Spin Seebeck effect induced by antiferromagnetic magnons and spin fluctuations in epitaxial FeF2 films

Recently, the spin Seebeck effect (SSE) in antiferromagnetic (AFM) Cr₂O₃ and MnF₂ has been reported and the signals were found to rise significantly above the spin-flop (SF) field. However, it is still unclear whether the SSE requires an induced magnetic moment or just the presence of AFM magnons. Here we report a study of the SSE of FeF₂/Pt heterostructures in which FeF₂ is an insulating AFM epitaxial film. The high SF field (42 T) of FeF₂ makes it a perfect candidate for studying the SSE below the SF field. The AFM properties of FeF₂ film are characterized by exchange bias. The SSE signal of FeF₂/Pt under different magnetic fields shows very similar temperature-dependent behaviors. First, there is a peak at around 11.6 K due to AFM-magnons' contribution. Second, we observe a ``bump'' at the AFM ordering temperature (70 K), beyond which the SSE signal decays and persists up to 250 K under high magnetic fields, indicating that short-range spin fluctuations also contribute to the SSE signal.

This work was supported as part of the SHINES, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Basic Energy Sciences under Award No. SC0012670. This work was also supported by the Startup Funds from University of California, Riverside.