Unlocking Spin to Boost Thermopower

Abstract

Figure 1. Illustrations of the mechanisms of spin-enhanced charge-based thermopower. (a) Spin entropy: a spin entropy flux is created by differences in spin–orbital degeneracies (g), flowing from high-degeneracy to low-degeneracy states, typically in transition metals (M), contributing to the total thermopower. Additionally, spin entropy arises from disordered spin orientations caused by the breakdown of long-range order at high temperatures, referred to as spin thermodynamic entropy. (b) Spin fluctuation: thermal fluctuations of the local spin density of itinerant electrons are most significant near T_C. These fluctuations are suppressed as the net magnetic moment stabilizes under a strong magnetic field. Reproduced with permission from ref (3). Copyright 2019 The Authors. (c) Magnon drag: magnons propagate in a magnetic material from the hot to the cold end, coupling with both electrons and phonons, contributing to thermopower through momentum transfer. Reproduced with permission from ref (4). Copyright 2021 The Authors. Figure 2. (a) Schematic illustration of spin entropy contributed by the localized electrons on Co ions transfer entropy via hopping transport due to the different degeneracy. Reproduced with permission from ref (6). Copyright 2020 The Authors. (b) The relative change in thermopower of Ca₃Co₄O_9+δ single crystal versus magnetic field for two directions (B along c axis and ab plane). Reproduced with permission from ref (8), Copyright 2013 John Wiley and Sons. (c) Calculated thermopower for different spin states as a function of cobalt valence in the CoO₂ layers. Reproduced with permission from ref (9), Copyright 2012 American Physical Society. (d) Schematic representation of spin orientation and thermodynamic entropy. Reproduced with permission from ref (10). Copyright 2021 The Authors. Figure 3. (a) Temperature dependent on thermopower with and without magnetic field in Fe₂V_0.9Cr_0.1Al_0.9Si_0.1. Reproduced with permission from ref (3). Copyright 2019 The Authors.. The inset displays the spin fluctuation contribution peaks at T_C. (b) −S/T of Fe₂V_0.9Cr_0.1Al_0.9Si_0.1, plotted as functions of magnetic field and temperature. −S/T has a sharp peak at T_C under zero magnetic field and is significantly suppressed with increasing H. Reproduced with permission from ref (3). Copyright 2019 The Authors. (c) Measured thermopower S_total and magnon drag induced thermopower S_M for Co₂TiAl. The area between the S_total and S_M lines represents the sum of S_sf and S_d. The inset displays the temperature-dependent thermopower of S_sf + S_d along with their respective values. Reproduced with permission from ref (14). Copyright 2023 The Authors. (d) Schematic illustration of interactions of spin fluctuations with electrons and phonons. Reproduced with permission from ref (4). Copyright 2021 The Authors. Figure 4. (a) Schematic representations of the two contributions to the magnon drag: hydrodynamic transport (no magnon decay, α → 0) and the geometric Berry phase drag (governed by magnon decay). Reproduced with permission from ref (20). Copyright 2016 EPLA. (b) Measured thermopower of Li_1–xMn_xTe. The magnon-drag thermopower significantly increases below T_N, with the paramagnon-drag thermopower remains elevated above T_N. Data in b taken from ref (4). (c) Total and partial specific heat capacities of MnTe, showing the magnon specific heat capacity C_m contribution, which exhibits a λ shape at T_N. Reproduced with permission from ref (4). Copyright 2021 The Authors. (d) Spin-dependent scattering in the FM and AFM systems along with their corresponding dispersion relations. Reproduced with permission from ref (21). Copyright 2020 RSC. Z.B.W. and J.Q.H. discussed the topic and proposed the outline. Z.B.W. organized and wrote the draft. J.Q.H. revised the manuscript. Zhong-Bin Wang is now a Ph.D. student at Southern University of Science and Technology (SUSTech). He obtained his Bachelor’s degree from Harbin Institute of Technology in 2021. His research focuses on the anomalous transport behaviors in magnetic thermoelectric materials. Jiaqing He is a chair professor at Southern University of Science and Technology (SUSTech). He received his joint Ph.D. degree in physics from both Juelich Research Center and Wuhan University in 2004. He was a postdoctor at Brookhaven National Laboratory (2004–2008), research associate (2008–2010), research assistant professor (2010–2012) at Northwestern University, and a professor at Xi’an Jiaotong University (2012–2013) and SUSTech (2013–present). His research interests include transmission electron microscopy, thermoelectric materials, and structure and property relationships. The authors thank the financial support of the National Natural Science Foundation of China (Grant No. 12434001, 11934007, 52461160258) and the Outstanding Talents Training Fund in Shenzhen (202108). This article references 27 other publications. This article has not yet been cited by other publications.