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Ag composite tape in high field
Applied SuperconductivityVol. 3, No. 11112, pp. 535-541,
Pergamon
SO964-1807(96)00004-X
1995 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0964-1807/95 $9.50 + 0.00
ANOMALY IN J,--B HYSTERESIS FOR Bi-22 12/Ag COMPOSITE TAPE IN HIGH FIELD HITOSHI KITAGUCHI, HIROAKI KUMAKURA and KAZUMASA TOGANO National Research Institute for Metals, 1-2-1 Sengen, Tsukuba 305, Japan (Received 23 November 1995)
Abstract--Transport J, is measured at 4.2 K in high magnetic fields up to 23 T for c-axis oriented Bi2212/Ag composite tape prepared by using doctor blade cast tape and melt-solidification techniques, and we find an anomaly in J=-B hysteresis around 19 T. The magnetic field is applied parallel to the tape surface (along the ab-plane). A typical J&3 hysteresis for polycrystalline oxide superconductor is observed below 18 T. Above 19 T, an inversion in J,-B hysteresis is observed. .J,-B curves in field increasing and field decreasing sequence cross each other at a field around 19 T. Between 18.7 T and 18.6 T in field decreasing sequence, J, increases 20% in value. At stable fields above 19 T after field increasing sequence, J, decreases with time.
INTRODUCTION
Hysteretic behavior of transport critical current density (J,) in magnetic fields is well known for polycrystalline (oxide superconductors. Transport J, in decreasing magnetic field is higher than that in increasing magnetic field at the same intensity of the field (except in a very low field). Many explanations for this J,-B hysteresis have been reported [l-5], which are based on the distribution of trapped flux in grains and boundaries. In this paper, a new kind of J,-B hysteresis in high field of around 19 T for Bi2Sr2CaCu20, (Bi-22 12)/Ag composite tape is reported. We found an anomalous J,-B hysteresis where J,-B curves in field increasing and field decreasing sequence cross each other at a field of around 19 T. We observed time-decay of J, values in stable magnetic fields higher than 19 T even in field increasing sequence while J, increases with time in the same aspect of a typical hysteresis. Moreover, we observed a large J, increases in field decreasing sequence. This anomalous behavior cannot be followed by existing explanations. EXPERIMENTAL
The samples are BL2212lAg composite tapes prepared through the doctor blade casting and melt-solidification technique [6-91. A slurry consisting of the Bi-2212 oxide powder with the nominal composition of Bi:Sr:Ca:Cu = 2.00:2.00:0.96:2.00, organic solvent (trichloroethlene), binder (poly(viny1 butyral)), and dispersant (sorditan trioleate) was cast under a doctor blade into a green tape. The green tape was cut into - 30 mm in length and 2-3 mm in width, and then mounted on a silver substrate of 30-50 ,um in thickness. Melt-solidification technique was employed in order to obtain an excellent J, property. The samples were put on the powder mixture of Bi2AL09 +A1203 and covered with an alumina crucible to suppress the vaporization of bismuth from Bi-2212 phase during the high temperature stage of the heat treatment. Then, the samples were heated to 892°C at a rate of 300”Ck, held for 5 min, cooled to 837°C at 5”C/h, held for 1 h, and rapidly cooled to room temperature (at a rate higher than 6OO”C/h in the temperature range above 600°C to suppress the deformation of Bi-2212 phase to BizSrzCuOX (Bi-2201) phase). The samples prepared in this way have a highly textured microstructure of the oxide layer, where the Bi-22 12 grains are oriented with the c-axes aligned perpendicular to the tape surface. The thickness of the oxide layer after the heat treatment was 25 ,um for each sample. The I-Vprofile was measured in various magnetic fields up to 23 T at 4.2 K (in liquid helium) by using the four-probe resistive method. Current and voltage leads were soldered to the silver substrate of each sample. A schematic illustration for samples is shown in Fig. 1. The criterion for 535
536
H. KITAGUCHI et al.
(mm) .--.-.-._.____.... _._._.._____ k
Voltage Tap
2-3 5-6 .---.-~_.._._._.___________. d Solder
C .urent Lead
Fig. 1. Schematic illustration of sample configuration for J, measurement.
critical current determination was 1 pV/cm and the cross-sectional area of the oxide layer was used to calculate the J, value. Magnetic field was applied parallel to the tape surface (thus, along with &-plane and perpendicular to the c-axis of Bi-22 12 grains) by using a water-cooled resistive magnet system or a hybrid magnet system. In the measurement for sample A by using a resistive magnet system in Laboratoire des Champs Magnetiques Intenses (LCMI), CNRS/MPI, Grenoble, France, the magnet system was charged or discharged at the rate of 3 T/mm, and the J, measurement was performed at stable magnetic fields. Another measurement for sample B was performed with a hybrid magnet system in High Field Laboratory for Superconducting Materials, Institute for Materials Research, Tohoku University, Japan.
