  An analytical expression is obtained for the mass of an isolated magnetic
skyrmion and its linearized equation of motion. The magnetic skyrmion is viewed
as a topologically protected spin-wave soliton in the magnetic ultrathin films
stabilized by the interfacial-Dzyaloshinskii-Moriya interaction. The equations
of motion are derived from the Landau-Lifshitz-Gilbert equation for both the
skyrmion charge and magnetization centers. They are generalized Thiele
equations, including the gyro-term, dissipation term, external force,
acceleration term with the tensorial mass, and time derivatives of the external
forces. The equation of motion of the center of the skyrmion charge essentially
shows the massless nature of the skyrmion. In contrast, the equation of motion
for the magnetization center results in a finite mass that is in the same order
as the Doring mass density for the linear domain wall. Furthermore, the time
derivative of the external force predominantly contributes to the immediate
response of the skyrmion motion, i.e., the mass-less property remains even
after the skyrmion acquires its kinetic mass.
  A micromagnetic simulation based on the LLG equation was performed for
various magnetic parameters. Obtained trajectories at 0 K are compared with the
theoretical predictions.
