This work reports on ab initio calculations and experiments on the half-metallic ferromagnetic Heusler compounds ${\text{Co}}_2\text{Ti}Z$ $\left(Z=\text{Si},\text{Ge},\text{Sn}\right)$. Aim is a comprehensive study of the electronic-structure and thermoelectric properties. The impact of the variation in the main group element $Z$ on those properties is discussed. X-ray diffraction was performed on the compounds and the lattice parameters are compared to other ${\text{Co}}_2$-based compounds. Hard x-ray photoemission measurements were carried out and the results are compared to the calculated electronic structure. The experimentally determined electronic structure, magnetic properties, and transport measurements agree well with the calculations. The electronic and thermoelectric properties were measured for temperatures from 2 to 950 K. The electrical resistivity exhibits a pronounced cusp-type anomaly at the Curie temperature $\left(T_C\right)$ where a distinct maximum appears. The strongest magnetic-scattering resistivity is found in ${\text{Co}}_2\text{TiSn}$ with $1.7\mu \Omega \text{m}$. The resistivity becomes nearly constant at higher temperatures. The measured thermal conductivity is on the order of $5\text{W}{\text{m}}^{-1}{\text{K}}^{-1}$. The measured Seebeck coefficient $\left[S\left(T\right)\right]$ is similar in all three compounds and exhibits also a very distinct anomaly at the magnetic phase transition. A maximum value of $-50\mu \text{V}{\text{K}}^{-1}$ is found for ${\text{Co}}_2\text{TiSn}$ at $T_C$. Most interesting is that $S$ stays constant above the Curie temperature in all three compounds.

• Received 11 September 2009

DOI:https://doi.org/10.1103/PhysRevB.81.064404