RESULTS
AND
DISCUSSION
Figure 2 shows J,-B hysteresis for the sample A in (OT-)23 T-10 T-23 T cycle. After cooling the sample to 4.2 K in zero field, magnetic field was generated up to 23 T, and the I-Vprofile was measured as soon as the field became stable. Z-V measurement was repeated at various stable fields in field decreasing sequence (from 23 to 10 T) and subsequent field increasing sequence (from 10 to 23 T). At each magnetic field, the I-Vmeasurement was performed as soon as the field became stable. A large drop in J, value is observed between 21 and 19 T in the J,-B curve for the field decreasing sequence. At 23 T, the sample carries 107 000 A/cm2. J, values decrease with decreasing the field to 21 T, and the J, values are 92 000 and 83 000 A/cm2 at 22 and 21 T, respectively. However, below 19 T, J, values increase with decreasing magnetic field. It is remarkable that J, jumps from 92 000 to 112 000 A/cm2 with decreasing the field over the narrow range from 18.7 to 18.6 T. In the field increasing sequence, J, value decreases with increasing the field. As in the case of a typical hysteresis, the J,-B curve for the field increasing sequence stays lower than that for the field decreasing sequence below 18 T. Above 19 T, J, decreases monotonically with increasing the field. However, J, value decreases steeply between 20 and 22 T: 106 000 A/cm2 at 20 T to 84 000 A/cm2 at 22 T. The relationship between J, and holding duration at 22 and 20 T for sample A is shown in Fig. 3. In this measurement, magnetic field was increased directly from 10 T (to 22 T) and from 14 T (to 20 T) without J, measurement at intermediate field. As in the case of 23 T (from 0 T) shown in Fig. 2, high J, values of 109000 and 108 000A/cm2 at 22 and 2OT, respectively, were obtained when we measured as soon as the magnetic field became stable after charging the magnet. In contrast with a typical hysteresis where J, increases with time in the field increasing sequence, J, value decreases with time both at 22 and 20 T in the present case. After holding for 3 min, the sample carries only about 75% of its initial J, value at each field. The steep slope in J,-B curves shown in Fig. 2 (from 23 to 2 1 T in the field decreasing sequence and from 20 to 22 T in the field increasing sequence) can be explained in terms of this time-decay of J, value. The decrease of J, value in the field decreasing sequence from 23 T (107 000 A/cm’) to 2 1 T (83 000 A/cm2) (see
Anomaly I
-
I
537
in ~~43 hysteresis -
I
*
I
-
I
-
I
-
I
-
Bi-22WAg Tape : 4.2K B//Tape Surface :
1.4 1
0.8 : :
Sample #A I.
1.
12
8
I.
I.
I.
16
I.
20
I
.
24
Magnetic Field (1) Fig. 2. J&3 hysteresis in 23T-TOT-23T cycle at 4.2 K. After zero field cooling, J, was measured as soon as the field became stable at 23 T. J, measurement was repeated at stable fields in field decreasing sequence to 10 T and subsequent field increasing sequence.
W-221 2/Ag Tape 4.2K B//Tape Surface :
0.8 Time (min) Fig. 3. Relatronship between J, and holding duration at 22 and 20 T. Magnetic field was increased directly from 10 and 14T, respectively, without J, measurement at intermediate field and then kept stable.
538
H. KITAGUCHI et al.
Fig. 2) does not originate in the decrease of magnetic field, but in the fact that the sample was kept in high field for a long period to carry out several I-Y measurements. However, the jump of J, between 18.7 and 18.6 T (see Fig. 2) cannot be explained in terms of this time decay of J,. Below 18 T, time dependence of J, is typical for polycrystalline oxide superconductors. J, decreases with time with keeping a stable field in field decreasing sequence. For example, at 10 T in field decreasing sequence, J,, which is 138200A/cm2 in the initial state, decreases to 1366OOA/cm* with keeping for 8 min. Time decay of J, was confirmed for another sample at 2 1,22, and 23 T in field increasing sequence and at 16 T in field decreasing sequence. Figure 4 shows current-voltage (I-V) curves obtained in the J, measurement for the J,-time plot shown in Fig. 3 and for field decreasing sequences subsequent to holding stages at 22 and 20T. In both holding stages, the slope of Z-V curves corresponding to the superconductingnormal transition become steeper after J, drops than that in the first run of I-Vmeasurement. This fact suggests that current path changes with keeping the sample in high field probably because of a rearrangement of magnetic flux lines around grain boundaries or inside grains. Above 18.7 T in field decreasing sequences, the remarkable increase in J, cannot be confirmed. However, at 18.6 T, notable increase in J, is observed and the slope of the 1-V curve corresponding to the superconducting-normal transition becomes less steep than that above 18.7T. The J, jump between 18.7 and 18.6 T reaches more than 20% of J, at 18.7 T.
Sample #A Bi-2212IAg Tape, t=25pm, w&9mm 4.2K, B//Tape Surface
Magnetic Field (T)
c
18.8 18 + ('? 20 lmin Pmin 3min 4min
18.8 18.7 18.8 18.5 14
e
--A
12 Fig. 4. Current-voltage (Z-V) curves obtained in the .I, measurements for J,-time plot shown in Fig. 3 and for field decreasing sequences subsequent to holding stages at 22 and 20 T.
