Composition-Dependent Voltage-Driven OFF-ON Switching of Ferromagnetism in Co-Ni Oxide Microdisks

Magneto-ionics, which refers to the modification of the magnetic properties of materials through electric-field-induced ion migration, is emerging as one of the most promising methods to develop nonvolatile energy-efficient memory and spintronic and magnetoelectric devices. Herein, the controlled generation of ferromagnetism from paramagnetic Co-Ni oxide patterned microdisks (prepared upon thermal oxidation of metallic microdisks with dissimilar Co-Ni ratios, i.e., Ni₂₅Co₇₅ and Ni₅₀Co₅₀) is demonstrated under the action of voltage. The effect is related to the partial reduction of the oxide phases to their metallic forms. Samples richer in Co show stronger magneto-ionic activity, which manifests in lower-onset threshold voltages, faster switching rates, and larger values of the attained saturation magnetization. By means of scanning electron microscopy, a cobalt segregation phenomenon has been experimentally observed upon thermal oxidation, which has been theoretically discussed from the diffusivities' viewpoint. X-ray diffraction characterization has revealed transitions between purely mixed Ni and Co oxides, in the OFF state, to a mixture of oxide and metallic phases, in the ON state, because of the oxygen ion motion outward/inward the Co-Ni oxide microdisks, depending on the voltage polarity. Ab initio calculations reveal that the energy barrier for oxygen vacancy migration is lower in CoO than in NiO, in agreement with the obtained magneto-ionic responses. The observation of magneto-ionic effects in patterned disks (and not only in archetypical continuous films) is a step further for the practical utilization of this phenomenon in real miniaturized devices.