  Placing a suitable spacer layer between two magnetic layers can lead to an
interaction between the magnetic layers known as Ruderman-Kittel-Kasuya-Yosida
(RKKY) coupling. Controlling RKKY coupling, particularly the ability to switch
between ferromagnetic and antiferromagnetic coupling, would enable novel
magnetic data storage devices. By combining solid-state Li ion battery
technology with an out-of-plane magnetized Co/Pt-based stack coupled through a
Ru interlayer we investigate the effects of the insertion of Li ions on the
magnetic properties of the stack. The RKKY coupling and its voltage dependence
is measured as a function of the Ru interlayer thickness, along with the
effects of repeated voltage cycling. The Li ions both change the amplitude of
the RKKY coupling and its phase, leading to the ability to switch the RKKY
coupling between ferromagnetic and antiferromagnetic with applied voltages.
