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In-plane anisotropy of critical current density in Bi1.6Pb0.6Sr1.8CaCu2.0Oy single crystal
PHYSICA ELSEVIER
Physica C 341-348 ( 2 0 0 0 ) 1 4 7 7 - 1 4 7 8 www.elsevier.nl/locate/physc
In-plane anisotropy of critical current density in Bil.6Pb0.6Srl.8CaCu2.0Oy single crystal Y. Nakayama, T. Motohashi, K. Otzschi, J. Shimoyama and K. Kishio Department of Superconductivity, University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8656, Japan
Magneto-optical imaging technique and magnetization measurements have been performed for a Bil.6Pb0.6Sr].sCaCu2.oOy single crystal. The second peak effects are found to be closely related to the in-plane anisotropy of J~, which is an evidence for the temperature- and field-induced directional pinning center. A large number of studies have been performed for developing the poor flux pinning in Bi2212, which is the most promising material as superconducting wire and tapes. Recently dramatically improved flux pinning properties at high temperatures have been observed in heavily Pb-doped Bi2212 single crystals[I], being accompanied by the reduction of electromagnetic anisotropy[2] and the micro-phase segregation with different Pb content[3]. However, characteristic properties such as the temperature-dependent second peak effect and the anomalous temperature-dependence of J~[4], are still controversial. In the present study, magnetic properties of the heavily Pb-doped Bi2212 single crystal were investigated by magneto-optical(MO) imaging technique, in order to elucidate the flux pinning mechanism in this system. A boule of Bil.6Pb06Srl.sCaCu2.0Oy grown by the floating zone method was cut and cleaved, and a plate-like crystal with the size of 1.12(//a)xl.lO(//b)xO.O35(//c)mm 3 was prepared. The crystal was annealed for 72h at 600°C under the effective pressure of P(O2)=3.9x10 -4 atm, and then a oxygen lightly overdoped crystal(T¢=86.7K) was obtained. Crystal was cooled down to various temperatures in zero field, and then magnetic field (B<1575G) was applied perpendicular to the ab-plane of the crystal. MO images were taken to visualize the normal field component B:(x,y) produced by the shielding currents, using a Bi-doped Y-garnet indicator film placed directly onto the crystal surface. Figure l(a) shows a typical MO image taken at 30K under B=630G. Despite the shape of this crystal 0921-4534/00/$ - see front matter © 2000 Elsevier Science B.~ PII S0921-4534(00)01096-0
is approximately square, the angle @ between the current discontinuity line and the crystal edge along a-axis is much smaller than 45 °, indicating that the flux penetration is anisotropic. Flux density profiles are represented in Fig. l(b), which clirectly shows that flux penetrates easier along a-axis than along b-axis. Jc values as fitting parameters[5] for these observed profiles were Jca=2.0xl05 A/cm 2 and Jcb=l.2xl05 A/cm2, where Jca and Jch represent the critical current density along a- and b-axis, respectively. Both Jfl and j b taken at various temperatures
o(a)
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0
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.
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,
.
.
.
[
.
.
.
.
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,
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1
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i
i
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0
500
x l l~m
J
IJ
Fig.1 (a) MO image for ZFC at T=30K and B=630G. (b) Magnetic flux profiles taken along the solid and broken line in (a). All rights reserved.
Y Nakayama et al./Physica C 341-348 (2000) 1477-1478
1478
I
J
B//c =
630G
I
I
,
I
3
2
,. ....
4
,,"
"-._._.
