Electrode processes and in situ magnetic measurements of FePt films in a LiPF6 based electrolyte

Reversible control of magnetism at the metal/electrolyte interface offers great application potential for multifunctional magnetic devices. Here, the electrode–electrolyte interactions and the charging of FePt alloy films in LiPF6 in dimethyl carbonate(DMC)–ethylene carbonate (EC) are analysed in comparison to the pure metals Fe and Pt. The main electrode processes are Li deposition and dissolution, Fe dissolution and redeposition, and reduction as well as oxidation processes of impurities and electrolyte. Fe is found to be more stable against anodic dissolution in FePt films. Pt attenuates electrode passivation and catalyses impurity reduction reactions. This results in a cathodic potential limit of 1.8 V vs. Li/Li+ for charging FePt films without side reactions. Subsequently, ultrathin textured FePt films promising large voltage induced magnetic surface effects where used as electrodes. Magnetic properties during charging were monitored by an in situ setup based on the anomalous Hall effect. Marginal Fe dissolution at the surface above 3 V vs. Li/Li+ is detected by the in situ measurement and limits the upper charging potential. In consequence, a potential window of 1.2 V for charging the FePt films is obtained. In this potential range a large reversible change of magnetic moment is detected, which is attributed to reversible redox processes in the native iron oxide surface layer.