  The penetration of transverse magnetic flux into a thin superconducting
square film in the flux flow state is considered by numerical simulation. Due
to the film self-field, the governing equations are nonlinear, and in
combination with the finite viscosity of the moving vortices, this sets up a
dynamical barrier for flux penetration into the sample. The corresponding
magnetization loop is hysteric, with the peak in magnetization shifted from the
zero position. The magnetic field in increasing applied field is found to form
a well-defined front of propagation. Numerical estimates shows that the
dynamical barrier should be measurable on films with low volume pinning.
