Pinning mechanisms in bulk high-Tc
superconductors
M. R. Koblischka
z and M. Murakami
Superconductivity Research Laboratory, International Superconductivity Technology
Center 16-25 Shibaura 1-chome, Minato-ku, Tokyo 105-0023, Japan
z
present address: Nordic Superconductor Technologies A/S, Priorparken 685, DK-2605 Brndby,
Denmark
Pinning mechanisms in bulk high-Tc superconductors
2
Volume pinning forces are determined for a variety of bulk high-Tc
superconductors of the 123-type from magnetization measurements. By means of
scaling of the pinning forces, the acting pinning mechanisms in various temperature
ranges can be identied. The Nd-based superconductors and some YBCO crystals
exhibit a dominating pinning of the ÆTc -type (i.e. small, superconducting pinning
sites).
This is ascribed to the presence of an Nd-rich phase with weaker
superconducting properties, leading to a spatial scatter of Tc , which can also be
provided by oxygen vacancy clusters. In contrast to this, the addition of insulating
211 particles provides pinning of the Æl-type. Measurements of the eld-cooled
magnetization show that the Nd-based superconductors exhibit two step transitions
if cooled/warmed in elds above 4 T. This secondary transition can be correlated
to the peak eect. This suggests that the peak eect is an unique property
of the superconducting matrix (i.e. oxygen vacancy clusters), whereas the 211
particles provide eective pinning in the entire temperature range acting quasi as a
"background" pinning mechanism for the peak eect. Based on these observations we
construct a pinning force diagram for bulk high-Tc superconductors.
Abstract.
PACS numbers: 74.60 Ge, 74.60 Jg
Pinning mechanisms in bulk high-Tc superconductors
3
1. Introduction
Flux pinning is one of the crucial problems in the development of technical high-Tc
superconductors, especially because of the high operation temperature (77 K) required
for many practical applications. In this respect, the development of the light rare earth
(LRE=Nd, Sm, Eu, Gd) superconductors of the 123-type provided samples with an
increased critical current density, jc , compared to YBa2 Cu3 O7 Æ (YBCO) is particularly
signicant. Characteristic of these samples is the strongly developed secondary peak or
shtail eect (FE) [1], thus yielding a large jc at elds of about 2.5 T [2, 3]. Therefore, the
high-eld performance of such superconductors is determined by the strength of the peak
eect; its origin clearly plays an important role in the design of even better materials.
Recently, we have prepared samples of the type (Nd0:33 Eu0:33 Gd0:33 )Ba2 Cu3 Oy ("NEG"),
which leads to an even further increase of jc . Furthermore, we could successfully embed
211 particles of submicron size into the superconducting matrix [4]. The presence of
these 211 particles inuences the jc (Ha ) behaviour drastically; but the position of the
secondary peak remains unchanged, as long as the concentration of the 211 particles
stays within a certain limit as discussed in Ref. [5]. This provides evidence that the
magnetization properties are inuenced by two dierent pinning mechanisms, acting
together. Following Blatter et al. [6], there are two fundamentally dierent pinning
mechanisms describing the interaction of a vortex core with a pinning site; the Ælpinning, which is due to a scatter of the electron mean free path, and the ÆTc -pinning,
which is associated with a spatial scatter of the superconducting transition temperature,
Tc , throughout the sample. A third mechanism, which played some role in conventional
superconductors, is the magnetic interaction between a ux line and a defect, can be
considered inactive in high-Tc materials with their large values of . In this case, the core
interaction Fp;core dominates the magnetic interaction, Fp;mag , by a factor of =4 ln ,
where denotes the Ginzburg-Landau parameter [7].