"" Jca/dcb
2
E2
~
t.9
"1 tzl
1 0
i 10
,
i 20
,
r , 30
r , 40
i 50
,
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60
TIK Fig.2 Temperature dependence of j a and job obtained from MO profiles. In-plane anisotropy was calculated as Jfl/Jc b at each temperature. under the field of 630G are demonstrated in Fig. 2. Jfl was always larger than j b in our study. It should be noted that decrease of Jfl with an increase of temperature is gentle, whereas j b drops rapidly, and in addition, a shoulder-like behavior was observed for ,/ca at around 40K. These results suggest that temperature-induced pinning centers suppress the flux penetration along the b-axis, which enhances only Jfl in a crystal. In-plane anisotropy of J~ was also plotted in Fig.2 as a function of temperature, which shows the maximum at 40K. While the values were calculated as J¢a/Jch in this figure, it is well known that in-plane anisotropy can be evaluated from the angle @ (see Fig.l(a)) as j , / j b =l/tan 8[6]. Both values were in good agreement each other in this study. Figure 3 illustrates the contour map of in-plane anisotropy in a B - T plane, whose values were estimated from angle 8. Irreversibility fields and second peak fields are also plotted in this figure, which were determined from the magnetization measurements for the same sample. Large in-plane anisotropy of J~ was observed above 30K especially below lkG, with the maximum value ofJcO/J¢ b =2.5. It is rather amazing that the temperature dependence of second peak fields well coincides with the area where in-plane anisotropy is relatively high. According to these results, one can naturally conclude that the second peak effect in this system is dominated by the temperature- and field-induced pinning centers, which have the large pinning effects only for the flux movement along b-axis. The most probable candidate for this pinning
0 2O
40
6O
8O
TIK Fig.3 Contour of in-plane anisotropy in a B - 7 plane. Dots represent the conditions where MO images were taken. Second peak fields(Bpk) and irreversibility fields(Bi~) were obtained from the magnetization measurements. center is the boundary of micro-phase segregation. Recently STS observation has revealed that there exists a difference of T¢ between two phases[7]. Though the separated microstructure has various shapes, such as lamella-type and bubble-type, they have basically the same direction as that of structural modulation in Bi2212. When the temperature or field goes up moderately, boundary of separated phases would act as directional pinning centers and improve the flux pinning property. In summary, we have found that second peak effects in this system is closely related to the in-plane anisotropy of Jc, which indicates that the micro-phase segregation improves the flux pinning property of heavily Pb-doped Bi2212, together with the reduction of electromagnetic anisotropy. The authors wish to thank A. A. Polyanskii for his kind cooperation. This study was supported by CREST of JST, and NEDO of Japan. [1] [2] [3] [4] [5] [6] [7]
I. Chong et al. Science 276, 770(1997) T. Motohashi et al. PRB 59, 14080(1999) Z. Hiroi et al. J.Solid State Chem. 138, 98(1998) J. Shimoyama et al. Physica C 281, 68(1997) E. H. Brandt et al. PRB 48, 12893(1993) Th. Schuster et al. PRB 56, 3413(1997) S. Nakao et al. Proc.of MOS'99 in press.
Physica C 341-348 ( 2 0 0 0 ) 1 4 7 7 - 1 4 7 8 www.elsevier.nl/locate/physc
In-plane anisotropy of critical current density in Bil.6Pb0.6Srl.8CaCu2.0Oy single crystal Y. Nakayama, T. Motohashi, K. Otzschi, J. Shimoyama and K. Kishio Department of Superconductivity, University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8656, Japan
Magneto-optical imaging technique and magnetization measurements have been performed for a Bil.6Pb0.6Sr].sCaCu2.oOy single crystal. The second peak effects are found to be closely related to the in-plane anisotropy of J~, which is an evidence for the temperature- and field-induced directional pinning center. A large number of studies have been performed for developing the poor flux pinning in Bi2212, which is the most promising material as superconducting wire and tapes. Recently dramatically improved flux pinning properties at high temperatures have been observed in heavily Pb-doped Bi2212 single crystals[I], being accompanied by the reduction of electromagnetic anisotropy[2] and the micro-phase segregation with different Pb content[3]. However, characteristic properties such as the temperature-dependent second peak effect and the anomalous temperature-dependence of J~[4], are still controversial. In the present study, magnetic properties of the heavily Pb-doped Bi2212 single crystal were investigated by magneto-optical(MO) imaging technique, in order to elucidate the flux pinning mechanism in this system. A boule of Bil.6Pb06Srl.sCaCu2.0Oy grown by the floating zone method was cut and cleaved, and a plate-like crystal with the size of 1.12(//a)xl.lO(//b)xO.O35(//c)mm 3 was prepared. The crystal was annealed for 72h at 600°C under the effective pressure of P(O2)=3.9x10 -4 atm, and then a oxygen lightly overdoped crystal(T¢=86.7K) was obtained. Crystal was cooled down to various temperatures in zero field, and then magnetic field (B<1575G) was applied perpendicular to the ab-plane of the crystal. MO images were taken to visualize the normal field component B:(x,y) produced by the shielding currents, using a Bi-doped Y-garnet indicator film placed directly onto the crystal surface. Figure l(a) shows a typical MO image taken at 30K under B=630G. Despite the shape of this crystal 0921-4534/00/$ - see front matter © 2000 Elsevier Science B.~ PII S0921-4534(00)01096-0
is approximately square, the angle @ between the current discontinuity line and the crystal edge along a-axis is much smaller than 45 °, indicating that the flux penetration is anisotropic. Flux density profiles are represented in Fig. l(b), which clirectly shows that flux penetrates easier along a-axis than along b-axis. Jc values as fitting parameters[5] for these observed profiles were Jca=2.0xl05 A/cm 2 and Jcb=l.2xl05 A/cm2, where Jca and Jch represent the critical current density along a- and b-axis, respectively. Both Jfl and j b taken at various temperatures
o(a)
%
~ , 0
0
'
I
I
.
