Voltage control of magnetism in Ni–Co oxide mesoporous films: impact of porosity on oxygen magneto-ionics performance

Control of magnetism through electric-field-driven migration of ions, referred to as magneto-ionics (MI), holds promise for the development of non-volatile energy-efficient memory storage, as well as spintronic, neuromorphic and magnetoelectric devices. Here, we study the MI phenomena in 350 nm thick Ni₅₅Co₄₅ oxide films with varying degrees of porosity, obtained by electrodeposition of the parent Ni-Co metallic alloy on metallized Si substrate and subsequent annealing in air. Annealing at 450 °C of the film electrodeposited from a P-123-containing electrolyte with Ni and Co sulfate salts yields a Ni-Co oxide that partially retains its mesoporosity. This sample exhibits a higher MI response compared to a low-porosity (nearly dense) Ni-Co oxide film, indicating that an increased surface-to-volume ratio enhances MI. Comprehensive characterization of the mesoporous Ni-Co oxide-coated Si/Ti/Au sample reveals that annealing not only oxidizes the top ≈100 nm of the Ni-Co film but also induces silicon diffusion. MI phenomena occur via O²⁻ migration out of and into the top Ni-Co oxide layer under negative and positive biasing, respectively. While the system shows some irreversibility, endurance improves significantly as cycling frequency increases, evidencing the potential of this material for voltage-tunable memory applications.