  The static and dynamic states of N\'eel skyrmions in ultrathin ferromagnetic
films with interfacial Dzyaloshinskii-Moriya interaction (DMI) have been
micromagnetically simulated as functions of the interfacial DMI strength and
applied static magnetic field. Findings reveal that while the breathing,
counterclockwise (CCW) and clockwise (CW) rotational eigenmodes exist in the
skyrmion lattice (SkL) phase, only the first two modes are present in the
isolated skyrmion (ISk) phase. Additionally, the eigenfrequency of the CCW mode
is insensitive to the magnetic-field driven SkL-ISk phase transition, and the
inter-skyrmion interaction is largely responsible for exciting the CW mode in
the SkL phase. The findings provide physical insight into the dynamics of the
phase transition and would be of use to potential skyrmion-based microwave
applications.
