Effect of Magnetic Entropy in The Thermoelectric Properties of Fe-Doped Fe₂VAl Full-Heusler

Spin entropy is involved in the transport of heat/charge carriers in magnetic materials, and can provide new opportunities to improve the conversion efficiency of thermoelectric materials over that of the conventional case [1-3]. Here, we have explored the effect of magnetic entropy on thermoelectric properties of well-characterized Fe-doped full-Heusler, Fe_2+xVAl_1-x with x = 0-0.1. These samples are prepared by arc melting, spark plasma sintering and annealing, producing high-density crystalline products. The low-temperature magneto-thermoelectric measurement shows exotic results, including a significantly high power factor near 300 K.<br/>The itinerant weak ferromagnetic behavior of studied samples is confirmed from the magnetization. A systematic increase in magnetic transition temperature (T_c; 40 K to 223 K) and saturation magnetization (M_s; 0.13 to 0.41μ_B/Fe) with increasing Fe doping (x = 0 to 0.1) is observed. Applying a magnetic field causes a clear suppression in the magnitude of thermopower (S) and S/T (entropy term) with a negative magnetoresistance near the T_c for all the samples, demonstrating a clear effect of spin fluctuation [2]. The spin fluctuation leads to an enhancement in S of ~34% than diffusion thermopower and hence a whopping enhancement in thermoelectric power factor of 18% for x=0.1 at the T_C. The temperature-dependent behavior of S ruled out the contribution of magnon drag. Interestingly, Fe doping can shift the T_c towards room temperature and lead to a spin fluctuations induced enhancement in the S, but, it also increases the overall carrier density and hence deteriorates the magnitude of S. Multiparabolic band model fitting reveals the formation of additional spin states with large effective mass near the Fermi level, which can be eliminated/suppressed by applying the magnetic field. Callaway model fitting of thermal conductivity reveals Fe doping increases the number of point defects, causing a significant reduction in lattice thermal conductivity. This study demonstrates the role of spin fluctuation in enhancing the thermopower/ thermoelectric performance of Fe-doped Fe₂VAl and opens a vista for the strategy's applicability for various thermoelectric materials.<br/><br/><i>References</i><br/>[1] S. Hébert, R. Daou, A. Maignan, S. Das, A. Banerjee, Y. Klein, C. Bourgès, N. Tsujii, T. Mori, <i>Sci. Technol. Adv. Mater.</i>, <b>22</b>, 583-596 (2021).<br/>[2] N. Tsujii, A. Nishide, J. Hayakawa, T. Mori, <i>Sci. Adv.</i> <b>5</b>, eaat5935 (2019).<br/>[3] K. Vandaele, S. J. Watzman, B. Flebus, A. Prakash,Y. Zheng, S. R. Boona, J. P. Heremans, <i>Materials Today Physics</i>, <b>1</b>, 39-49 (2017).<br/><br/><i>Acknowledgments</i><br/>This work was supported by the Japan Science and Technology Agency (JST), MIRAI program grant (JPMJMI19A1).