Physica C 469 (2009) 1218–1220

Contents lists available at ScienceDirect

Physica C
journal homepage: www.elsevier.com/locate/physc

Vortex pinning properties of ðY1x Lax Þ–Ba–Cu–O and ðY1x Prx Þ–Ba–Cu–O
superconducting bulks
T. Naito *, K. Sato, D. Yamaguchi, H. Fujishiro
Faculty of Engineering, Iwate University, Morioka 020-8551, Japana r t i c l ei n f o

Article history:
Available online 2 June 2009
PACS:
74.25.Bt
74.25.Qt
74.25.Sv
74.72.Bka b s t r a c t
We have studied the effect of a small amount of Y-site substitution by La or Pr ions on the vortex pinningin the Y–Ba–Cu–O system. ðY1x Lax Þ–Ba–Cu–O and ðY1x Prx Þ–Ba–Cu–O bulks were fabricated by the melttextured growth, in which x was varied from 0 to 0.01. The critical current density Jc at 77 K is improvedin magnetic ﬁelds parallel to the c-axis above 2–4.5 T and the corresponding irreversibility ﬁeld, Hirr ,shifts to the higher value in both bulks.
Ó 2009 Elsevier B.V. All rights reserved.

Keywords:
ðY1x Lax Þ–Ba–Cu–O bulk
ðY1x Prx Þ–Ba–Cu–O bulk
Critical current density
Irreversibility ﬁeld

1. Introduction
It is well known that RE–Ba–Cu–O (REBCO) superconducting
bulks (RE = rare earth elements) demonstrate a high critical current density J c at 77 K and under high magnetic ﬁelds. REBCO bulk
is a composite material which consists of the matrix phase ofREBa2 Cu3 O7d (RE123) and secondary phase of RE2 BaCuO5(RE211). Such a high J c is brought about by various pinning centers,such as RE211 particles, oxygen vacancies in RE123, and local RErich regions. To achieve a higher J c in REBCO bulks, many studies
relating with the introduction of other pinning centers have beenperformed. Zn substitution for Cu-site in RE123 is very effectivefor increasing J c [1]. However, at the same time the superconducting transition T c is suppressed by this substitution, because Zn isintroduced to the superconducting CuO2 plane. Ishii et al. foundthat a very small amount of impurity (Fe, Co, Ga) substituted toCu-site in the CuO chain gives very improved J c ðHÞ properties inYBCO bulks [2]. Recently, in ðY1x Lux ÞBCO [3] and ðDy1x Tbx ÞBCO[4] bulks the increase of J c ðHÞ was found at a small amount of x,demonstrating that the impurity substituting to the RE-site is also
effective for improving the pinning properties. These previous reports strongly suggest that the dilute impurity substituting to theRE-site and Cu-site in the CuO chain is a promising way to improvepinning properties of REBCO bulks.
* Corresponding author. Tel./fax: +81 19 621 6362.
E-mail address: tnaito@iwate-u.ac.jp (T. Naito).
0921-4534/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.physc.2009.05.241

In this paper, we report on the effect of the Y-site substitution
by La or Pr ions on the vortex pinning properties such as Jc ðHÞand the irreversibility ﬁeld, Hirr , at 77 K under the magnetic ﬁelds
parallel to the c-axis in ðY1x Lax ÞBCO and ðY1x Prx ÞBCO bulks. Theseparameters were improved by the substitution and an origin of theimprovement was discussed.
2. Experimental procedureandðY1x Prx ÞBCObulks(x ¼ 0; 0:0005;
ðY1x Lax ÞBCO0:001; 0:003, and 0.01) were grown by a melt-texture technique.
(Y,La)123, (Y,Pr)123 and Y211 powders were prepared by a solidstate reaction method. Raw powders of RE2 O3 (RE = Y, La),
Pr6 O11 ; BaCO3 and CuO were weighted with proper molar ratios
and ground by an agate mortar and pestle for 1 h. For the RE123
phase, the mixed powder was calcined at 900 °C for 24 h in air.
The calcined powder was pulverized and ground, and was re-sintered at 920 °C for 72 h in air. For the Y211 phase, the mixture
was sintered at 940 °C for 24 h in air. The RE123 powder mixed
with the RE211 powder with a molar ratio of 1:0.3, was pressed
into pellets with 20 mm in diameter and about 10 mm in thickness. The 0.5 wt% CeO2 powder was added in all samples. We useda Nd123 single crystal as the seed crystal, which was set on thecenter of top surface of each pellet before the heating up, i.e., thecold seeding. The pellet was heated up to 1010 °C, kept for 2 h,and slowly cooled down from 1005 °C to 970 °C at a rate of
0.5 °C/h in air. The rectangular shaped samples with typical1219

