  A skyrmion can be stabilized in a nanodisc geometry in a ferromagnetic
material with Dzyaloshinskii-Moriya (DM) interaction. We apply spin torque
uniform in space and time and observe numerically that the skyrmion is set in
steady rotational motion around a point off the nanodisc center. We give a
theoretical description of the emerging auto-oscillation dynamics based on the
coupling of the rotational motion to the breathing mode of the skyrmion and to
the associated oscillations of the in-plane magnetization. The analysis shows
that the achievement of auto-oscillations in this simple set-up is due to the
chiral symmetry breaking. Thus, we argue that the system is turned into a
spin-torque oscillator due to the chiral DM interaction.
