Microscopic origin of the spin Seebeck effect
In plain words
A temperature difference across a magnetic insulator like yttrium iron garnet drives a spin current into an attached platinum strip, where it appears as a voltage. Which spin waves carry this current, and how lattice vibrations help, is not settled.
Precise statement
In YIG/Pt the longitudinal spin Seebeck voltage depends on YIG thickness (saturating over lengths of order 1e-4 cm), magnetic field (suppressed at high field), and temperature, with anomalies where magnons and phonons hybridize (magnon polarons, 2016). Determine the roles of bulk thermal-magnon accumulation, the interfacial magnon-electron temperature difference, and magnon-phonon coupling, and predict the voltage versus thickness, T and field. An answer is one quantitative model reproducing all three dependences.
What would settle it
A parameter-free model using measured magnon and phonon spectra that reproduces the thickness, field and temperature dependence of the voltage in YIG/Pt.
Status in the literature
Bulk magnon accumulation models describe the thickness dependence and magnon-polaron anomalies show phonons matter; a unified quantitative account is lacking.