In this work, systematic X-band electron magnetic resonance (EMR) studies for YMnO 3 /Si ferroelectric gate structures were performed to trace a variation of interface characteristics as different sputtering condition of O 2 /(Ar+O 2 ) ratio. Our result showed that the EMR signal intensities were increased with increasing O 2 /(Ar+O 2 ) ratio. In addition, it was suggested from detailed analyses that the observed EMR signals could be originated from Mn nanoclusters existing in both the polycrystalline Y 2 O 3 layer and the amorphous Si-enriched Y–Si interface layer in YMnO 3 /Si thin film structure. And also, a correlation between the decrease of crystallinity in YMnO 3 /Si film and the content of Mn nanoclusters within the polycrystalline Y 2 O 3 layer and/or the amorphous Y–Si layer was discussed.