Electric-field control of ferromagnetism is demonstrated in a mechanically flexible solid-state system through a proton-induced redox reaction at room temperature. Protons transported through a perfluorosulfonic acid (PFSA) proton exchange membrane (PEM) trigger the reduction of CoO by reacting with lattice oxygen, which enables reversible switching between paramagnetic and ferromagnetic states starting at voltages below 10 V. The proton supply is sustained by ambient humidity and water splitting at a Pt thin film counter electrode. Due to its flexibility, the device architecture—a sandwich of CoO and Pt thin films integrated with the polymeric PEM—retains full magneto-ionic functionality under mechanical bending. Three CoO films, synthesised via reactive sputtering under varying conditions, are compared: amorphous, crystalline, and mixed-phase CoO/Co. While the amorphous film exhibits weak response, both the crystalline and mixed-phase films show pronounced electric-field-dependent magnetic switching, highlighting the critical role of microstructure in magneto-ionic performance.

Magneto-ionics, an emerging approach to manipulate magnetism that relies on voltage-driven ion motion, holds the promise to boost energy efficiency in information technologies such as spintronic devices or future non-von Neumann computing architectures. For this purpose, stability, reversibility, endurance, and ion motion rates need to be synergistically optimized. Among various ions, nitrogen has demonstrated superior magneto-ionic performance compared to classical species such as oxygen or lithium. Here, we show that ternary Co_1-xFe_xN compound exhibits an unprecedented nitrogen magneto-ionic response. Partial substitution of Co by Fe in binary CoN is shown to be favorable in terms of generated magnetization, cyclability and ion motion rates. Specifically, the Co_0.35Fe_0.65N films exhibit an induced saturation magnetization of 1,500 emu/cm³, a magneto-ionic rate of 35.5 emu/(cm³·s) and endurance exceeding 10³ cycles. These values significantly surpass those of other existing nitride and oxide systems. This improvement can be attributed to the larger saturation magnetization of Co_0.35Fe_0.65 compared to individual Co and Fe, the nature and size of structural defects in as-grown films of different composition, and the dissimilar formation energies of Fe and Co with N in the various developed crystallographic structures.