Magnon diffusion lengths in bulk and thin film Fe3O4 for spin Seebeck applications

The magnon diffusion length (MDL) is understood to play a major role in the thickness dependence of spin Seebeck effect (SSE) voltages in Fe3O4/Pt thin films. Here we extract the MDL in an Fe3O4 single crystal using inelastic neutron scattering (INS) and in Fe3O4/Pt thin films using accurate heat flux SSE and static magnetization measurements. The INS MDLs were 34 ± 8 nm at 300 K and 27 ± 6 nm at 50 K. The SSE MDLs decreased with temperature (19 ± 2 nm at 300 K and 13 ± 4 nm at 50 K), but were markedly smaller. Whilst the bulk MDL is expected to be an upper limit of the MDL in thin films, we show that the film magnetization must be considered to obtain MDLs from SSE measurements. This study highlights the importance of disentangling the role of various effects in SSE measurements which is crucial in increasing the efficiencies of thermomagnetic energy harvesting devices.