Hydrogen-based magneto-ionic effect in Ni foils and films in acidic and alkaline media

Hydrogen-based magneto-ionic systems are among the most promising ones for the energy-efficient and fast voltage-control of magnetic materials. Hydrogen-based magneto-ionic control is mainly reported for Co-based materials, often combined with a Pd interlayer with high hydrogen solubility. We study the impact of reversible electrochemical hydrogenation on the magnetic properties of Ni foils and films in acidic electrolyte and in alkaline electrolytes with different pH values. For Ni foils, vibrating sample magnetometry shows a voltage-induced decrease in saturation moment after electrochemical hydrogenation in an acidic electrolyte. For Ni films, having significantly smaller volume than the foils, we demonstrate reversible hydrogen-based magneto-ionic changes of magnetization using in situ magnetotransport measurements. For both foils and films, magneto-ionic effects are enhanced in acidic and strongly alkaline electrolytes but diminish under mildly alkaline conditions. Electrochemical analysis indicates that this may be related to surface nickel oxide species, which are more stable at intermediate than at high or low pH values and which hinder hydrogen absorption. Our study reveals the fundamental importance of electrolyte and interface effects for magneto-ionic mechanisms.