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MMM 2022

November 07, 2022

Minneapolis, United States

Magnon drag observed via field

Recent experimental and theoretical work has focused new interest on how magnons can influence the transport properties of metallic ferromagnets (FM). Here we demonstrate the influence the Gilbert damping parameter, αGD, has on both the electron and magnon populations available for transport in two distinctly different CoxFe1-x alloys. The absolute Seebeck coefficient, αabs, is measured for each of the films 1, and we compare to our calculated total thermopower, αtot. To calculate the electronic and magnonic components, we employ a semi-classical approach; the classical free-electron model estimates the diffusive thermopower component, αMott 2, while we compute the relativistic magnon-drag component, αmd, based on the spin transfer mechanism 3,4. From these results, we obtain quantitatively comparable measured Seebeck coefficients for both alloys, subsequently showing αmd dominates the low-damping sample, while the alloy with normal damping follows αMott well over our temperature regime. We further present anisotropic magnetoresistance (AMR) 5 and magnetothermopower (MTP) 6 measurements over the temperature range of interest from 125 to 250 K, where we previously reported a significant non-electronic thermal conductivity 7. AMR measurements align with expected FM metal behavior, while MTP shows a field-dependence as H ∥ ▽T in the low-damping FM metal. We estimate the magnitude of this behavior using the relationship between α(H,⊥) and the inverse resistance, R -1, and calculated the expected α(H,∥) 6. This argues for an additional field-direction dependent contribution to thermopower, which could be a novel form of magnon-drag.
1. S. J. Mason, A. Hojem, and D. J. Wesenberg, Journ. of Appl. Phys. 127 (2020).
2. M. Jonson and D. Mahan, Phys. Rev. B 21 (1980).
3. M. E. Lucassen, C. H. Wong, and R. A. Duine, Appl. Phys. Let. 99 (2011).
4. B. Flebus, R. A. Duine, and Y. Tserkovnyak, EPL 115 (2016).
5. J. Shi, S. S. Parkin, and L. Xing, Journ. Of Mag. and Mag. Mater. 125, (1993).
6. A. D. Avery, M. R. Pufall, and B. L. Zink, Phys. Rev. B – Cond. Mat. and Mater. Phys. 86,(2012).
7. M. R. Natale, D. J. Wesenberg, and E. R. Edwards, Phys. Rev. Mater. 5, (2021).
8. M. R. Natale, D. J. Wesenberg, and E. R. Edwards, in preparation

Fig. 1: Temperature dependence of absolute thermopower for Co25Fe75 alloy. Calculated diffusive, αMott, (green), magnon-drag, αmd, (orange), and total thermopower, αtot, (marron)overlayed with experimental measurements, αmeas , from 125-250K.

Fig. 2: MTP measurement for Co25Fe75 at 200K. αabs(H) is measured in-plane for Hext ⊥ ▽T and Hext ∥ ▽T. Curved arrows indicate the field sweep direction and are color coordinated to distinguish between field-orientation. Dotted lines are expected values.

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