Magneto-optical studies of the critical states in c-axis oriented MgB2 thin film and bulk MgB2/Mg nano-composites

We present magneto optical (MO) studies of the critical states in a c-axis oriented MgB₂ thin film with a critical current density (J_c) of 6 MA/cm² at 5 K, and bulk MgB₂/Mg nano-composites with J_c over 1 MA/cm² at 5 K. The magnetic flux penetration and trapping in the thin film are found remarkably different from that in the bulk samples. A tree-like magnetic flux pattern was observed entering the superconducting film, as well as various complex dendritic flux jumps due to the thermo-magnetic instability. These behaviors indicate the collapse of the critical state of a type II superconductor at local level. In contrast, a regular flux penetration into the critical state was observed in the bulk nano-composites, which indicates that the addition of pure Mg helps to stabilize the critical state.