Magnon-drag thermopower and Nernst coefficient in Fe, Co, and Ni

Abstract

Magnon-drag is shown to dominate the thermopower of elemental Fe from 2 to 80 K and of elemental Co from 150 to 600 K; it is also shown to contribute to the thermopower of elemental Ni from 50 to 500 K. Two theoretical models are presented for magnon-drag thermopower. One is a hydrodynamic theory based purely on non-relativistic, Galilean, spinpreserving electron-magnon scattering. The second is based on spin-motive forces, where the thermopower results from the electric current pumped by the dynamic magnetization associated with a magnon heat flux. In spite of their very different microscopic origins, the two give similar predictions for pure metals at low temperature, allowing us to semi-quantitatively explain the observed thermopower of elemental Fe and Co without adjustable parameters. We also find that magnon-drag may contribute to the thermopower of Ni. A spin-mixing model is presented that describes the magnon-drag contribution to the Anomalous Nernst Effect in Fe, again enabling a semi-quantitative match to the experimental data without fitting parameters. Our work suggests 2 that particle non-conserving processes may play an important role in other types of drag phenomena, and also gives a predicative theory for improving metals as thermoelectric materials.

Theory of the magnon-mediated tunnel magneto-Seebeck effect

“…[29] and [30], respectively, the room-temperature magnonic contribution S to the thermopower of symmetric tunnel junctions is obtained then to be significant when compared with the free-electron one. Similar estimates have been made in the context of the measured bulk thermopower in metallic ferromagnets [8].…”

“…1. At low temperatures, i.e., T T C ,T F (with T C and T F being the Curie and Fermi temperature, respectively), the electric current is controlled by the voltage V applied to the 2469-9950/2017/96(9)/094429 (8) 094429-1 ©2017 American Physical Society…”

“…On another front, new experimental evidence suggests that the magnon drag in the ferromagnetic bulk is playing a fundamental role in the thermopower: Watzman et al [8] observe a thermopower scaling as T 3/2 (and thus dominating over the single-electron diffusive contribution ∝T ) over a broad temperature range in elemental transition metals, in agreement with the theoretically predicted magnon-drag contribution associated with the magnonic heat flux [9,10]. These findings call for a reassessment of the mechanism of the Seebeck effect in MTJs, where the relative importance of the electronic and magnonic contributions may be expected to parallel that in the bulk [11].…”

Magnon-polaron transport in magnetic insulators

“…According to Eqs. (38) and (40) the function y 1 is also stationary at H = H ⊥ , but it has a minimum only for η < 1, while an inflection point for η = 1, and a maximum otherwise. The resulting dependence on η of Eq.…”

Scalable Nernst thermoelectric power using a coiled galfenol wire

“…1 To that end, phonon drag had been shown to enhance the Nernst coefficient at low temperatures in certain material systems. 3,4 More recently, Watzman et al reported that the anomalous Nernst coefficient also benefits from magnon drag as shown in single crystal Fe where N ANE is dominated by magnon drag up to 200 K. 5 In this study, we propose using a magnetostrictive metallic metal, Galfenol (Fe 100-x Ga x ), as a pathway to increase the power factor due to its enhanced Nernst coefficient from phonon and magnon coupling, as well as its large intrinsic electrical conductivity. We demonstrate Nernst voltage generation in a coiled Galfenol wire, characterize its Nernst coefficient, and offer an experimental proof-of-concept for a novel and scalable thermoelectric generator ideal for cylindrical heaters.…”