Chaotic vortex dynamics and low current phases in the remanent state of MgB₂ thin films

The current carrying critical state in thin superconducting films can become unstable at low temperatures. Collective motion of flux lines in the form of thermomagnetic avalanches leads to a substantial suppression of the critical current density and magnetization in the remanent state. Since the formation of these chaotic processes is strongly dependent on the local magnetic flux density, these effects can be restricted to particular areas of a thin film superconductor. We show magneto-optical images of the remanent state of thin magnesium diboride films. By producing difference images of remanent states subsequently prepared under the same conditions, the areas of chaotic flux line behaviour can be easily identified. Temperature-dependent investigations show that the area of chaotic motion grows with decreasing temperature below T = 10 K.