Defects-curated bipolar resistive switching and magnetism in Cu_xO films

Recently, the resistive switching (RS) and room-temperature ferromagnetism (RTFM) exhibited in copper oxide (Cu_xO) films and nanostructures have received extensive interest due to its promising application potential in various fields. Although there is consensus that these intriguing observations may have been resulting from structural defects, the exact type of defect responsible, however, still remains a matter of debate. In this study, we conduct systematic and comparative investigations on both RS and magnetic properties of Cu_xO films deposited under different atmospheric conditions using magnetron sputtering method with a single CuO ceramic target. X-ray diffraction and Raman spectroscopy show that the Cu_xO films deposited under O₂ gas possesses a pure CuO phase, while the films deposited under N₂ gas exhibit mixture of CuO and Cu₂O phases. All films displayed bipolar RS characteristics, with the temperature dependence of current–voltage curves showing that the transport mechanisms of the low- and high-resistance states (LRS and HRS) are being governed by semiconductor-like ohmic conduction and space charge limited conduction, respectively. Interestingly, the pure-phased CuO thin film in both LRS and HRS states only displayed paramagnetic (PM) responses, whereas the CuO/Cu₂O mixture film exhibited PM and PM + ferromagnetic (FM) behaviors in LRS and HRS, respectively. Detailed comparative analyses suggest that positive doubly charged oxygen vacancy (V_O^∙∙) curated conducting filaments may have dominated the bipolar RS, and electrons bound to V_O^∙∙ may further promote the FM coupling between neighboring Cu¹⁺ and Cu²⁺ ions.