The scaling of the volume pinning force, Fp , is an important tool to analyze the
data for a priori unknown pinning mechanisms [8, 9, 10]. The scaling also works well
in most high-Tc samples (see e.g. Ref. [10]), however, the appropriate scaling eld is
the irreversibility eld, Hirr (where Fp = 0 by denition) instead of the upper critical
eld, Hc2 . For YBCO, most authors found h0 0.33 [10, 11], which is in accordance
with pinning provided by normal-conducting or insulating regions, i.e. Æl-pinning. For
some YBCO materials, however, higher peak positions are found as reported recently
[12]. In the case of the LRE-123 superconductors an excellent scaling is found to hold;
this leads to peak positions h0 > 0.4 for NdBa2 Cu3 O7 Æ (NdBCO) [2]. For pure NEG
we obtained h0 0.5 [3] and 0.54 for a Gd-rich NEG [13].
In this paper, we present experimental evidence on a variety of bulk samples of the
123-type that the peak eect is due to the ÆTc -pinning mechanism, using the results
of the pinning force scaling and of measurements of temperature scans of m(T ) in
eld-cooled cooling (FCC) and -warming (FCW) modes, which yield direct evidence
for the existence of a weaker superconducting phase within the LRE-123 samples.
Pinning mechanisms in bulk high-Tc superconductors
4
Based on these observations, we construct a pinning force diagram for bulk high-Tc
superconductors of the 123-type.
2. Experimental procedure
As a variety of samples are studied here, detailed descriptions of the preparation
techniques can be found elsewhere (NdBCO single crystals [14, 15], YBCO single
crystals, see e.g. Ref. [16], OCMG (oxygen-controlled melt-growth) NdBCO [17, 18]
and OCMG NEG-123 [4]).
Magnetization loops (MHLs) are measured using commercial SQUID magnetometers [19] with a maximum eld of 7 T; Ha k c axis. To minimize eld inhomogeneities,
the scan length is set to 1 cm. Temperature scans of m(T ) in both FCC and FCW
modes were carried out in various elds between 10 mT and 7 T. The measurements
were performed using the model XL magnetometer, enabling measurements in a continuous temperature sweep mode with a controlled temperature sweep rate dT =dt = 35
mK/min in the transition region; the datapoints are recorded in steps of 50 mK. Note
that the temperature sweep is not interrupted for data recording as in a conventional
SQUID magnetometer. No averaging of the signal is performed, and the scan length
is only minimal. This procedure ensures a large number of datapoints even in a sharp
superconducting transition. All curves are measured between 1.7 K and 120 K. More
details of the measurement procedure are given by Koblischka et al. [20].
3. Experimental results
Recently, it was shown by Koblischka et al. [2, 3] that the scaling of the volume
pinning forces works very well for Nd-123 samples, and enables the determination of
the underlying microscopic pinning mechanisms [8, 9]. A recent literature survey [10]
showed that a scaling of Fp , normalized by its maximum value, Fp;max , versus Hirr holds
in many high-Tc systems. Peak positions, h0 , larger than 0.33 cannot be explained
by pinning at normal-conducting or insulating particles; this is an indication of the
ÆTc -pinning activity [2, 8].
Figures 1 (a{f) shows the scaling of Fp versus Hirr for ve dierent Nd-based
superconductors and one YBCO single crystal. In (a), data of an NdBCO single crystal
("crystal A") with a Tc of 93.8 K are presented; in (c), data of the OCMG NdBCO sample
(Tc 94.7 K). Both samples exhibit a very good scaling, with peak positions h0 0.4.
For pure OCMG-processed NEG (b), the scaling works perfectly for all temperatures
between 60 K and 90 K, and the peak position is obtained at h0 = 0.51, which is even
higher than in the NdBCO samples (a,b). The small amount of large 211 particles
formed in this compound has no apparent inuence on the scaling in this temperature
and eld range. The NEG + 40 mol% NEG-211 sample (d) also shows a good scaling at
temperatures between 60 and 77 K. These curves nearly all fall on a common line, also
with the peak position at h0 = 0.5. The data taken at higher temperatures, however,
Pinning mechanisms in bulk high-Tc superconductors
5
do not scale in the same manner (dashed line). This is a direct consequence of the
shape change of the jc (Ha ) curves. Such a change of shape reects the change in the
basic active pinning mechanism. The secondary peak eect almost disappears at 77 K.
