Physica C 296 Ž1998. 43–48
Comparative study of magnetic properties between melt-textured
and single crystalline Y–Ba–Cu–O
M. Watahiki ) , W.J. Jang, N. Sakai, M. Murakami
SuperconductiÕity Research Laboratory, 1-16-25 Shibaura, Minato-ku, Tokyo 105, Japan
Received 12 April 1997; revised 25 July 1997; accepted 5 October 1997
Abstract
We studied magnetic properties of a melt-textured Y–Ba–Cu–O with Y2 BaCuO5 inclusions and a single crystalline
YBa 2 Cu 3 O y grown by a crystal pulling technique by changing oxygen annealing conditions with the emphasis placed on the
effect of doping level on flux pinning. Magnetization hysteresis loop measurements at 20 K and 77 K showed that the
critical currents of the melt-textured sample were three times as large as those of a single crystal, which confirms the
contribution of Y2 BaCuO5 to pinning enhancement. In order to enhance flux pinning, it is important to introduce effective
pinning centers in the good matrix with sufficiently large oxygen content. q 1998 Elsevier Science B.V.
Keywords: YBa 2 Cu 3 O y ; Crystal pulling; Melt process; Y2 BaCuO5 ; Critical current
1. Introduction
Since the discovery of high-temperature superconductors, extensive research has been conducted for
achieving a large critical current density Ž Jc . in large
sample dimensions, which is one of the most important properties for practical applications. It is well
known that melt-processed YBa 2 Cu 3 O y ŽY123. superconductors, including fine Y2 BaCuO5 ŽY211. particles, exhibit large Jc values w1,2x even in a sample
with diameter exceeding 5 cm w3x. It is now generally
accepted that Y211 particles play an important role
in achieving large Jc values, although it is still
controversial whether the flux pinning center is Y211
itself or the defect around the particle.
Flux pinning of high-temperature superconductors
)
Corresponding author. E-mail: watahiki@istec.or.jp
is also dominated by the electronic anisotropy between the ab-plane and the c-axis. First, an idea of
pancake vortex was introduced to explain weak pinning of highly anisotropic superconductors w4x. Recently, Matsushita et al. w5x proposed that the interaction distance along the flux line Ž l 44 . becomes very
small for the sample with large anisotropy so that the
interaction volume for pinning becomes small, leading to weak pinning. The anisotropy is strongly
dependent on the doping level, which determines the
strength of the interlayer coupling. It has been reported that overdoped samples exhibit strong pinning
due to the fact that the interlayer coupling is greatly
improved even when the superconducting transition
temperature ŽTc . is lowered w6x. It is known that the
superconducting properties of the Y123 single crystal are strongly dependent on the oxygen composition w7,8x. Therefore, in order to clarify the role of
Y211 inclusions for flux pinning, it is very important
0921-4534r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.
PII S 0 9 2 1 - 4 5 3 4 Ž 9 7 . 0 1 7 7 1 - 1
M. Watahiki et al.r Physica C 296 (1998) 43–48
44
Table 1
Oxygen annealing conditions
Process a
2
3
6
1
4
5
7
Temperature schedule
6208C Ž2 h., 5008C Ž24 h., 4508C Ž48 h., 4008C Ž72 h.
6208C Ž2 h., 5008C Ž4 h., 4508C Ž8 h., 4008C Ž12 h.
6208C Ž2 h., 4208C Ž330 h.
6208C Ž2 h., 4908C Ž300 h.
6208C Ž2 h., 5408C Ž150 h.
6208C Ž2 h., 6008C Ž20 h., 5508C Ž10 h.
6208C Ž2 h., 6008C Ž100 h.
MPMG
SRL–CP
Tc ŽK.
DTc ŽK.
Tc ŽK.
DTc ŽK.
91
91
91.5
89
82.5
82.5
67
1
2
0.5
0.8
2.5
7.5
4
89.5
89.5
89
92
90.5
90.5
79
2
3.2
1.3
0.7
1
1.5
6
Both samples were annealed repeatedly in the order of numbers listed here.
