  Magnetic instability in both ultra-pure and carbon-doped MgB2 films is
investigated by magneto-optical imaging, transport and bulk magnetization
measurements. In the carbon-doped MgB2 thin film, familiar dendritic flux-jump
patterns were observed at low temperature as reported in previous experiments.
In the ultra-pure MgB2 thin film, however, a remarkably stable flux penetration
was observed, clearly showing the classic behavior of the critical state model.
Such different behavior indicates that the electron scattering ultimately
controls the magnetic stability of MgB2 films.