As a consequence, the data at the elevated temperatures tend to move towards lower
h0 which implies that dominant pinning centers are normal-conducting or insulating
particles. In (e), a second NdBCO single crystal ("crystal B") with a very high-Tc of
95.7 K is presented. The high Tc of this sample indicates that the Nd/Ba solid solution
is practically suppressed. Note that the peak of the Fp diagram is at h0 0.36, which is
very close to the 0.33 found in YBCO. A reason for this behaviour is given by the large
values of Hirr; consequently, only data between 80 and 93 K can be used for the scaling.
For comparison, (f) presents the scaling of a YBCO single crystal, which exhibits also
a pronounced FE and even the intermediate peak as seen in several NdBCO crystals
[21]. Consequently, the scaling of this sample is similar to that of crystal A. Note also
that the scaling is less convincing than that for the NdBCO samples. To summarize the
results of the Fp scaling of NdBCO, we clearly observe very high peak positions above
0.4, which cannot be explained assuming a dominant Æl-pinning.
Another important feature is the dependence of the superconducting properties and
the peak eect on the oxygenation state as discussed by several authors [16, 22]. Fig.
2 presents the oxygen reduction eect on an NdBCO single crystal. The crystal was
fully oxygenated, exhibiting a very high transition temperature, Tc of 95.7 K (crystal
B). Magnetization loops were measured at 77 K and 60 K (left column). The sample
shows no shtail eect at 77 K, and only a slight eect at 60 K. After oxygen reduction,
resulting in a decrease of Tc to 94.1 K, the magnetization loops shown in the right
column were measured. The oxygen reduction causes a decrease of M , but the shtail
shape clearly develops upon oxygen reduction. This demonstrates that the peak eect
can indeed be created by oxygenation/deoxygenation procedures, although this FE will
be always a weak one [22] as compared to samples containing LRE/Ba solid solution.
The shtail peak appears in the LRE-123 superconductors at higher elds compared
with most YBCO samples, thus suggesting that the underlying mechanism of peak
formation may be dierent. As shown by Erb et al. [16], based on experiments
controlling the oxygen content in single crystals of YBCO prepared in BaZrO3 crucibles,
the formation of oxygen vacancy clusters in conjunction with metal impurities stemming
from the crucible material appears to be responsible responsible for the FE. In the
NdBCO system (and the other LRE-123), the solid solution between the LRE and Ba
provides another source of pinning which is not present in YBCO. The ngerprint of
this additional pinning is the shift of the peak position towards higher elds, and also
the width of the peak is considerably increased [23].
A very important piece of information comes from the m(T ) behaviour of bulk
superconductors, measured in FCC and FCW conditions as already shown in Ref. [20].
In Figs. 3 (a) { (d), temperature scans of the magnetic moment m(T ) are plotted for
(a) a NdBCO single crystal (sample "C"), which exhibits a relatively low Tc of 87 K
[14], a OCMG melt-textured NdBCO sample, (c) crystal "B" at various applied elds
Pinning mechanisms in bulk high-Tc superconductors
6
between 0.1 T and 7 T and (d) the YBCO single crystal. All data shown were recorded
during FCC runs; the FCW data are omitted for clarity as only very small dierences
were observed. Crystal "C" and the melt-processed sample are found to exhibit a clear
step in the superconducting transitions in elds above 4 T, similar to the NEG samples
as presented in Ref. [20]. Therefore, we ascribe this step to the presence of an LRErich phase with weaker superconducting properties, providing a spatial scatter of Tc
throughout the sample. Also the YBCO crystal shows a kink in the superconducting
transition in elds above 4 T (see also Ref. [24]), which is due to the presence of oxygen
vacancy clusters. As presented in Ref. [25], the same m(T ) scans on polycrystalline
YBCO do not show such kinks or steps. In contrast to this, crystal "B" does not
show such a step within the available eld range. This is a clear indication that in
this sample, the solid solution between Nd and Ba is suppressed; therefore, we do not
expect to observe such a second step in the superconducting transitions. Furthermore,
crystal "C", the OCMG-NdBCO sample and all the NEG samples exhibit the most
pronounced peak (shtail) eect. In crystal "C", the shtail eect is most pronounced
at temperatures between 50 and 60 K; whereas the OCMG-NdBCO and NEG samples
(see Ref. [20] show the most pronounced peak eect at about 77 K. Correspondingly, the
second step in the FCC curves of crystal "C" is found at 62 K, whereas the OCMGNdBCO sample shows an onset of the second step at around 84 K. The magnetization
loops of crystal "B" are monotonously decreasing on increasing eld in the temperature
range between 50 K and Tc . A slight shtail eect can, however, be observed at 40 K.
