  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, spin-preserving 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 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.