T. Naito et al. / Physica C 469 (2009) 1218–1220

dimensions of 2  2  1 mm3 were cut from an as-grown bulk at
1 mm and 3 mm just from the seed crystal, as schematically shownin the inset of Fig. 1a. We call these samples as RE-1 mm and RE3 mm or 1 mm-sample and 3 mm-sample, respectively. The
amount of oxygen deﬁciency, d, in the RE123 phase is varied bycontrolling the annealing temperature [5]. In this study, all thesamples were annealed at 400 °C for 168 h in 0.1 MPa ﬂowing oxygen gas. The magnetization was measured in magnetic ﬁelds parallel to the c-axis up to 5 T using a commercial SQUID magnetometer(MPMS-5S and MPMS-XL, Quantum Design Inc.). The critical current density, J c , was estimated from the MðHÞ hysteresis loop using
the extended Bean model, J c ¼ 20DM=að1  a=3bÞ, where DM is thewidth of the MðHÞ loop, and a and b (a < b) are the cross-sectional
dimensions of the sample perpendicular to the applied magneticﬁeld.
3. Results
Fig. 1a–d shows the temperature dependence of the normalized
magnetization, MðTÞ, under the applied ﬁeld of l0 H ¼ 0:4 mT parallel to the c-axis after the zero-ﬁeld cooling (ZFC) for (Y,La)BCOand (Y,Pr)BCO bulks. The onset of the superconducting transition
temperature, T c , is about 90.5–91.2 K for all the samples, meaningthat the dilute La- and Pr-substitution up to 1% slightly reduces thesuperconductivity.
Fig. 2a–d shows the magnetic ﬁeld dependence of the criticalcurrent density, J c ðHÞ, at 77 K for Hkc-axis for (Y,La)BCO and(Y,Pr)BCO bulks. For the La-1 mm samples, J c ðHÞ ofðY0:999 La0:001 ÞBCO is slightly higher than that of the non-substituted
YBCO above 4.5 T and the corresponding irreversibility ﬁeld Hirr deﬁned at J c ¼ 4 A=cm2 exceeds 5 T. However, J c of other La-1 mmsamples are quite smaller than that of YBCO. For the Pr-1 mm samples, both J c and Hirr are improved for the samples with x ¼ 0:001
and 0.003. J c of the La-3 mm samples with x ¼ 0:0005 and 0.001and the Pr-3 mm samples with x ¼ 0:0005  0:003 is higher thanthat of YBCO above approximately 2 T. In addition, Hirr of thesesamples is also higher than 5 T. The x dependence of Hirr is summarized in Fig. 3.

