Electrochemically driven variation of magnetic properties in ultrathin CoPt films

Controlled variation of magnetism in ultrathin metallic films by external electric fields is a promising route towards advanced multifunctional devices. In situ magnetic measurements of ultrathin CoPt films in a liquid electrolyte (LiClO4 in dimethyl carbonate-ethylene carbonate) have been performed, dependent on the applied external voltage. Huge hysteretic changes of coercivity (above 200%) and smaller variations of saturation magnetisation (∼4 %) have been identified. The commonly considered concept of changing the electron density of states by double layer charging is not applicable as the changes observed are mainly irreversible. Rather, a strong influence of redox processes, namely surface oxide reduction and Co dissolution, is considered. We argue that by exploiting these redox reactions much higher changes of the magnetic properties are achievable than for double layer charging.