JOURNAL OF APPLIED PHYSICS
VOLUME 96, NUMBER 6
15 SEPTEMBER 2004
Observation of dendritic flux instabilities in YNi2B2C thin films
S. C. Wimbusha) and B. Holzapfel
IFW Dresden, P.O. Box 270016, 01171 Dresden, Germany
Ch. Jooss
Institut für Materialphysik, University of Göttingen, Tammanstrasse 1, 37077 Göttingen, Germany
(Received 10 March 2004; accepted 14 June 2004)
Magneto-optical imaging and magnetization measurements performed on thin films of the
borocarbide superconductor YNi2B2C reveal the occurrence of magnetic flux instabilities upon
reducing the applied magnetic field towards the remanent state. In contrast to other low-Tc materials
such as Nb and MgB2, where similar instabilities occur in both increasing and decreasing magnetic
fields, dendritic flux patterns are observed in YNi2B2C for decreasing fields only. Also in the
magnetization measurements, a distinct asymmetry is evident between increasing and decreasing
fields. The effect does not depend on the sweep rate of the field, but is strongly dependent on the
maximum field applied before reduction. The observation of spontaneous flux instabilities in this
additional family of low-temperature superconductors suggests that the responsible mechanism is
universal to this class of materials. © 2004 American Institute of Physics.
[DOI: 10.1063/1.1778816]
cases. The addition of the borocarbide family of superconductors to those in which such patterns have been observed
generalizes the phenomenon and opens up new possibilities
for its investigation. A question of particular interest is
whether a similar behavior can be observed in bulk samples,
since the connection between its occurrence in thin films and
bulk material is currently not clear.
Thin film samples of YNi2B2C were prepared by pulsed
laser deposition, as described in Ref. 10. The deposition conditions were chosen to produce samples with a high degree
of c-axis orientation and optimized Tc values lying between
14.5 and 14.6 K (at 90% of the normal state resistivity, measured inductively and thus representative of the entire sample
volume, as appropriate for the other measurement techniques
used) with a transition width (90%–10%) less than 0.3 K.
The in-plane orientation was here of secondary consideration
and samples having only a weak in-plane texture (predominant fibre texture) were selected. After preparation, the
samples were cleaved to an approximate size of 5 ⫻ 5 mm2
for further investigation.
Two distinct measurement techniques were applied for
investigation of the samples: bulk magnetization measurements via a dc extraction technique, and high-resolution
magneto-optical imaging (MOI). These techniques reveal
different features of interest in the investigation of flux
jumps. A primary advantage of the magnetization measurements is that they allow easy access to lower temperatures
and higher fields than the magneto-optical technique can attain, whereas MOI provides a spatially resolved visualization
of the flux patterns leading to the overall magnetization
signal.
The MOI technique is based on the principle of Faraday
rotation. By placing a doped iron garnet layer as a magnetooptically active sensing element on top of the superconductor, the normal component of the magnetic flux density
distribution can be measured with a spatial resolution of up
The observation of dendritic flux penetration in thin
films of the superconductor MgB2 (Refs. 1 and 2) triggered
renewed interest in the origin and mechanisms of this
effect,3,4 which exhibits a surprising variety of features in
different materials. Flux jumps were first observed magnetooptically in superconducting Nb disks,5 and later, with the
invention of high-resolution magneto-optical imaging, were
shown to be dendritic in nature.6 Dendritic flux patterns have
been observed to occur spontaneously in thin films of MgB2
for both increasing and decreasing external fields,1 and similar patterns can also be triggered by pulsed transport
currents,7 whereas in thin films of YBa2Cu3O7−␦, spontaneous dendritic flux patterns are usually not observed but may
be induced by means of local laser pulse heating.8 A common
feature in all cases is the requirement for low temperatures—
dendritic flux patterns occur only up to a fraction of the
superconducting transition temperature Tc (reported to be
around 0.65 for Nb and 0.25 for MgB2), being replaced by a
usual critical state flux penetration at higher temperatures.
Conventionally, flux jumps are explained in terms of a
thermomagnetic instability arising from the heat generated
by vortex motion.9 A rise in the local temperature lowers the
local critical current density Jc, encouraging further vortex
motion and thus resulting in a macroscopic vortex avalanche.