4. Discussion
The obtained results indicate that the substitution of Y-site by
La or Pr ions improves both J c and Hirr under relatively high magnetic ﬁeld. To discuss an effect of the substitution, the role of theLa or Pr ions in the matrix phase should be considered, i.e., whether
La123 or Pr 123 phases are created in the matrix of bulks or the Laor Pr ions form other compounds. T c of La123 is much higher thanthat of Y123 [6], on the other hand, Pr123 phase is believed not toshow the superconductivity [7]. As seen in Fig. 1, the T c ’s of bothLa- and Pr-substituted samples are basically lower than that ofYBCO, meaning that at least La123 phase is not created. In addition,
as described in the preceding paragraph, the starting materials of(Y,La)123 and (Y,Pr)123 were ﬁred at 920 °C, which might be lowto create La123 or Pr123 phases. Consequently, we consider that
La123 and Pr123 phases are absent in the samples.
The observed ﬁeld-induced enhancement of pinning properties
indicates that the weak superconducting regions are introduced.
One possible origin is the appearance of the RE-rich region, because it is generally accepted that the RE-rich region in REBCO suppresses T c of the matrix phase. It is well known that in RE123 bulkswith RE = La, Nd, Sm a part of the RE ions is substituted for a Ba-site[8], which gives rise to form the RE1þx Ba2x Cu3 O7d -type solid solution, because both ionic radii are comparable. This migration increases the weak superconducting regions. This RE-rich region
acts as a ﬁeld-induced pinning center. In usual, such a solid solution cannot occur in the Y123 bulks, because the ionic radius ofY3þ is quite smaller than that of Ba2þ . The La- or Pr-substitution enable us to make the ﬁeld-induced pinning sites in YBCO bulks.
Recently, Shimoyama et al. [3] reported that the Lu-substitution
for Y-site in the YBCO bulk is effective for the increase of J c ðHÞ at
77 K in relatively high magnetic ﬁeld, which is basically consistentwith our results. Tazaki et al. [4] reported that in ðDy1x Tbx ÞBCObulks ðx ¼ 0; 0:1Þ the J c ðHÞ values at 77 K are enhanced by the substitution for Dy-site by Tb. This result was interpreted as follows.
First, the BaTbO3 particles are created by the Tb-substitution,
which causes the substitution of Dy for Ba-site. Second, Tb issubstituted for Dy-site. These ideas are also consistent with our0(a) La-1mm(Y1-xLax)-Ba-Cu-Oµ0H(||c)=0.4 mT-0.5xseed00.0005
0.001
0.003
0.01

1 mm
3 mmcbulk-1

Normalized M

Normalized M

0x00.0005
0.001
0.003
0.01

-0.50(b) La-3mm(Y1-xLax)-Ba-Cu-Oµ0H(||c)=0.4 mT

x00.0005
0.001
0.003
0.01

-0.5-1899091

T (K)

92

Normalized M

Normalized M(Y1-xPrx)-Ba-Cu-Oµ0H(||c)=0.4 mT-1088(c) Pr-1mm(d) Pr-3mm(Y1-xPrx)-Ba-Cu-Oµ0H(||c)=0.4 mT

x00.0005
0.001
0.003
0.01-0.5-193

88899091

92

93

T (K)

Fig. 1. The temperature dependence of the magnetization, MðTÞ, after the zero-ﬁeld cooling in l0 HðkcÞ ¼ 0:4 mT for ðY1x Lax ÞBCO (a and b) and ðY1x Prx ÞBCO (c and d) bulks
ðx ¼ 0  0:01Þ. The RE-1 mm and RE-3 mm samples are cut from the position 1 mm and 3 mm below the bottom of the seed crystal, respectively, as shown schematically in
the inset of (a).

T. Naito et al. / Physica C 469 (2009) 1218–1220

410310210110

0T=77 K, H||c (a)(Y1-xLax)-Ba-Cu-Ox-110410Jc (A/cm2)

Jc (A/cm2)

10La-1mm00.0005
seed0.001 1 mm
3 mm0.003
c0.01bulk

3102

10110010-1(Y1-xLax)-Ba-Cu-O

0x00.0005
0.001
0.003
0.01
1La-3mm

234103102

101100

T=77 K, H||c (c)(Y1-xPrx)-Ba-Cu-O

x00.0005
0.001
0.003
0.01-1

T=77 K, H||c (b)101010410Jc (A/cm2)

