Annealing-temperature-dependent voltage-sign reversal in all-oxide spin Seebeck devices using RuO₂

Thermoelectric converters based on the spin Seebeck effect (SSE) have attracted great attention due to their potential to offer novel applications such as energy harvesting and heat-flow sensing. For converting a SSE-induced spin current into an electric current, a transition metal film such as Pt, which exhibits large inverse spin-Hall effect (ISHE), has been typically used. In this work, we show an all-oxide SSE device using ruthenium oxide (RuO₂) as a conductive film. We found that both the sign and magnitude of the SSE-induced ISHE voltage V appearing in the RuO₂ film changes depending on the post annealing temperature, and that the magnitude can become larger than that of a standard SSE device using Pt. The similar sign change was also observed in Hall-resistance measurements of the RuO₂ films. X-ray absorption fine structure (XAFS) spectra of as-deposited and annealed RuO₂ revealed that the annealing process substantially improved the long-range crystalline order in RuO₂. This suggests that change in the crystalline order may modify the dominant ISHE mechanism or electronic states in RuO₂, leading to the sign reversal of V as well as the Hall coefficient. Our result demonstrates that RuO₂ is an interesting material not only as a practical ISHE film but also as a testbed to study physics of spin-to-charge converters that depend on their crystalline order.