Large Improvement of Thermoelectric Performance by Magnetism in Co-Based Full-Heusler Alloys.

Full-Heusler alloys (fHAs) exhibit high mechanical strength with earth-abundant elements, but their metallic properties tend to display small electron diffusion thermopower, limiting potential applications as excellent thermoelectric (TE) materials. Here, it is demonstrated that the Co-based fHAs Co XAl (X = Ti, V, Nb) exhibit relatively high thermoelectric performance due to spin and charge coupling. Thermopower contributions from different magnetic mechanisms, including spin fluctuation and magnon drag are extracted. A significant contribution to thermopower from magnetism compared to that from electron diffusion is demonstrated. In Co TiAl, the contribution to thermopower from spin fluctuation is higher than that from electron diffusion, resulting in an increment of 280 µW m  K in the power factor value. Interestingly, the thermopower contribution from magnon drag can reach up to -47 µV K , which is over 2400% larger than the electron diffusion thermopower. The power factor of Co TiAl can reach 4000 µW m  K which is comparable to that of conventional semiconducting TE materials. Moreover, the corresponding figure of merit zT can reach ≈0.1 at room temperature, which is significantly larger than that of traditional metallic materials. The work shows a promising unconventional way to create and optimize TE materials by introducing magnetism.