Jc (A/cm2)

1220

45Pr-1mm

T=77 K, H||c (d)10310210110010-1(Y1-xPrx)-Ba-Cu-O

0x00.0005
0.001
0.003
0.01
1µ0H (T)Pr-3mm

234

5µ0H (T)Fig. 2. The magnetic ﬁeld dependence of the critical current density, Jc ðHÞ, at 77 K and Hkc for ðY1x Lax ÞBCO (a and b) and ðY1x Prx ÞBCO (c and d) bulks ðx ¼ 0  0:01Þ. The
dotted lines represent Jc ðHÞ ¼ 4 A=cm2 which is the criterion for the irreversibility ﬁeld, Hirr .

ated the critical current density, J c ðHÞ, and the irreversibility
ﬁeld, Hirr , at 77 K from measuring the magnetization using theðY1x Lax ÞBCO and ðY1x Prx ÞBCO bulks ðx ¼ 0  0:01Þ. The low level of RE-substitution tends to improve Jc ðHÞ and the corresponding Hirr in the relatively high ﬁeld. This improvement ofthe vortex pinning properties is expected to originate from introducing the ﬁeld-induced pinning centers such as the RE-rich region caused by the RE1þx Ba2x Cu3 O7d -type solid solution(RE = La, Pr).
Acknowledgements
This work was supported in part by the JGC-S Scholarship foundation (No. 1907) and a Grant-in-Aid for Scientiﬁc Research (C)from Japan Society for the Promotion of Science, Japan (Nos.
19560003 and 20560001). A part of the magnetization measurements was performed using a SQUID magnetometer at the Non
Destructive Evaluation and Science Research Center, Faculty ofEngineering, Iwate University.
Fig. 3. x dependence of the irreversibility ﬁeld, Hirr ðxÞ, at 77 K and Hkc for ðY1x Lax ÞBCO and ðY1x Prx Þ BCO bulks ðx ¼ 0  0:01Þ.

interpretation, although the mechanisms causing the substitution
between RE and Ba sites is different from each other.
We note that the relation between J c and the size and distribution of the Y211 parts evaluated from the SEM images (not shownhere) was not observed, which supports that the improvement of J coriginates from the effect of the La- or Pr-substitution.
5. Summary
To study the effect of the impurity substitution for the Y-site
in YBCO bulk on the vortex pinning properties, we have evalu-References[1] G. Krabbes, G. Fuchs, P. Schatzle, S. Grus, J.W. Park, F. Hardinghaus, G. Stover, R.
Hayn, S.-L. Drechsler, T. Fahr, Physica C 330 (2000) 181.
[2] Y. Ishii, J. Shimoyama, Y. Tazaki, T. Nakashima, S. Horii, K. Kishio, Appl. Phys.
Lett. 89 (2006) 202514.
[3] J. Shimoyama, Y. Tazaki, Y. Ishii, T. Nakashima, S. Horii, K. Kishio, J. Phys.: Conf.
Ser. 43 (2006) 235.
[4] Y. Tazaki, Y. Ishii, T. Nakashima, J. Shimoyama, S. Horii, K. Kishio, Physica C 460–462 (2007) 1343.
[5] K. Kishio, J. Shimoyama, T. Hasegawa, K. Kitazawa, K. Fueki, Jpn. J. Appl. Phys. 26(1987) L1228.
[6] For instance, A. Gantis, M. Calamiotou, D. Palles, D. Lampakis, E. Liarokapis,Phys. Rev. B 68 (2003) 064502.
[7] L. Soderholm, K. Zhang, D.G. Hinks, M.A. Beno, J.D. Jorgensen, C.U. Segre, I.K.
Schuller, Nature 328 (1987) 604.
[8] M. Murakami, N. Sakai, T. Higuchi, S.I. Yoo, Supercond. Sci. Technol. 9 (1996)
1015.