Figure 4 presents a summary of the FCC/FCW measurements performed on a
variety of samples. The second step can be observed mainly at temperatures above 70
K, and in elds above 4 T. Plotted in the graph are the onset temperatures of the second
transition as a function of eld. As shown in Ref. [26], this onset corresponds to a large
"bump" in the resistance curves, and is always located well below the irreversibility lines.
The NdBCO ("A") and YBCO single crystals show practically the same behaviour; they
also exhibit the peak in the Fp -scaling at 0.4. The data for the NEG samples show
the highest onset temperatures of all samples studied here, which consequently leads to
the high peak positions in the Fp -diagrams. Only in two crystals studied (e.g. crystal
B) was any secondary step or kink apparent. Note that that the FCC data and the
MHL data cannot be compared directly with each other due to the dierences in ux
distributions in the two experiments. However, it is clearly visible that the lower the
secondary step occurs in the FCC data, the lower the most pronounced FE will occur
in the MHLs. The step or kink correlates, therefore, directly with the FE. This also
conrms that the enhanced pinning in the LRE-123 samples is indeed due to a spatial
distribution of Tc , not due to BaCuO2 layers as discussed by Wu et al. [27].
In this way, our measurements reveal a clear relation between the peak eect and
the presence of a LRE-rich phase. This again provides evidence that the peak eect
may be due to the spatial scatter of Tc , i.e. due to the activity of the ÆTc -pinning.
Pinning mechanisms in bulk high-Tc superconductors
7
4. Discussion
All the observations on our NEG-123 samples give evidence how the dierent pinning
mechanisms act together in one sample.
In their authoritative review paper, Blatter et al. [6] state that oxygen vacancies
could either act as a Æl- or ÆTc pinning sites. Further, they mentioned that no
experimental evidence could be found identifying which of the two pinning mechanism
is the dominating one. Following this work, Griessen et al. [28] provided evidence for a
dominant Æl-pinning in YBa2 Cu3 O7 Æ thin lms, and lms with various oxygen contents.
The same conclusion was reached by van Dalen et al. [29] for very thin, twin-free DyBCO
single crystals. More recently, some evidence was found for a dominating ÆTc -pinning
in (Ba,K)BiO3 [31], Pr-doped YBCO single crystals [30] and Zn-doped YBCO melttextured bulks [32]. However, it remained an open question which pinning mechanism
plays the most important role.
It is now generally accepted that the peak eect in jc (Ha ) is due to oxygen vacancy
clusters in conjunction with metal impurities as demonstrated by Erb et al. [16] using
ultra-pure YBCO single crystals. It is important to note that such oxygen vacancy
clusters are, strictly speaking, just ÆTc -pinning sites providing locally a reduction of Tc .
Tc eectively drops only in the case of oxygen vacancy clusters, thus leading to a spatial
scatter of Tc . Point defects cannot provide a scatter of Tc due to the proximity eect.
The presence of the LRE/Ba solid solution in the LRE superconductors may either lead
to an increase of the disorder in the oxygen sublattice or provide directly regions with
weaker superconducting properties, and hence to an increase of the ÆTc -pinning. This
is indicated in the jc (Ha ) curves by larger values of Hpeak , and in the pinning force
scaling by the increased h0 [5]. Due to the proximity eect, the presence of the LRErich phase with a smaller Tc is masked when measuring the samples in a typically small
eld ( 1 mT). Therefore, this secondary phase cannot be observed in "standard" Tc
determinations.
