  Ionic control of magnetism gives rise to high magneto-electric coupling
efficiencies at low voltages, which is essential for low-power magnetism-based
non-conventional computing technologies. However, for on-chip applications,
magneto-ionic devices typically suffer from slow kinetics, poor cyclability,
impractical liquid architectures or strong ambient effects. As a route to
overcoming these problems, we demonstrate an LiPON-based solid-state ionic
supercapacitor with a magnetic Pt/Co$_{40}$Fe$_{40}$B$_{20}$/Pt thin-film
electrode which enables voltage control of a magnetic skyrmion state. Skyrmion
nucleation and annihilation are caused by Li ion accumulation and depletion at
the magnetic interface under an applied voltage. The skyrmion density can be
controlled through dc applied fields or through voltage pulses. The skyrmions
are nucleated by single 60-$\mu$s voltage pulses and devices are cycled 750,000
times without loss of electrical performance. Our results demonstrate a simple
and robust approach to ionic control of magnetism in spin-based devices.
