Magneto-optical imaging studies of flux propagation in ultra-pure and carbon-doped MgB₂ thin films

The mixed state in c-axis oriented ultra-pure MgB₂ thin films and carbon-doped MgB₂ thin films produced by hybrid physical-chemical vapor deposition was studied using magneto-optical imaging (MOI) technique. In the ultra-pure MgB₂ films, a regular magnetic flux penetration pattern was observed, as expected from the Bean critical state model. This is in striking contrast to earlier MOI observations of MgB₂ films made by pulse laser deposition, where dentritic flux jumps were usually abundant at temperature below 10 K. In our carbon-doped MgB₂ thin films, similar dentritic flux jumps were also observed at low temperature and low applied magnetic field. It is evident that the dendritic flux jumps or stability of the film is strongly influenced by the material parameters of the films, rather than inherent in all MgB₂ thin films.