Note that the contribution of the ÆTc -pinning is only weak as compared to the
pinning provided by the insulating inclusions. Therefore, in thin lms with their much
higher jc , for example the pinning is only provided by the Æl-pinning type as found by
Griessen et al [28]. Further, the ÆTc -pinning is only eective at elevated temperatures.
The importance of the ÆTc -pinning mechanism in bulk high-Tc superconductors is further
illustrated by the possibility of constructing the jc (Ha ) curves of a sample exhibiting a
secondary peak eect from two dierent contributions, as demonstrated by Jirsa et al.
[33] One contribution is responsible for the central peak [jc (0T)], and decays quickly
with increasing eld. The other contribution is negligible at low elds, but increases
with increasing eld and is responsible for the formation of the secondary peak.
It is important to point out that these conclusions are reached by investigating
a variety of samples in order to ensure a general validity. In conclusion, we may
state that the secondary peak eect (and hence, the ÆTc-pinning) is a property of the
superconducting matrix. The presence of the 211 particles does not aect the shtail
Pinning mechanisms in bulk high-Tc superconductors
8
shape, as long as their concentration is below a certain limit. This demonstrates the
eectivity of the submicron-sized pinning sites achieved here. These particles provide a
very eective pinning, forming quasi the "background" for the peak eect [34]. In low
elds, pinning is only due to these insulating 211 particles. This leads to the pinning
diagram presented in Fig. 5. The arrows show the degree to which the ux pinning sites
may be engineered in the 123 materials. The peak position, Hpeak , can be moved due
to composition variations in the matrix (LRE/Ba solid solution, Gd-rich NEG). The
peak height can be varied by oxygenation procedures (which is possibly reversible as
suggested by Erb et al.). Finally, the border line between the two pinning mechanisms
can be inuenced by e.g. irradiation (see e.g. Ref. [35]) or by the addition of small 211
particles. Furthermore, thin YBCO lms with their natural strong pinning sites [36, 37]
have a very strong Æl-pinning, so that a possible contribution of the weaker ÆTc -pinning
mechanism is negligible, even in oxygen-decient thin lms. It should be noted here
that YBCO thin lms also do exhibit the peak eect.
5. Conclusions
In summary, we can state that the newly developed ternary compounds of the type
NEG-123 allow the engineering of isotropic pinning sites by controlling the processing
conditions, so such ternary 123 systems are ideal systems for bulk applications. From the
theoretical point of view, an important issue will be the development of a new theory for
ux pinning, which has to include the low-Tc systems as a special case with Hirr ! Hc2
and to give a complete description of the pinning force scaling and I/V characteristics.
First such attempts can already be found in literature [38].
Acknowledgments
We would like to thank M. Muralidhar, T. Mochida, T. Higuchi, K. Waki, Th. Wolf and
B. Veal for the several samples used in this study, and M. Jirsa, A. Erb, M. Daumling,
H.W. Weber, and G. Crabtree for stimulating discussions. We also acknowledge the
assistance of A. Veneva, K. Ogasawara, and T. Matano during some of the experiments.
This work was partially supported by NEDO. MRK gratefully acknowledges the support
from the Japanese Science and Technology Agency (STA).
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Pinning mechanisms in bulk high-Tc superconductors
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Scaling of the volume pinning forces, Fp versus h = Ha =Hirr of an NdBCO
single crystal ("crystal A") (a), an OCMG processed NdBCO sample (b), pure NEG
(c), the NEG sample with 40 mol% NEG-211 (d), a YBCO single crystal (e) and an
NdBCO single crystal (crystal "B") (f).
Figure 1.
Oxygen reduction eect on an NdBCO single crystal. The crystal was
fully oxygenated, showing a Tc of 95.7 K. Magnetization loops were measured at 77 K
and 60 K (left column). The sample shows no shtail eect at 77 K, and only a very
slight one at 60 K. After oxygen reduction resulting in a decrease of Tc to 94.1 K, the
magnetization loops shown in the right column were measured. The oxygen reduction
causes a decrease of M , but the shtail shape clearly develops upon oxygen reduction.
