  Voltage manipulation of skyrmions is a promising path towards low-energy
spintronic devices. Here, voltage effects on skyrmions in a GdOx/Gd/Co/Pt
heterostructure are observed experimentally. The results show that the skyrmion
density can be both enhanced and depleted by the application of an electric
field, along with the ability, at certain magnetic fields to completely switch
the skyrmion state on and off. Further, a zero magnetic field skyrmion state
can be stablized under a negative bias voltage using a defined voltage and
magnetic field sequence. The voltage effects measured here occur on a
few-second timescale, suggesting an origin in voltage-controlled magnetic
anisotropy rather than ionic effects. By investigating the skyrmion nucleation
rate as a function of temperature, we extract the energy barrier to skyrmion
nucleation in our sample. Further, micromagnetic simulations are used to
explore the effect of changing the anisotropy and Dzyaloshinskii-Moriya
interaction on skyrmion density. Our work demonstrates the control of skyrmions
by voltages, showing functionalities desirable for commercial devices.