More recent theoretical work3 is also able to explain the
dendritic shape of the flux patterns. Experiments on MgB2
thin films under varying thermal conditions have shown that
this mechanism is in fact at work in this material.4
We report the observation of spontaneous dendritic flux
patterns on approaching the remanent state in YNi2B2C thin
films, similar to those seen in MgB2. We also highlight the
important differences in the flux patterns observed in the two
a)
Present address: National Institute for Materials Science, International
Center for Young Scientists, 1-1 Namiki, Tsukuba, Ibaraki 305-0044,
Japan; electronic mail: s.c.wimbush@ifw-dresden.de
0021-8979/2004/96(6)/3589/3/$22.00
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© 2004 American Institute of Physics
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3590
J. Appl. Phys., Vol. 96, No. 6, 15 September 2004
Wimbush, Holzapfel, and Jooss
FIG. 2. Magneto-optical images of the flux distribution within the sample at
4 K under decreasing applied fields after application of ␮0Hex = 66.0 mT. No
flux instability is observed down to the remanent state.
FIG. 1. Magneto-optical images of the flux distribution within the sample at
4 K under decreasing applied fields after application of ␮0Hex = 92.4 mT,
revealing the occurrence of a dendritic flux instability around ␮0Hex
⬇ 20 mT and its persistence down to the remanent state. The irregularity
visible in the upper right corner of all the images is a structural defect
resulting from the cleaving of the sample.
to 1 ␮m.11 A varying external magnetic field was applied
perpendicular to the plane of the film (parallel to the c axis of
the borocarbide unit cell) with a sweep rate between 150 and
300 mT s−1, and the resulting flux penetration into the superconductor (held at 4 K) observed.
Magnetization measurements were performed at different temperatures after zero-field cooling of the sample. As
for the MOI measurements, the field was oriented parallel to
the c axis of the borocarbide unit cell. An applied field sweep
rate of 1 mT s−1 was used, and magnetization data were collected at the maximum possible rate, without averaging. The
data are presented as measured, without correcting for the
background (substrate, sample holder), although this too was
measured (just above Tc) and found to be negligible.
The series of magneto-optical images in Fig. 1 shows the
development of the magnetic instability in decreasing magnetic field. After zero-field cooling and application of a
maximum external field ␮0Hex = 92.4 mT, this external field
was gradually reduced. Down to ␮0Hex = 26.4 mT, a typical
critical state rooflike pattern of trapped flux is visible. The
different sizes of the four current domains (separated by
bright lines in the image, the so-called discontinuity lines)
suggest that the critical current in the sample is slightly inhomogeneous, most likely due to a thickness variation across
the film. Upon further reduction of Hex the instability
abruptly appears at ␮0Hex ⬇ 20 mT. As Hex is then reduced to
zero, the dendritic flux pattern remains unchanged, with critical state behavior occurring around it. The flux pattern can
be overridden by remagnetization of the sample, and no instability was ever observed while increasing Hex. The appearance of the instability in decreasing Hex, however, depends on the value of the maximum applied external field.
This can be seen in Fig. 2, where the sample is shown in
decreasing Hex after application of a lower maximum
␮0Hex = 66.0 mT. In this case, the rooflike pattern is visible
right down to the remanent state 共␮0Hex = 0 mT兲. This implies that a minimum flux density (flux line proximity)
within the sample is necessary for the instability to develop.
A further interesting feature of the instability is that it
always develops towards the same region of the sample, the
region of maximum trapped flux. On repeating the experiment with the same maximum ␮0Hex = 92.4 mT, but using
different sweep rates of the external field, the position, size,
and general features of the dendritic flux pattern remain unchanged. However, its finer structure is different in each experiment.
Figure 3 shows the magnetization loops of the sample
measured at different temperatures. An irregular scattering of
the data points at low fields, typically interpreted as fluxjump behavior, is seen for temperatures as high as 10 K
共0.7Tc兲. With decreasing temperature, the scattering becomes
more pronounced, and extends to higher applied fields (up to
200 mT at 2 K). This is similar to the temperature dependence of the flux instability in other materials, which is seen
to occur only below a specific temperature T ⬍ Tc.
