(color online) (a) Repeated unipolar RS characteristics of Au/NiO/STO/Pt cell under negative bias. (b) Under positive bias the cell just shows a few switching cycles (about 4–7 cycles). Inset in panel (c) exhibits I–V curve of the last cycle, in which high resistance becomes higher at about 24 V. After this, the Au/NiO/STO/Pt cell cannot be recovered to the RS possible state even with 30-V bias as shown in panel (c). (d) If positive bias is applied to the cell after the electroforming processes, the non-switching state is still observed after RS has been performed a few times.