SUMMARY Magneto-optical studies of a c-oriented epitaxial MgB 2 film show that below 10K the global penetration of vortices is dominated by complex dendritic structures abruptly entering the film. We suggest that the observed behavior is due to a thermo-magnetic instability, which is supported by vortex dynamics simulations. This complex flux dynamics is shown to be responsible for large fluctuations in the M(H) curves and can limit practical applications of MgB 2. SUMMARY Magneto-optical studies of a c-oriented epitaxial MgB 2 film show that below 10K the global penetration of vortices is dominated by complex dendritic structures abruptly entering the film. We suggest that the observed behavior is due to a thermo-magnetic instability, which is supported by vortex dynamics simulations. This complex flux dynamics is shown to be responsible for large fluctuations in the M(H) curves and can limit practical applications of MgB 2. D.V. Shantsev, T.H. Johansen, M. Baziljevich, P.E. Goa, Y.M. Galperin Superconductivity Lab., Department of Physics, University of Oslo, Norway Dendritic flux penetration in MgB 2 films We acknowledge samples from S.I. Lee et al., Pohang Superconductivity Center, Korea

Noisy M(H) in MgB 2 Z. W. Zhao et al., cond-mat/ Thin film Polycrystalline sample Y. Bugoslavsky et al., Nature 410, 563 plotted: flux creep rate Large-grain tape S.X. Dou et al., cond-mat/ What’s the origin of noise??

cold stage magnetic field Magneto-optical imaging setup Bi:FG MsMs B  MsMs Faraday rotation  F = V t M s sin  B z / B A  F /  sat B x =0B x = 0.5 B A Anisotropy field B A = 2K / M s PRB 54, (1996)

1 mm 3.4 mT 21 mT 60 mT17 mT 8.5 mT 0 mT T = 5 K cond-mat/ MgB 2 thin film sample from S.I. Lee et al., Korea MgB 2 thin film sample from S.I. Lee et al., Korea c-axis oriented MgB 2 grown by PLD on (1 1 02) Al 2 O 3 thickness = 400 nm J c > 10 7 A/cm T< 8 K As the applied field increases, magnetic flux abruptly enters the film in the form of irregular dendritic structures When the field decreases, flux of the opposite polarity enters the film in a similar manner

Properties of dendrites: nucleate at random places near the sample edge develop in less than 1 ms the pattern never repeats itself in detail => the pattern is not related to film defects usually stop near d-lines => their growth is driven by Meissner current

Robust dendrites a “tree” grown during field increase becomes “frozen” and remains unchanged during subsequent field decrease down to the remanent state increasing field remanent state

Field-Temperature Phase Diagram Dendrites never appear above 11 K Dendrites sometimes appear for 10K<T<11K Dendrites are much more branching at higher T Most of these features are reproduced by computer simulations of vortex avalanches due to thermo- magnetic instability

exp(-U i /T i )  r i F ij Q  T T = 9.9 KT = 3.3 KT = 10.5 K Simulations: 1/r 2 - forces T  r i 1) Evaluate P i = exp(-U i /T i ) U i = U pin –  [  j F ij (r ij ) + F M (r i )] 2) Displace  r i  P i 3)  T =  r i F i /c (T)

Microstructure of dendrites: experiment and simulations Simulations: the central dendrite is growing; the region of high T (most intense vortex motion) is shown as red; vortices get pinned after they leave this region leading to an empty core of the dendrite Experiment: Some dendrites have low- field region in the middle.