To confirm the thermomagnetic origin of the instability,
an experiment was performed on the sample, whereby it was
successively coated in an increasing quantity of gold and
remeasured at each stage. The effect on the magnetization
loop at 2 K is shown in Fig. 4. An initial gold coating is seen
to strongly suppress the flux-jump behavior, while further
coating leads the behavior closer to the “ideal” (non-fluxjump) state. This is consistent with the interpretation given
above, with the gold layer facilitating the transport of heat
away from the critical site, allowing sufficient dissipation to
prevent the occurrence of the instability.
FIG. 3. Magnetization loops measured at temperatures from 14 to 2 K in
2 K intervals. The inset shows in detail the region of irregular behavior at
low fields, highlighting the asymmetry of the curves, the varying extent of
the flux-jump regime, and the absence of irregularity at higher temperatures.
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J. Appl. Phys., Vol. 96, No. 6, 15 September 2004
FIG. 4. Effect on the magnetization loop of successive coating with an
increasing quantity of gold, measured at 2 K. The inset highlights the effect
within the flux-jump regime.
A distinct asymmetry is also evident in all of the magnetization results, with far more scattering occurring in the
quadrants of decreasing applied field than increasing. In the
increasing field quadrants, the curves are close to “ideal.”
This asymmetry, evidenced by both measurement techniques,
can again be related to the requirement for a minimum flux
density within the sample before an instability will occur.
For maximum applied fields higher than those at which the
instability can occur (which is the case for all the measurements presented here, and appears from the result of Fig. 2 to
be a requirement for instabilities to occur at all), there will
always be a higher flux density within the sample during
decreasing field than increasing. Thus, flux instabilities are
generally observed in this material only when decreasing the
applied field.
Our observation of dendritic flux instabilities in
YNi2B2C superconducting films raises the general question
as to which materials are capable of exhibiting this behavior,
and under which conditions it arises. Comparing Nb, MgB2,
and YNi2B2C with the high-Tc superconductors, in which
such spontaneous instabilities are usually not observed, we
note that the strength of thermal fluctuations (quantified in
terms of the Ginzburg number12) differs by orders of magnitude in the different materials (10−9 for Nb, 10−4 for
YNi2B2C and MgB2, and 100 for YBa2Cu3O7−␦). This suggests that strong thermal relaxation in the high-Tc’s helps to
prevent such instabilities, since locally generated “hot spots”
can easily be thermalized in the vortex system (the exception
to this being experiments where the formation of a “hot spot”
is artificially forced8). Furthermore, in general, stronger flux
Wimbush, Holzapfel, and Jooss
3591
creep is present at higher temperatures and larger magnetic
fields, where no instability is observed. This ties in with the
theoretical treatment of Aranson, Gurevich, and Vinokur3
who define a parameter ␶ relating the thermal and magnetic
flux diffusivities. Strong flux creep implies a very small ␶
and reduced Jc, resulting in the absence of flux instabilities.
For small ␶, the instability will be localized, while for ␶
large, it will grow to consume the entire sample. This may
explain the different scale of the dendritic patterns observed
with respect to MgB2, with YNi2B2C having a comparatively
large ␶. Aranson also considers a parameter ␣, defining a
threshold Jc for the instability to occur in terms of the thermal conditions and the resistivity of the sample. The occurrence of flux instabilities predominantly in decreasing field
suggests that this value ␣ is close to the critical value; this
explains why flux instabilities have first been observed in
borocarbides in these samples, having Bean model zero-field
Jc values of the order of 106 A cm−2, some two orders of
magnitude higher than previously reported samples.
In summary, the families of low-Tc superconductors
combining moderate Tc values with large coherence lengths
provide experimental access to a fuller understanding of the
well-established phenomenon of thermomagnetic flux instability.
ACKNOWLEDGMENTS
Work at the IFW Dresden was supported by the Deutsche Forschungsgemeinschaft as part of SFB463, “Rare earth
transition metal compounds: structure, magnetism, and transport.” The work in Göttingen was also supported by the
Deutsche Forschungsgemeinschaft.
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