Spin-to-charge conversion at metal/oxide interfaces with stacked structures of permalloy/(Cu or Ag)/Bi2O3 were systematically investigated by using the spin-pumping technique and cross-sectional transmission electron microscopy. Although the transport measurement reproduced the results in previous studies, the interfacial structure of (Cu or Ag)/Bi2O3 was found to change depending on the crystalline orientation of the Cu or Ag underlayer. While the Ag/Bi2O3 stacks with Ag(111) had steep interfaces, the formation of a nanometer-scale Cu-O layer can be found in the Cu/Bi2O3 interface, which should be the main origin of the sign inversion of the conversion coefficients between Cu/Bi2O3 and Ag/Bi2O3. This study stresses the importance of nanostructure identification for discussing spin-to-charge conversions at metal/oxide interfaces.
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