Magnetic flux penetration in MgB2 thin films produced by pulsed laser deposition

Two types of MgB2 thin films produced by pulsed laser deposition with different in situ and ex situ sintering routes have been studied. Using the magneto-optical (MO) imaging technique, the magnetic flux penetration behaviour in the films has been investigated. In the case of the in situ film, the MO observations reveal conventional flux jumps below the corresponding threshold temperature, whereas in the case of the ex situ film the flux jumps appear to take the form of unusual, structurally driven 'blob'-like patterns. The underlying structural features of the films have been investigated by scanning electron and atomic force microscopy. The critical current density dependence on applied magnetic field obtained from magnetization measurements is consistent with the local behaviour of the magnetic flux, as well as with flux pinning properties expected from the obtained crystalline structure of the ex situ film and the amorphous-like surface of the in situ film. On the basis of these structural and electromagnetic observations, a mechanism for the structurally driven, spatially reproducible flux jumps is proposed