Electron-magnon Umklapp Scattering in FePt Films Measured by Thermopower

Abstract

We report unconventional thermopower (Seebeck coefficient, $S$) behavior of $L1_{0}$ FePt films. The electron diffusion and magnon scattering are found to be the major contributions to the thermopower at low temperatures. The temperature dependence of $S$ is fitted by a phenomenological expression. The extracted electron diffusion coefficient is positive, instead of negative for conventional metals. An overall concave curvature of the Fermi surface specific to FePt is found to be responsible. More interestingly, the magnon drag coefficient carries an opposite sign to that of electron diffusion, suggesting dominant contribution from 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minority spins among two of those bands, greatly enhancing the electron-magnon scattering.Comment: This paper has been withdrawn by the authors due to a measurement erro