INSTITUTE OF PHYSICS PUBLISHING
SUPERCONDUCTOR SCIENCE AND TECHNOLOGY
Supercond. Sci. Technol. 14 (2001) 839–843
PII: S0953-2048(01)18812-3
Magnetic field penetration in
YBa2Cu3O7−δ single crystals under
conditions of a reduced surface barrier
L Dorosinskii1 , H Bocuk, U Topal and C Birlikseven
Ulusal Metroloji Enstitusu (UME—National Metrology Institute), PO Box 21,
41470 Gebze-Kocaeli, Turkey
E-mail: dorosins@ume.tubitak.gov.tr
Received 9 November 2000, in final form 29 June 2001
Published 13 September 2001
Online at stacks.iop.org/SUST/14/839
Abstract
Using a magneto-optic technique, we have studied field penetration in a
sample consisting of several YBCO single crystals merged together on their
basal plane. The bending of field stream lines around crystal edges in such a
geometry creates an orientation of the local applied field that is nearly
perpendicular to the sample surface and thus results in an almost
surface-barrier-free field penetration. The penetration field was measured
under conditions of a fully developed and a reduced surface barrier, and the
values obtained were compared in these two cases. The results obtained
show that the surface barrier causes a substantial delay of field penetration in
YBCO single crystals over the whole temperature range.
(Some figures in this article are in colour only in the electronic version)
1. Introduction
It is well known that the penetration of a magnetic field in
type-II superconductors is affected by the Bean–Livingston
surface barrier [1]. The effect of this barrier was found to
be especially strong in single crystals of high-temperature
superconductors [2–4], which is connected with a large value
of the anisotropy parameter γ and with the high quality of
the surface in high-temperature superconductor single crystals.
Because of the effect of the surface barrier, the magnetic
field starts to penetrate into a superconductor not when it
reaches the first critical field, HC1 , but at a higher field
value which, in the ideal case, is close to the thermodynamic
critical field, HC . It was shown that the unusual upturn in
the temperature dependences of the penetration field in hightemperature superconductors, which develops on decreasing
temperature [3, 5–7], is connected with an increased effect
of the surface barrier at lower temperatures [2]. At higher
temperatures the surface barrier is completely surmounted
due to the fast thermal activation [2, 3], and the penetration
field is equal HC1 , whereas at lower temperatures it becomes
closer to HC . Another reason for a delayed (compared to the
equilibrium situation) field penetration in superconductors can
1
Author to whom correspondence should be addressed.
0953-2048/01/100839+05$30.00
© 2001 IOP Publishing Ltd
be the recently discovered geometrical barrier [8, 9]. As a
result, it becomes very difficult to measure the temperature
dependence of HC1 for high-temperature superconductors.
In this work we have found a way to bypass the surface barrier
and to measure the penetration field unaffected (or affected to
a lesser extent) by the barrier. We will present a comparison
of the results of field penetration into the same crystal for two
situations: with a fully developed surface barrier and under
conditions of a reduced barrier.
The effect of the surface and geometrical barriers could be
bypassed if the applied field was perpendicular to the sample
surface. In typical experiments this is not the case because
the field starts to penetrate into the sample from its edges,
where it is nearly parallel to the lateral surface. In principle, a
perpendicular field orientation could be created using a small
coil placed on top of the sample being investigated (if the
coil is much smaller than the sample, it creates a local field
which is perpendicular to the sample surface). However,
it is very difficult to realize such an arrangement for hightemperature superconductor single crystals because of their
small dimensions. We have found a different way to create
such a field orientation, which was given by the natural
shape of a sample consisting of several crystals which are
merged together (see figure 1). The way in which a field
Printed in the UK
839
L Dorosinskii et al
(a)
(b)
(c)
Figure 1. (a) General view of the sample studied. The sample consists of several YBCO single crystals merged together on their basal
plane. (b) Field penetration into the sample. T = 18 K, a field of −143 Oe is applied after field cooling in H = +315 Oe. The field starts to
penetrate into each crystal at the area around the edge of the adjacent crystal. (c) Schematic representation of field stream lines around the
studied sample in an applied field.
streamlines around such a sample in a perpendicular external
field is shown schematically in figure 1(c). They bend near
the edges of the upper crystal, thus creating a component
perpendicular to the surface of the lower crystal. We expect
840
that this should result in easier field penetration in the lower
crystal in the region which is in contact with the upper crystal
(below, for brevity sake, we will call this region an interface
region).