The superconducting transition temperature ŽTc . and the width of transition Ž DTc . determined in the magnetic field of 10 Oe are also
summarized.
to take account of the doping level of the Y123
matrix.
In this paper, we measured superconducting properties of Y–Ba–Cu–O sample grown by the meltpowder-melt-growth ŽMPMG. process w1x and Y123
single crystals grown by a crystal pulling technique
w9x. Recently, it has been reported that the doping
level can easily be controlled in the Y123 single
crystal grown by a crystal pulling technique simply
by changing the oxygen annealing temperature w7x. In
order to find out an optimum oxygen content by
control of annealing condition, the sample grown by
each method was repeatedly subjected to the various
annealing conditions.
By comparing the superconducting properties of a
sample grown by the MPMG process with those of a
sample grown by a crystal pulling technique, it is
possible to separate out the contributions to the
pinning strength due to changes in the doping level
by the annealing condition and Y211 particles which
play an important role in achieving large Jc values,
since the Y123 single crystal grown by a crystal
pulling technique has a small amount of impurities.
We will discuss the role of the Y211 particles with
the emphasis placed on the difference of doping
level for the Y123 matrix.
main bulk sample prepared by the MPMG method,
the details of which are described in Ref. w1x. The
sample contains about 30% Y2 BaCuO5 about 1 mm
in diameter. Y123 single crystal with dimensions of
1.91 = 2.48 = 1.13 mm3 with its wide surface perpendicular to the c-axis was cut from a crystal grown
by a crystal pulling technique w9x. The cation composition of the Y123 single crystal was determined by
ICP analysis to be Y:Ba:Cus 1:2.06:3.07 w10x.
Both samples were heated to 6208C in 2 h, and
annealed under various conditions tabulated in Table
1. Oxygen annealing was performed in the order of
numbers listed in the table. The samples were subjected to magnetic measurements, which enabled us
to minimize the chemical impurity effect as well as
the shape dependence or the demagnetizing effect,
though it was time consuming.
Magnetization measurements were performed with
a commercial SQUID magnetometer ŽQuantum design. for fields parallel to the c-axis. The temperature dependence of magnetization was measured between 5 and 100 K in the magnetic field of 10 Oe
both in the field-cooled and zero-field-cooled processes. The magnetization curves Ž M–H loops. at 20
K and 77 K were measured with fields applied
parallel to the c-axis and a maximum field was 7 T.
2. Experimental
3. Results and discussion
The Y–Ba–Cu–O sample with dimensions of
1.84 = 2.42 = 1.14 mm3 with its wide surface perpendicular to the c-axis was cut from a single do-
3.1. Effect of annealing condition on Tc
The temperature dependence of magnetization for
an MPMG Y–Ba–Cu–O and a Y123 single crystal
M. Watahiki et al.r Physica C 296 (1998) 43–48
annealed under various conditions was measured respectively. The superconducting transition temperature ŽTc . and the width of transition Ž DTc . determined in the magnetic field of 10 Oe are summarized in Table 1.
Here, we notice that the superconducting transition behavior is mainly determined by the final
annealing temperature for both MPMG and single
crystals, however, the annealing condition which
yields good superconducting transition is different.
For MPMG Y–Ba–Cu–O, the highest Tc with sharp
transition is achieved with the annealing temperature
of 4208C, while that is achieved with the annealing
at 4908C for the single crystal. We can also say that
for MPMG Y–Ba–Cu–O, Tc monotonously decreases with increasing the annealing temperature,
while, for Y123 single crystal, Tc first increases and
then decreases, implying that the optimum doping
level is achieved by annealing at 4908C.
Furthermore, it is notable that although Tc of
MPMG Y–Ba–Cu–O is depressed significantly with
annealing at temperatures above 5408C, single crystal exhibits high-Tc above 90 K under the same
annealing condition, suggesting that oxygen distribution is different.
