Simultaneous Enhancement of Electron and Phonon Transport Properties via Magnetism Phase Transition in Fe<i>_x</i>Co<i>_2‐x</i>TiGe Heusler Alloy

Magnetism offers an additional degree of freedom to control the electron and phonon transport properties of thermoelectric materials through various thermoelectromagnetic effects. To better understand the interplay among spin, carriers, and phonons, it is important to design material systems with intrinsic magnetic and thermoelectric properties. Here, it is demonstrated that Fe doping of the full Heusler alloy Co₂TiGe simultaneously enhances its magnetic and thermoelectric performance. Aberration-corrected scanning transmission electron microscopy (STEM) reveals that Fe atoms preferentially substitute for Co atoms within the Co₂TiGe lattice, leading to increased saturation magnetization and a higher Curie temperature. This Fe substitution reduces the carrier concentration, enhancing the Seebeck coefficient and power factor, while also increasing magnetic scattering due to spin fluctuations during the ferromagnetic-paramagnetic phase transition. Furthermore, the thermal conductivity is significantly reduced by enhanced phonon scattering from secondary phases and point defects. Consequently, a maximum ZT value of 0.066 is achieved at 600 K for Fe_0.4Co_1.6TiGe, representing a 663% increase compared to the undoped Co₂TiGe.