the predicted possibility of tailoring the minority band gap using substitutional quaternary Heusler compounds. The results maybe of general importance for the understanding of the electronic structures in complex intermetallic compounds.