Anomalous Thermopower and High ZT in GeMnTe₂ Driven by Spin's Thermodynamic Entropy

Na CoO₂ was known 20 years ago as a unique example in which spin entropy dominates the thermoelectric behavior. Hitherto, however, little has been learned about how to manipulate the spin degree of freedom in thermoelectrics. Here, we report the enhanced thermoelectric performance of GeMnTe₂ by controlling the spin's thermodynamic entropy. The anomalously large thermopower of GeMnTe₂ is demonstrated to originate from the disordering of spin orientation under finite temperature. Based on the careful analysis of Heisenberg model, it is indicated that the spin-system entropy can be tuned by modifying the hybridization between Te-p and Mn-d orbitals. As a consequent strategy, Se doping enlarges the thermopower effectively, while neither carrier concentration nor band gap is affected. The measurement of magnetic susceptibility provides a solid evidence for the inherent relationship between the spin's thermodynamic entropy and thermopower. By further introducing Bi doing, the maximum ZT in Ge_0.94Bi_0.06MnTe_1.94Se_0.06 reaches 1.4 at 840 K, which is 45% higher than the previous report of Bi-doped GeMnTe₂. This work reveals the high thermoelectric performance of GeMnTe₂ and also provides an insightful understanding of the spin degree of freedom in thermoelectrics.