Burning of high T c bridges
M. E. Gaevski,a) T. H. Johansen, Yu. Galperin,a) and H. Bratsberg
Department of Physics, University of Oslo, P. O. Box 1048 Blindern, N 0316 Oslo, Norway
A. V. Bobyl, D. V. Shantsev, and S. F. Karmanenko
State Technical University, 195251 St. Petersburg, Russia
~Received 4 February 1997; accepted for publication 24 September 1997!
Burning of superconducting thin film bridges by large transport currents ~up to densities of 2
3107 A/cm2 ) is investigated by magneto-optical imaging of flux distribution and low-temperature
scanning electron microscopy providing T c maps. It is shown that the destruction is preceded by
significant penetration of magnetic field inside a weak-pinning region. In bridges containing
extended defects magneto-optic investigation is sufficient to locate the incipient burning region. In
high-quality bridges free from such defects only a combination of the two techniques will allow
prediction of the place of fatal destruction. © 1997 American Institute of Physics.
@S0003-6951~97!01847-0#
In order to optimize the performance of devices made
from high T c superconductors ~HTSs! it is important to have
space-resolved information about the material and its response properties. Today, several techniques have been developed for this purpose. One of them is low-temperature
scanning electron microscopy ~LTSEM!,1 which monitors
the transition into the normal state under the local heating by
an electronic beam. This method allows one to determine the
local values of the critical temperature T c . Another powerful
technique—magneto-optical ~MO! imaging ~see e.g., Ref. 2
and references therein!—enables one to monitor the distribution of magnetic field perpendicular to a plane adjacent to the
HTS surface. In this work we combine these two nondestructive techniques to investigate the performance of HTS
bridges carrying large transport currents.
When an HTS device carries a transport current, I T ,
approaching the critical value, I c , one expects that local
variations in the material become increasingly important and
eventually determines where the device is destroyed. The
usual source of destruction is thermally induced motion of
magnetic flux which leads to further heat dissipation and
local increase in the temperature. This, in turn, causes reduced flux pinning locally, and a process having the character of an avalanche3 leads to strong overheating or burning.
While HTS films carrying small transport currents have been
studied previously by the MO method,4–7 there exists so far
no such study focusing on the behavior when I T ;I c . In this
work we show that MO images of self-fields from I T correlated with T c maps obtained by the LTSEM technique8 can
be used to predict the location of burning in HTS films.
Superconducting YBa2 Cu3 O72 d ~YBCO! films were
prepared by dc magnetron sputtering of a ceramic stoichiometric target. The films were grown on ~100! LaAlO3 substrates at a rate of 2 nm/min at T5680 K in a gaseous mixture of Ar and oxygen. Subsequent heat treatment at T
5500 K for 20 min in pure oxygen at a pressure of 1 atm
and cooling for 1 h resulted in a high oxygen saturation of
the YBCO lattice.9 The film thickness was 300 nm. X-ray
a!
Also with A. F. Ioffe Physico-Technical Institute, 194021 St. Petersburg,
Russia.
Appl. Phys. Lett. 71 (21), 24 November 1997
analysis confirmed good quality and a ~100! orientation of
the films. According to scanning electron microscope ~SEM!
studies, the films contained no inclusions or outgrowths >1
m m normal to the surface.
Micro-bridges 1003500 ( m m!2 were formed by a standard photolithographic procedure. To optimize conditions for
MO studies we fabricated samples with all contact plates
placed on the same side of the structure at a sufficient distance from the bridge. The contact pads were covered by Ag,
and Au wires were attached by a thermal compression. Two
typical bridges were studied. One of them ~see Fig. 1! referred to as ‘‘good,’’ had a critical current density ;106
A/cm2 at 77 K as determined by current-voltage (I-V) measurements. The other sample, referred to as ‘‘bad’’ had critical current ;53104 A/cm2 at 77 K. As shown below, the
burning process for the two samples showed distinctly different features.
MO images of the ‘‘good’’ sample at 15 K are shown in
Figs. 1~b!–1~f!. Here bright areas correspond to large absolute values of the perpendicular magnetic field. As an MO
indicator we use Bi-doped YIG film with in-plane anisotropy, and an optical cryostat system described in Ref. 10. In
the current-carrying states with I T 53 A and 6 A, Figs. 1~b!
and 1~d!, respectively, the field is seen to concentrate near
the edges. Note that the MO indicator does not discriminate
between the ‘‘up’’ and ‘‘down’’ directions of the perpendicular field when using the present crossed polarizer setting
on the microscope, and hence the images are essentially
symmetric about the center line of the bridge. ~By changing
the polarizer settings the two directions are easily distinguished.! The increased intensity near the corners is due to
bending of the current flowlines as they are governed by the
bridge geometry.11 The uniform dark appearance of the inner
part of the bridge demonstrates a very good homogeneous
screening.
In the remanent state after switching off the current, Fig.
1~c!, one can clearly observe two pairs of bright bands near
the edges separated by the dark shielded area. The inner two
bands show the trapped flux, which has opposite direction in
the two bands. The outer two bands show re-magnetized regions, i.e., the return field of the trapped flux partly penetrat-
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© 1997 American Institute of Physics
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FIG. 1. ~a! Sketch of the bridge geometry. ~b!–~f! MO images of the ‘‘good’’ bridge @scale bar in ~b! is 100 m m long#. ~b!,~d! The self-field distributions for
currents I T 53 and 6 A, respectively. ~c!,~e! The remanent state after switching off the 3 and 6 A currents, respectively. ~f! Destroyed bridge in an external
field of 5 mT. In this frame the the bright areas near the contact pads are due to the magnetic prehistory of the sample, and thus irrelevant for the illustration
of the burned region, which appears as a bright channel near the center. ~Right! T c map of the ‘‘good’’ sample: ~black! 90 K, ~white! 92 K.
ing into the film. The flux bands therefore represent an oscillating flux profile across the bridge where the field
changes sign three times, as predicted by theory.12 A more
detailed comparison is presented in a forthcoming paper.