Anomaly
in J,-B hysteresis
539
All the results, which are shown in Figs 2-4, are summarized in Fig. 5. J,-B profiles for several sequences show good agreement and the jump of J, in field decreasing sequence between 19 and 18 T is strongly confirmed. This jump is observed in all of four field decreasing sequences. J,-B curves for field increasing sequences and field decreasing ones cross at a field between 18.6 and 18.7 T. Below 18 T, the J,-B curve for field decreasing sequences stays higher than that for field increasing sequences. This J,-B hysteresis shows similar behavior as a typical hysteresis for polycrystalline oxide superconductors containing weak-coupling between grains [l-5]. Above 20 T, initial J, values at each field, which were measured as soon as the field was generated from the lower magnetic field no more than 14 T without intermediate measurement, lie on the extrapolated curve of the J,-B relationship for field increase sequences below 18 T. With repeating the measurement and/or time, J, decreases and reaches the value corresponding to the J,-B curve for field decreasing sequences. Figure 6 shows J,-B hysteresis for sample B measured by using a hybrid magnet system installed in Tohoku University. Anomalous J,-B hysteresis such as time decay of J, at 22 T in the field increasing sequence and a jump of J, in the field decreasing sequence was observed. However, in this case, the jump J, is observed around 19.5 T, while it was observed between 18.6 and 18.7 T for sample A. The inversion and the anomaly of J,-B hysteresis and time decay of J, in high field which we report in this paper cannot be explained simply in terms of the distribution of trapped flux in grains and boundaries [l-5]. The anomaly in J,-B hysteresis suggests that there exists some transformation on pinning and/or electric coupling at the grain boundary. One possible explanation is as follows. A thin layer of secondary superconducting phase whose irreversible field or upper critical field is around 19 T forms at the grain boundary. Experimental results suggest that the magnitude of the magnetic field where the jump of J, appears depends on the geometric configuration of magnetic fields and/or samples. Anomalous J,-B hysteresis is thought
0.8 8
12
16
20
24
Magnetic Field (T) Fig. 5. JC-B profiles for several sequences show good agreement with each other. The jump ofJ, in field decreasing sequence between 19 and 18 T is strongly confirmed. An inversion of J,-B hysteresis can be seen. J,-B curves for field increasing and field decreasing sequence cross at a field between 18.6 and 18.7T.
540
H. KITAGUCHIet al.
I
Bi-2212IAg Tape B//Tape Surface
1
:
Sample#B
0.88 . ’ * ’ . ’ ’ ’ ’ ’ - ’ . ’ 12 16 20 24 Magnetic Field (T) Fig. 6. &-g profile for sample B. J, jump and the inversion of&B hysteresis were observed for another sample measured by using another magnet system.
to be related to magnetic field distribution in the sample. Further study on these points and the relationship between J,-B hysteresis and microstructure should be continued.
CONCLUSION
J,-B hysteresis for Bi-22 12/Ag tape at 4.2 K in high magnetic field applied parallel to the tape surface up to 23 T was studied. An anomaly in J,--B hysteresis was found around 19 T. Below 18 T, the J,-B curve for field decreasing sequences stays higher than that for field increasing sequences as in a typical hysteresis for polycrystalline oxide superconductor containing weak-coupling between each grains. However, J,-B curves for field increasing sequences and field decreasing ones cross at a field between 18.6 and 18.7 T. Above 20 T in field increasing sequence, initial J, values at each field, which were measured as soon as the field was generated from the lower magnetic field of no more than 14 T without intermediate measurement, lie on the extrapolated curve of the J,-B relationship for field increasing sequences below 18 T, however, J, decreases with time in contrast with a typical hysteresis where J, increases with time in the field increasing sequence. A J, jump of about 20% in its value was observed between 18.7 and 18.6 T in the field decreasing sequence. authors wish to express their appreciation to Dr J. C. Vallier, Laboratoire des Champs Magnetiques Intenses (LCMI), CNRSMPI, Grenoble, and Dr K. Watanabe, Institute for Materials Research, Tohoku University, for providing them with frequent opportunities to perform high field measurements. They are grateful also to Dr J. P Senateur, Laboratoire des Mattrlaux et du Genie Physique, INPG-ENSPG-URA, CNRS for tiuitful discussion and his assistance in the sample preparation.
Acknowledgements-The
REFERENCES 1. K. Noto, K. Morita, K. Watanabe, T, Murakami, Y, Koyanagi, I. Yoshii, I. Sato, H. Sugawara, N. Kobayashi, H. Fujimori and Y. Muto, Physica B 148, 239 (1987). 2. J. E. Evetts and B. A. Glowacki, Cvogeaics 28, 641 (1988). 3. K. Watanabe, K. Noto, H. Morita, H. Fujimori K. Mizuno, T. Aomine, B. