  Electrically induced ionic motion offers a new way to realize
voltage-controlled magnetism, opening the door to a new generation of logic,
sensor, and data storage technologies. Here, we demonstrate an effective
approach to magneto-ionically and electrically tune exchange bias in
Gd/Ni$_{1-x}$Co$_{x}$O thin films (x=0.50, 0.67), where neither of the layers
alone is ferromagnetic at room temperature. The Gd capping layer deposited onto
antiferromagnetic Ni$_{1-x}$Co$_{x}$O initiates a solid-state redox reaction
that reduces an interfacial region of the oxide to ferromagnetic NiCo. Exchange
bias is established after field cooling, which can be enhanced by up to 35%
after a voltage conditioning and subsequently reset with a second field
cooling. These effects are caused by the presence of an interfacial
ferromagnetic NiCo layer, which further alloys with the Gd layer upon field
cooling and voltage application, as confirmed by electron microscopy and
polarized neutron reflectometry studies. These results highlight the viability
of the solid-state magneto-ionic approach to achieve electric control of
exchange bias, with potentials for energy-efficient magneto-ionic devices.