Increasing the current above 6 A caused burning, i.e.,
J c 523107 A/cm2 at 15 K, leaving the bridge in the remanent state shown in Fig. 1~e!. More than 50% of the bridge
appears dark because the trapped flux escaped this region
during the heating. In order to localize where fatal destruction took place an external field was applied to the sample.
Figure 1~f! shows such an MO image with an applied field of
5 mT. The destroyed region is the bright channel crossing the
bridge.
From the details of the pre-burning MO images the
bridge had two regions near the edge, marked by arrows,
showing enhanced flux penetration in the current carrying
state. Other irregularities can also be seen in the flux front,
but these are of less magnitude. Of the two major defects the
one associated with region 1 is the more pronounced, and
should therefore be the most probable candidate to initiate
the burning. Surprisingly, the bridge ‘‘chooses’’ instead region 2, as evident from Fig. 1~f!. Note also that none of the
defects at any pre-burning stage extends across a substantial
part of the bridge even when J T .J c , as in Fig. 1~d!. It is
therefore clear that MO imaging alone is not capable of pre3148
Appl. Phys. Lett., Vol. 71, No. 21, 24 November 1997
dicting the burning location in a bridge which is free from
extended defects. This situation is quite different from the
case studied in Ref. 5, and also for the ‘‘bad’’ sample discussed below, where pronounced extended defects dominated the behavior.
Using the LTSEM technique, described in detail
elsewhere,8 we made prior to the MO investigation, a map of
the local T c for each bridge. Such a T c map of the ‘‘good’’
bridge is shown in Fig. 1~right! where the dark regions correspond to reduced values of T c . Here one clearly recognizes regions 1 and 2 from the MO images as also being
areas with relatively low T c . If one assumes that regions
with low T c also have reduced J c it is evident that region 2 is
the more probable candidate as the area of low T c here percolates across the entire bridge. Indeed, the channel of destruction seen in the MO image strongly correlates with the
percolating path of low T c .
One can also notice that a low T c path exists near the
upper boundary of the bridge. The reason why we believe
that this region is not fatal is as follows. First, the MO images show no distinct features in this part of the bridge,
indicating that the region has an enhanced concentration of
pinning centers which prevents reduction in J c in spite of
low T c . Second, the central part of the bridge is likely to be
in the worst situation with respect to thermal stability. Thus
Gaevski et al.
FIG. 2. ~a!–~e! MO images of the ‘‘bad’’ sample. ~a! Remanent state obtained by switching off a field of 150 mT. ~b!–~d! Increase of transport
current up to 3 A. ~e! The destroyed bridge in an external field of 5 mT. The
scale bar is 100 m m long. ~f! SEM image of the area marked by white
rectangle on ~e!.
one expects that the device will burn at some ‘‘weak region’’
close to the central part.
The MO images of the ‘‘bad’’ bridge are shown in Figs.
2~a!–2~e!. In 2~a! one sees an image of the remanent state
after applying a magnetic field of 150 mT which caused full
penetration. In this case, the pair of dark lines separate the
uni-directed trapped flux in the center from the reversely
directed return flux. Adding now a transport current, the field
distribution becomes asymmetric, as shown in 2~b!–2~d!.
According to Ref. 12 an additional line with B z 50, i.e., with
dark appearance, should exist within an interval of I T values.
Figure 2~b! gives clear evidence for this prediction of the
theory, not previously verified for a state with such a combined magnetic prehistory.
The main observation here, however, is that in this
sample an extended defect is visible in the MO images. The
flux pattern in Fig. 2~c! is severely distorted near a ‘‘weak
region’’ where the field penetrates more than half of the
bridge width. Note however that, at large I T close to the
critical value as in 2~d!, the field distribution becomes more
symmetric. We believe that in this regime the transport current drives the cross section containing the ‘‘weak region’’
into a resistive state with resistivity of the same order of
magnitude as in the nondefected part. Finally, at I5I c 53 A,
the bridge burns out. From 2~e! showing how the destroyed
Appl. Phys. Lett., Vol. 71, No. 21, 24 November 1997
bridge shields an external field it is clear that the burning
took place in the ‘‘weak region.’’ The micro-photograph of
Fig. 2~f! shows that the damaged part of the film consists of
a narrow straight line, not directly across the bridge, but
instead forming a certain angle. The ‘‘bad’’ bridge was evidently destroyed along a linear defect present in the film.
We conclude therefore that MO imaging can be used to
locate extended defects which become fatal for HTS bridges
carrying large transport currents. In films showing no extended defects, like the ‘‘good’’ sample in our study, the
situation is more complex. From MO imaging one can find
several candidates for the location of burning, and hence
additional diagnostics is required. The method of LTSEM
mapping of T c provides another set of candidates, which in
general do not completely overlap with the ones from MO
images. We have shown that bands of low T c extending
across the bridge, and also overlapping with edge defects
seen by MO imaging, are the actual place of fatal destruction
when I T >I c . The burning is usually seen to take place near
the central part of the bridge. This indicates that in order to
optimize the performance of superconducting bridge devices
it is important to design them in a convex shape in order to
increase the heat transfer. In spite of the considerable field
amplification near the sharp corners, see e.g., Fig. 1~d!, we
found that burning never takes place here. Thus, a design
with rounded corners will probably not improve the strength
of the bridge with respect to burning.
The authors thank The Norwegian Research Council and
the Russian State Program on HTSC ~Grant No. 94048! for
financial support.
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