I
,
.
.
.
[
.
.
.
.
I
'
,
~
1
-500
,
i
i
,I
I
0
500
x l l~m
J
IJ
Fig.1 (a) MO image for ZFC at T=30K and B=630G. (b) Magnetic flux profiles taken along the solid and broken line in (a). All rights reserved.
Y Nakayama et al./Physica C 341-348 (2000) 1477-1478
1478
I
J
B//c =
630G
I
I
,
I
3
2
,. ....
4
,,"
"-._._.
"" Jca/dcb
2
E2
~
t.9
"1 tzl
1 0
i 10
,
i 20
,
r , 30
r , 40
i 50
,
R
0
60
TIK Fig.2 Temperature dependence of j a and job obtained from MO profiles. In-plane anisotropy was calculated as Jfl/Jc b at each temperature. under the field of 630G are demonstrated in Fig. 2. Jfl was always larger than j b in our study. It should be noted that decrease of Jfl with an increase of temperature is gentle, whereas j b drops rapidly, and in addition, a shoulder-like behavior was observed for ,/ca at around 40K. These results suggest that temperature-induced pinning centers suppress the flux penetration along the b-axis, which enhances only Jfl in a crystal. In-plane anisotropy of J~ was also plotted in Fig.2 as a function of temperature, which shows the maximum at 40K. While the values were calculated as J¢a/Jch in this figure, it is well known that in-plane anisotropy can be evaluated from the angle @ (see Fig.l(a)) as j , / j b =l/tan 8[6]. Both values were in good agreement each other in this study. Figure 3 illustrates the contour map of in-plane anisotropy in a B - T plane, whose values were estimated from angle 8. Irreversibility fields and second peak fields are also plotted in this figure, which were determined from the magnetization measurements for the same sample. Large in-plane anisotropy of J~ was observed above 30K especially below lkG, with the maximum value ofJcO/J¢ b =2.5. It is rather amazing that the temperature dependence of second peak fields well coincides with the area where in-plane anisotropy is relatively high. According to these results, one can naturally conclude that the second peak effect in this system is dominated by the temperature- and field-induced pinning centers, which have the large pinning effects only for the flux movement along b-axis. The most probable candidate for this pinning
0 2O
40
6O
8O
TIK Fig.3 Contour of in-plane anisotropy in a B - 7 plane. Dots represent the conditions where MO images were taken. Second peak fields(Bpk) and irreversibility fields(Bi~) were obtained from the magnetization measurements. center is the boundary of micro-phase segregation. Recently STS observation has revealed that there exists a difference of T¢ between two phases[7]. Though the separated microstructure has various shapes, such as lamella-type and bubble-type, they have basically the same direction as that of structural modulation in Bi2212. When the temperature or field goes up moderately, boundary of separated phases would act as directional pinning centers and improve the flux pinning property. In summary, we have found that second peak effects in this system is closely related to the in-plane anisotropy of Jc, which indicates that the micro-phase segregation improves the flux pinning property of heavily Pb-doped Bi2212, together with the reduction of electromagnetic anisotropy. The authors wish to thank A. A. Polyanskii for his kind cooperation. This study was supported by CREST of JST, and NEDO of Japan. [1] [2] [3] [4] [5] [6] [7]
I. Chong et al. Science 276, 770(1997) T. Motohashi et al. PRB 59, 14080(1999) Z. Hiroi et al. J.Solid State Chem. 138, 98(1998) J. Shimoyama et al. Physica C 281, 68(1997) E. H. Brandt et al. PRB 48, 12893(1993) Th. Schuster et al. PRB 56, 3413(1997) S. Nakao et al. Proc.of MOS'99 in press.