Magnetic field penetration in YBCO single crystals
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Figure 3. Local ac susceptibility curves measured at the points
designated in figure 1(a).
Figure 2. Detailed view of the pattern of field penetration in part of
the sample. T = 21 K, H = 483 Oe, zero-field cooling.
2. Experiment
Studies of field penetration in superconducting crystals have
been carried out using the magneto-optic method [10]. This
method allows observation and quantitative measurements
of the field patterns with high spatial resolution, down to
1 µm. Also, we can measure local ac susceptibility curves
which show the spatial distribution of transition temperatures
in an inhomogeneous sample. Quantitative measurements
of local ac and dc fields are carried out by measuring the
light intensity with a photomultiplier connected to a lock-in
amplifier [10]. The local ac field is measured directly, and for
dc measurements a special modulation technique is used [10].
YBa2 Cu3 O7−δ samples were grown in gold crucibles using the
method of [11]. The samples were annealed for 72 h at 450 ◦ C
in flowing oxygen, which resulted in a sharp superconducting
transition. The critical temperature of the crystals, 84 K,
measured in a Quantum Design SQUID magnetometer, was
found to be somewhat depressed compared to the best YBCO
samples. This may be connected with some uncontrolled
impurities.
3. Results and discussion
The structure of the sample and the field penetration pattern
are shown in figure 1(a) and (b). One can clearly see that the
easiest field penetration occurs in the crystal regions around the
edge of an adjacent crystal (each single crystal photographed
in figure 1(a) is visible also in figure 1(b), all crystal edges are
manifested as long bright strips corresponding to the higher
local field). The same field penetration pattern is shown for
a part of the sample in figure 2 with a higher resolution. The
effect of interface regions on the field penetration is clearly
visible2 . At first sight, these regions look like weak links
where the field can easily enter the sample. However, further
analysis will show that this is not the case. To demonstrate
this, let us first consider local ac susceptibility curves (figure 3)
measured in an ac field of 510 Hz and 5 Oe amplitude. We took
measurements at three different points marked in figure 1(a),
each measurement was taken from a spot 2.7 µm in diameter.
Local TC values are approximately the same at all three points,
the local TC is even a little higher at the interface region
(point 2). Also, we have measured local magnetization curves
at the interface region in the lower crystal (point 2) and in
the upper crystal near its edge (point 4). The two points
are located close to each other and far from the outer edge
of the sample. Therefore, all possible differences should
be connected with the effect of the interface between the
two crystals. The results are shown in figure 4 for several
temperatures. First of all, we noticed that at all temperatures
the residual hysteresis width (in zero field) for the interface
region is not smaller, and normally even larger, than in the
adjacent crystal. This directly demonstrates that the interface
region is not a weak link because there is strong pinning
there. On the other hand, the penetration field (determined by
extrapolating the ascending branch of the magnetization curve
to zero, as shown in figure 4(a)) is always lower for the interface
region. The temperature dependences of the penetration field
for the interface region and for the adjacent crystal are shown
in figure 4(f).
2 Notice that due to the step at the crystal edge the gap between the indicator
film and the lower crystal becomes larger. It becomes even larger at every
next crystal edge according to the staircase profile of our sample. Therefore,
the spatial resolution becomes worse than the ideal value of 1 µm; however,
as one can see from figures 1 and 2, it is still sufficient to see the effect of the
edges on the field penetration.
841
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Figure 4. (a)–(e) Local magnetization curves measured at the interface region of the lower crystal (the full symbols denote point 2 in
figure 1(a)) and close to the edge of the upper crystal (the open symbols denote point 4 in figure 1(a)). (f) Temperature dependences of the
penetration field for the two points.
842
Magnetic field penetration in YBCO single crystals
To understand the reason why the penetration field is
lower in the interface region, let us consider again the
field penetration into the sample as shown schematically
in figure 1(c). The field bends and concentrates near the
edges of each crystal. Therefore, the local applied field is
higher at the interface region, and penetration should start
earlier there in accordance with our experimental observations.