In a study on high quality Au-free Y123 single
crystal, Claus et al. w11x demonstrated that overdoping, as manifested by a maximum in Tc versus the
oxygen concentration exists in Au-doped crystals but
45
to a smaller extent than in Au-free crystals. They
proposed that this difference should be due to a
decreased oxygen solubility in the Au-doped crystals. Kobayashi et al. w12x performed the magnetic
measurements on Y123 single crystals grown by the
flux method. The Tc of Y123 single crystals with
different oxygen content monotonously increases
with decreasing annealing temperature.
Terasaki et al. w13x measured the transport properties of Y123 crystals grown by a CuO flux method
and by the crystal pulling technique and found that
after annealing in oxygen at 4008C for 4–10 days,
the crystal grown by the crystal pulling technique
showed slightly overdoped behavior. In contrast, the
flux grown crystal could not be overdoped. Although
the origin of such peculiar characteristic of the crystal is not clear, the difference in the superconducting
transition behavior between the MPMG Y–Ba–Cu–
O and the Y123 single crystal grown by the crystal
pulling technique may be due to oxygen solubility.
Here, it is also important to note that like most other
Y123 single crystals, MPMG Y–Ba–Cu–O could
not be overdoped.
3.2. Effect of annealing condition on M–H loops
Fig. 1a,b shows respective M–H loops of MPMG
Y–Ba–Cu–O and Y123 single crystal at 77 K. It is
interesting to notice that the irreversibility fields
Ž Hirr ., at which the magnetization curve becomes
Fig. 1. M–H loops Ž H 5 c-axis. at 77 K for Ža. an MPMG Y–Ba–Cu–O and Žb. a Y123 single crystal grown by a crystal pulling technique
annealed under the conditions listed in Table 1.
46
M. Watahiki et al.r Physica C 296 (1998) 43–48
reversible, for both samples monotonously increase
with decreasing the annealing temperature. We interpret that such change is due to the difference in the
doped level. For both the highly oxygenated samples, which are annealed the lower temperature, the
Hirr is located at the higher fields due to enhanced
interlayer coupling.
M–H loops of MPMG sample preserve the same
shape over all the annealing conditions, though the
annealing above 5408C drastically suppressed the
hysteresis width Ž D M .. It is also clear that high-Tc
values are critical in achieving large flux pinning. A
comparison of annealing conditions at 4008C Ža2.
and 4908C Ža1. for the MPMG sample shows that a
small change in Tc from 91 to 89 K depressed D M
dramatically. It is also important to note that although a small hump can be observed in M–H loop,
anomalous secondary peak effect, which is commonly observed in Y123 single crystals, is absent in
the MPMG sample.
In contrast, a drastic change in the shape of the
M–H loop is observed for Y123 single crystal depending on the annealing temperature, which can be
classified into two groups: one for annealing at
relatively low temperatures Ž- 4208C. characterized
by high Hirr and small hysteresis without secondary
peak effect and the other for annealing at high
temperatures Ž) 4908C. characterized by large hysteresis in a low field region and relatively low Hirr
with secondary peak effect. For the overdoped sam-
ples, which are annealed below 4208C, the Hirr is
located at the higher fields. However, since the
oxygen defect density must be small in the overdoped condition, the number of pinning centers is
small so that pinning in a low field region is lower.
It may then be concluded that in order to achieve
high irreversibility and large flux pinning, the increase of the defects which can act as pinning centers is important in addition to the control of the
doping level toward overdoped direction.
