electrons move past atoms in a solid, their charge distorts the nearby
lattice and can create a wave. Reciprocally, a wave in the lattice
affects the electrons motion, in analogy to a wave in the sea that
pushes a surfer riding it. This interaction results in a thermoelectric
effect that was first observed during the 1950´s and has come to be
known as phonon-drag, because it can be quantified from the flow of
lattice-wave quanta (phonons) that occurs over the temperature gradient.
after the discovery of the phonon drag, an analogous phenomenon was
predicted to appear in magnetic materials: the so called magnon drag. In
a magnetic material the intrinsic magnetic moment or spin of the
electrons arrange in an organized fashion. In ferromagnets, the spins
maintain a parallel orientation. If a distortion in the preferred spin
orientation occurs, a spin wave is created that could affect electron
motion. It is therefore reasonable to expect that the flow of magnons
(spin-wave quanta) could also drag the electrons.
the similarities with phonon drag, the observation of the magnon drag
has been elusive, and only a few indirect indications of its existence
have been reported over the years. The main reason being the presence of
other thermoelectric effects, most notably the phonon drag, that make
it difficult to discriminate its contribution to the thermopower.
of the Catalan Institute of Nanotechnology´s Physics and Engineering of
Nanodevices Group, Marius V. Costache, Germán Bridoux, Ingmar Neumann
and group leader ICREA Prof. Sergio O. Valenzuela used a unique device
geometry to discriminate the magnon drag from other thermoelectric
effects. The device resembles a thermopile formed by a large number of
pairs of ferromagnetic wires placed between a hot and a cold source and
connected thermally in parallel and electrically in series. By
controlling the relative orientation of the magnetization in pairs of
wires, the magnon drag can be studied independently of the electron and
phonon drag thermoelectric effects.
work is very timely as thermoelectric effects in spin-electronics
(spintronics) are gathering increasing attention as a means of managing
heat in nanoscale structures and of controlling spin information by
using heat flow. Measurements as a function of temperature reveal the
effect on magnon drag following a variation of magnon and phonon
populations. This information is crucial to understand the physics of
thermal spin transport. It both provides invaluable opportunities to
gather knowledge about electron-magnon interactions and may be
beneficial for energy conversion applications and for the search of
novel pathways towards transporting spin information.