SUMMARY

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Dendritic flux penetration in MgB2 films
D.V. Shantsev, T.H. Johansen, M. Baziljevich,
P.E. Goa, Y.M. Galperin Superconductivity Lab.,
Department of Physics, University of Oslo, Norway
SUMMARY Magneto-optical studies of a c-oriented
epitaxial MgB2 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 MgB2.
We acknowledge samples from S.I. Lee et al.,
Pohang Superconductivity Center, Korea
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Noisy M(H) in MgB2
Large-grain tape
S.X. Dou et al., cond-mat/0102320
Thin film
Polycrystalline sample Y. Bugoslavsky et al.,
Nature 410, 563 plotted flux creep rate
Z. W. Zhao et al. , cond-mat/0104249
Whats the origin of noise??
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Magneto-optical imaging setup
Faraday rotation qF V t Ms sinf
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MgB2 thin film sample from S.I. Lee et al., Korea
c-axis oriented MgB2 grown by PLD on (1102)
Al2O3 thickness 400 nm Jc gt 107 A/cm2 _at_ Tlt
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
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Properties of dendrites

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

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Robust dendrites
a tree grown during field increase becomes
frozen and remains unchanged during subsequent
field decrease down to the remanent state
remanent state
increasing field
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Field-Temperature Phase Diagram

• Dendrites never appear above 11 K
• Dendrites sometimes appear for 10KltTlt11K
• 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
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Microstructure of dendrites experiment and
simulations
Experiment Some dendrites have low-field region
in the middle.
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