Moreover, in order to penetrate into the upper crystal, flux lines
should cross its edge, where they will experience the surface
barrier. However, the field penetrating into the lower crystal is
almost perpendicular to its surface, resulting in a barrier-free
penetration. This is why early penetration is observed only in
the interface region of the lower crystal and not near the edge
of the upper crystal, where the local applied field has the same
high magnitude. However, in order to make sure that what we
observed in the interface region is really field penetration into
the lower crystal, we should first consider another possible
variant. Since the sample has a stepwise shape, there is a
gap between the indicator film and the lower crystal (see the
footnote above). Therefore, there is a possibility that what
we observe is just the field which bends near the edge of the
upper crystal but does not penetrate into the lower one (from
the scheme of the field stream lines in figure 1(c) one can really
see that the normal field component increases on increasing the
distance between the lower crystal and the point where the field
is measured). However, the local magnetization curves shown
in figure 4 demonstrate that this is not the case. In reality, as we
have already mentioned above, the hysteresis width is larger in
the interface region than near the edge of the adjacent crystal;
this difference in width becomes more pronounced at higher
temperatures. This clearly demonstrates that the higher field
observed in the interface region is pinned there, i.e. it has really
penetrated there (the field concentration due to the bending of
stream lines is not subject to pinning). More evidence that there
is stronger field penetration in the interface region comes from
observations at higher temperatures. Images taken at higher
temperatures (not shown because of their low contrast) reveal
a wider area of field penetration in the interface region. This
widening of the penetration area at higher temperatures can
only be explained by weaker pinning. If the observed effect
was coming just from the bending of the field stream lines, we
would not observe such a widening.
Thus, due to the specific shape of the multiple-crystal
sample, we could achieve a situation where the surface barrier
is reduced and the penetration field is closer to the first critical
field. Notice, however, that both curves in figure 4(f) have a
characteristic shape with an upturn at lower temperatures. As
mentioned above, this upturn is characteristic of the surface
barrier. If the curve for the interface region is multiplied by
an appropriate scaling factor it would perfectly coincide with
the curve for the upper crystal. This shows that penetration in
the interface region of the lower crystal is also affected by the
surface barrier. This is not surprising because the local field
there is not exactly perpendicular to the surface, and the parallel
component should result in the formation of the surface barrier.
A better way to create a barrier-free configuration would be,
as mentioned above, to place a miniature coil on top of a
superconducting sample. However, this is very difficult due
to the small dimensions of high-temperature superconductor
single crystals. Nevertheless, it should be possible if a crystal
with the lateral dimensions of a few millimetres is taken. This
will be the subject of our future work.
As was shown in our earlier work [2], at higher
temperatures the surface barrier is surmounted due to thermal
fluctuations. However, from the present results it is evident
that it persists to some extent even at high temperatures: the
difference between the two curves in figure 4(f) is observed
in the whole temperature range. In all probability, this can
be explained by the fact that the barrier height (and therefore
the rate of activation over the barrier) decreases (although
logarithmically slowly) with the applied field [3]. When
the applied field is equal to HC1 , the barrier height is large
and the thermal activation over it is slow even at elevated
temperatures. This is why even at higher temperatures a field
larger than HC1 should be applied to surmount the barrier,
i.e. the effect of the barrier can persist, to some extent, in
the whole temperature range. Another, maybe more probable,
explanation is connected with the geometrical barrier. The
field configuration achieved in the present experiment results
not only in a reduced surface barrier but also in a reduced
geometrical barrier. However, because of the macroscopic
width of the latter, there is no thermal activation over it.
This is why the effect of a reduction of the geometrical
barrier at the interface region should be observed at all
temperatures.
4. Conclusion
In conclusion, we have created a field configuration which
results in a reduction of surface and geometrical barriers for
field penetration into type-II superconductors. A comparison
of the field penetration under conditions of a fully developed
and a reduced surface barrier shows that the effect of the
surface barrier persists in the whole temperature range below
TC . This refines the conclusions of our earlier work where the
surface barrier was supposed to vanish completely at elevated
temperatures.
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