Transport properties of Co2HfSn Heusler alloy obtained by rapid solidification and sintering

Co-based Heusler alloys are attracting considerable interest as they were found to be suitable for spin-injection processes, and for the generation of spin-polarized currents via spin-Seebeck effect. Co2HfSn has been proved to be one of the most promising candidates for this application, since it combines remarkable half-metallic properties, compositional and doping versatility, ease of preparation, high Curie temperature, and sufficiently high Seebeck coefficient. In this work, the Co2HfSn Heusler compound was synthesized by rapid solidification (meltspinning) followed by spark plasma sintering. Electron backscattered diffraction (EBSD) analysis was performed, providing useful information on the microstructure and grain size distribution which result from such processing route. An average grain size of approximately 5 mu m was observed. The electronic and thermal transport properties were then measured, and the thermoelectric figure of merit zT was experimentally estimated for the first time, having a maximum value of 0.040 at 800 K. The electrical conductivity and the charge carrier concentration were measured down to 2 K to collect evidence on the shape of the electronic density of states in proximity of the Fermi level at temperatures close to absolute zero. A change of regime of the electrical conductivity at low temperatures was found and explained in terms of a modification of the electron scattering mechanisms, due to a crossover from a half-metallic to a conductive state at 75 K. Finally, the transport, elastic and vibrational properties were calculated using Density Functional Theory (DFT). Properties such as the Seebeck coefficient and the electronic thermal conductivity were evaluated using DFT-calculated electronic bands within the framework of Boltzmann transport theory. Vibrational properties, such as phonon band structure, density of states and heat capacity were computed in the harmonic approximation, using DFT to calculate the force constants matrix. Results from calculations of the elastic moduli enabled us to apply Slack's model for the estimation the lattice thermal conductivity. By combining experimental measurements with DFT calculations, we obtained consistent results that offer a deeper understanding of the properties of this compound at both low and high temperatures.