  Magnetic ultrathin films grown on heavy metal substrates often exhibit rich
spin structures due to the competition between various magnetic interactions
such as Heisenberg exchange, Dzyaloshinskii-Moriya interaction and higher-order
spin interactions. Here we employ spin-polarized scanning tunneling microscopy
to study magnetic nano-skyrmion phase in Fe monolayer grown on Ir(111)
substrate. Our observations show that the formation of nano-skyrmion lattice in
the Fe/Ir(111) system depends sensitively on the growth conditions and various
non-skyrmion spin states can be formed. Remarkably, the application of voltage
pulses between the tip and the sample can trigger a non-skyrmion to skyrmion
phase transition. The fact that nano-skyrmions can be created using voltage
pulse indicates that the balance between the competing magnetic interactions
can be affected by an external electric field, which is highly useful to design
skyrmion-based spintronic devices with low energy consumption.
