  We report unconventional thermoelectric power (Seebeck coefficient, S) in L10
structured FePt films. The temperature dependence of S can be well fitted by a
phenomenological expression consisting of electron diffusion and magnon-drag
contributions. Interestingly, the magnon drag coefficient carries an opposite
sign to that of electron diffusion, revealing a dominant contribution from the
elusive electron-magnon Umklapp scattering. DFT calculations identify several
bands crossing the Brillouin zone boundaries, facilitating the Umklapp process.
The large spin-orbit coupling in FePt results in strong mixing of majority and
minority spins among some of those bands, greatly enhancing the electron-magnon
scattering.