Figure 2.
FCC/FCW transitions of various bulk high-Tc samples in elds between
0.5 T and 7 T; the FCW transitions are omitted for clarity. (a): NdBCO single crystal
(sample "C"), (b): OCMG NdBCO melt-textured sample. In (a) and (b), very clear
secondary transitions can be seen above 4 T. These samples also exhibit the most
pronounced shtail eect. (c): NdBCO single crystal (sample "B"), which does not
exhibit the shtail eect. (d): YBCO single crystal with a very pronounced shtail
eect. Note that in this case only a slight kink in the m(T ) curves above 4 T can be
observed. The MHLs below each FCC plot are measured at 77 K, Ha k c.
Figure 3.
Extracted onset temperatures of the secondary transitions as function of the
applied eld. All transition temperatures are well below the respective irreversibility
lines. Note that crystal "C" shows very low onset temperatures, which is reected also
in the peak eect, which is most pronounced at 60 K. The NEG samples show the
highest onset temperatures, which reects their high peak positions in the Fp -scaling
and their strongly developed FE at 77 K and above. The YBCO crystal is very similar
to the NdBCO crystals of type "A".
Figure 4.
Pinning diagram diagram, deduced from the NEG data with various NEG211 additions. The borderline between the two pinning mechanisms can be inuenced
by, e.g., neutron irradiation or addition of 211 particles. Hpeak can be inuenced by e.g
changing the matrix composition. The peak height can be inuenced by oxygenation,
or as in the case of NEG, by increased disorder within the matrix.
Figure 5.
1.0
F p/F p,max
0.8
(a)
(b)
(c)
(d)
(e)
(f)
0.6
0.4
0.2
0.0
1.0
F p/F p,max
0.8
0.6
0.4
0.2
0.0
1.0
F p/F p,max
0.8
0.6
0.4
0.2
0.0
0.0
0.2
0.4
0.6
h
fig.1
0.8
1.00.0
0.2
0.4
0.6
h
0.8
1.0
m [10 -5 Am 2]
2
77 K
77 K
0.5
1
0
0.0
-1
-2
m [10 -5 Am 2]
3
-0.5
-3
8
60 K
2.0
60 K
m [10 -5 Am 2]
4
1.0
2
0.0
0
-2
-1.0
-4
-2.0
-6
-4
-2
0
2
µ0Ha [T]
fig.2
4
6
8 -4
-2
0
2
µ0Ha [T]
4
6
8
m [10 -5 Am 2]
6
(a)
6
m [10 -4 Am 2]
5
4
3
2
1
0
7T
6T
5T
4T
3T
2T
1T
0.5 T
20
40
60
T [K]
80
100
6
m [10 -3 Am 2]
4
2
0
-2
-4
-6
-1
0
µ0Ha [T]
1
120
(b)
20
7T
6T
m [10 -5 Am 2]
15
5T
4T
10
3T
2T
5
0
1T
0.5 T
0
20
40
60
80
100
120
T [K]
m [10 -5 Am 2]
10
5
0
-5
-10
-4
-2
0
2
µ0Ha [T]
4
6
8
(c)
7T
m [10 -5 Am 2]
1.5
6T
5T
1.0
4T
3T
0.5
2T
1T
0.0
0.5 T
20
40
60
80 100 120
T [K]
m [10 -5 Am 2]
2
0
-2
-4
-2
0
2
µ0Ha [T]
4
6
8
(d)
7T
6T
5T
4T
3T
2T
1T
0.5 T
1
m [10 -5 Am 2]
0
-1
-2
-3
-4
0
20
40
60
T [K]
80 100 120
m [10 -4 Am 2]
10
5
0
-5
-10
-4
-2
0
2
µ0Ha [T]
4
6
8
8
µ 0H a [T]
6
4
NdBCO OCMG
NEG
NdBCO crystals "C"
,
YBCO sc, NdBCO crystals "A"
2
60
fig.4
70
T [K]
80
90
jc
δTc-pinning
δl-pinning
Hpeak
H