Voltage-Driven Generation of Ferromagnetism in a Magneto-Ionically Active Antiferromagnet Enabling Room-Temperature Exchange Bias.

Magneto-ionics-as the voltage-driven control of magnetic properties through ionic motion and redox processes-offers a promising route toward energy-efficient spintronic devices. Exchange bias, being the unidirectional anisotropy arising from interfacial coupling between antiferromagnets and ferromagnets, plays a central role in spintronics. Here, we demonstrate reversible, room-temperature magneto-ionic generation, suppression, and modulation of exchange bias within a 50 nm-thick antiferromagnetic, magneto-ionically active NiCoO layer. Instead of relying on field cooling to set exchange bias, an applied magnetic field during the growth promotes alignment of the antiferromagnetic spin sublattices, producing a preferential unidirectional orientation. Gating drives oxygen-ion migration along columnar grain boundaries, partially reducing NiCoO and forming ferromagnetic NiCo clusters that couple to the antiferromagnetic matrix. The exchange bias can be controlled by tuning the Ni/Co ratio, which adjusts the Néel temperature, and by varying the actuation time and voltage amplitude which control ferromagnetic cluster size. Micromagnetic simulations reveal that the exchange bias originates from the interfacial uncompensated spins exhibiting partial ferromagnetic-like behavior. This single-layer approach, together with the voltage-controlled formation and tuning of exchange bias without heat treatments, simplifies fabrication and offers a framework for low-power antiferromagnetic spintronic devices.