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, α_GD, has on both the electron and magnon populations available for transport in two distinctly different Co_xFe_1-x alloys. The absolute Seebeck coefficient, α_abs, is measured for each of the films 1, and we compare to our calculated total thermopower, α_tot. To calculate the electronic and magnonic components, we employ a semi-classical approach; the classical free-electron model estimates the diffusive thermopower component, α_Mott 2, while we compute the relativistic magnon-drag component, α_md, based on the spin transfer mechanism 3,4. From these results, we obtain quantitatively comparable measured Seebeck coefficients for both alloys, subsequently showing α_md 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.
References
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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, α_Mott, (green), magnon-drag, α_md, (orange), and total thermopower, α_tot, (marron)overlayed with experimental measurements, α_meas , from 125-250K.

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