The magnon diffusion length (MDL) is understood to play a major role in the thickness dependence of spin Seebeck effect (SSE) voltages in ${\mathrm{Fe}}_3{\mathrm{O}}_4/\mathrm{Pt}$ thin films. Here we extract the MDL in an ${\mathrm{Fe}}_3{\mathrm{O}}_4$ single crystal using inelastic neutron scattering (INS) and in ${\mathrm{Fe}}_3{\mathrm{O}}_4/\mathrm{Pt}$ thin films using accurate heat flux SSE and static magnetization measurements. The INS MDLs were $34\pm 8\mathrm{nm}$ at $300\mathrm{K}$ and $27\pm 6\mathrm{nm}$ at $50\mathrm{K}$. The SSE MDLs decreased with temperature ($19\pm 2\mathrm{nm}$ at $300\mathrm{K}$ and $13\pm 4\mathrm{nm}$ at $50\mathrm{K}$), 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.

• Received 13 February 2020
• Revised 13 May 2020
• Accepted 16 June 2020

DOI:https://doi.org/10.1103/PhysRevMaterials.4.075402

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Published by the American Physical Society