3.3. Temperature dependence of M–H loop
Fig. 2a,b shows the magnetization curves of the
MPMG Y–Ba–Cu–O and the Y123 single crystal at
20 K, respectively. D M of the M–H loops at 20 K
were almost twenty times as large as those at 77 K
for both samples. It should also be noted that for
MPMG samples annealed at 400 and 4208C, flux
pinning is so large that the external field of 7 T is
not sufficient for full penetration and only minor
loops were obtained. For single crystals, although a
drastic change in the shape of M–H loops was
observed at 77 K, at 20 K no such dramatic change
is observable, suggesting that the doping level is not
so important for pinning at lower temperatures. This
may be related to the fact that the interlayer coupling
is enhanced with decreasing temperature, and therefore at sufficiently low temperatures like 20 K, the
strength of coupling is not strongly affected by the
Fig. 2. M–H loops Ž H 5 c-axis. at 20 K for Ža. an MPMG Y–Ba–Cu–O and Žb. a Y123 single crystal grown by a crystal pulling technique
annealed under the conditions listed in Table 1.
M. Watahiki et al.r Physica C 296 (1998) 43–48
doping level. It is then notable that D M of MPMG
samples are three times larger than that of single
crystal for all the annealing conditions, indicating
that the dispersion of Y2 BaCuO5 inclusions contributes to the enhancement of flux pinning.
3.4. Jc –B property at 77 K
Fig. 3 shows Jc –B properties Ž H 5 c-axis. for
MPMG Y–Ba–Cu–O and Y123 single crystal at 77
K. Here Jc was estimated based on the extended
Bean model from the following equation: Jc s
D Mra 1 y ar3b4 where a and b are the lengths of
two sides of the rectangular surface perpendicular to
the applied field Ž a ) b ..
It is evident that MPMG sample annealed at
4208C exhibits the best Jc –B property, although the
Jc –B curve merges with that of a single crystal
annealed at 4208C in a high field region. There is no
cross-over of Jc for MPMG, whereas low field Jc
values for annealing temperature of 4908C are superior in a single crystal. As discussed before, flux
pinning depends both on the number of defects and
the interlayer coupling. Here, we believe that the
main pinning centers in Y123 single crystal are
oxygen deficient regions, which can be increased by
annealing at relatively high temperatures. On the
47
other hand, the interlayer coupling can be enhanced
by increasing the oxygen content and therefore by
annealing at lower temperatures. It is thus impossible
to enhance both parameters simultaneously for a
single crystal, resulting in a cross-over of Jc values.
In contrast, MPMG Y–Ba–Cu–O sample contains
extra pinning centers provided by Y211 dispersion,
and therefore, if the oxygen content of the matrix is
increased, both pinning and interlayer coupling can
be enhanced, leading to good Jc –B properties. However, in a high field region, the pinning by Y211
particles is not so effective that Jc –B curve merges
with that of single crystal. These results suggest that
for further enhancement of Jc –B properties at 77 K,
on one hand, it is important to introduce some
controllable pinning centers other than oxygen defects, and on the other hand, the oxygen content of
the matrix should be sufficiently large —preferably
in a slightly overdoped range.
4. Conclusions
We studied magnetic properties of an MPMG
Y–Ba–Cu–O and a Y123 single crystal grown by a
crystal pulling technique by changing the oxygen
annealing conditions. The M–H loops of a single
crystal were drastically changed by changing the
doping level. In particular, the irreversibility field
could be greatly enhanced by controlling the oxygen
content in the overdoped level. In contrast, M–H
properties of MPMG Y–Ba–Cu–O were mainly determined by Tc values.
Magnetization measurements both at 20 and 77 K
suggest that Y211 particles can contribute to flux
pinning enhancement. It can be concluded that for
Jc –B improvement it is important to increase the
defect density and the oxygen content of the matrix
simultaneously.
Acknowledgements
Fig. 3. Magnetic field dependence of Jc at 77 K Ž H 5 c-axis. for an
MPMG Y–Ba–Cu–O Žopen symbols. and a Y123 single crystal
grown by a crystal pulling technique Žsolid symbols..
This work was supported by New Energy and
Industrial Technology Development Organization
ŽNEDO. for the Research and Development of Industrial Science and Technology Frontier Program.
48
M. Watahiki et al.r Physica C 296 (1998) 43–48
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