Electric-field-controlled magnetism can boost energy efficiency in widespread applications. However, technologically, this effect is facing important challenges: mechanical failure in strain-mediated piezoelectric/magnetostrictive devices, dearth of room-temperature multiferroics, or stringent thickness limitations in electrically charged metallic films. Voltage-driven ionic motion (magneto-ionics) circumvents most of these drawbacks while exhibiting interesting magnetoelectric phenomena. Nevertheless, magneto-ionics typically requires heat treatments and multicomponent heterostructures. Here we report on the electrolytegated and defect-mediated O and Co transport in a Co 3 O 4 single layer which allows for room-temperature voltage-controlled ON−OFF ferromagnetism (magnetic switch) via internal reduction/oxidation processes. Negative voltages partially reduce Co 3 O 4 to Co (ferromagnetism: ON), resulting in graded films including Co-and O-rich areas. Positive bias oxidizes Co back to Co 3 O 4 (paramagnetism: OFF). This electric-field-induced atomic-scale reconfiguration process is compositionally, structurally, and magnetically reversible and self-sustained, since no oxygen source other than the Co 3 O 4 itself is required. This process could lead to electric-field-controlled device concepts for spintronics.

The topological nature of magnetic skyrmions leads to extraordinary properties that provide new insights into fundamental problems of magnetism and exciting potentials for novel magnetic technologies. Prerequisite are systems exhibiting skyrmion lattices at ambient conditions, which have been elusive so far. Here, we demonstrate the realization of artificial Bloch skyrmion lattices over extended areas in their ground state at room temperature by patterning asymmetric magnetic nanodots with controlled circularity on an underlayer with perpendicular magnetic anisotropy (PMA). Polarity is controlled by a tailored magnetic field sequence and demonstrated in magnetometry measurements. The vortex structure is imprinted from the dots into the interfacial region of the underlayer via suppression of the PMA by a critical ion-irradiation step. The imprinted skyrmion lattices are identified directly with polarized neutron reflectometry and confirmed by magnetoresistance measurements. Our results demonstrate an exciting platform to explore room-temperature ground-state skyrmion lattices.