Reversible, Electric-Field Induced Magneto-Ionic Control of Magnetism in Mesoporous Cobalt Ferrite Thin Films

The magnetic properties of mesoporous cobalt ferrite films can be largely tuned by the application of an electric field using a liquid dielectric electrolyte. By applying a negative voltage, the cobalt ferrite becomes reduced, leading to an increase in saturation magnetization of 15% (M_S) and reduction in coercivity (H_C) between 5-28%, depending on the voltage applied (-10 V to -50 V). These changes are mainly non-volatile so after removal of -10 V M_S remains 12% higher (and H_C 5% smaller) than the pristine sample. All changes can then be reversed with a positive voltage to recover the initial properties even after the application of -50 V. Similar studies were done on analogous films without induced porosity and the effects were much smaller, underscoring the importance of nanoporosity in our system. The different mechanisms possibly responsible for the observed effects are discussed and we conclude that our observations are compatible with voltage-driven oxygen migration (i.e., the magneto-ionic effect).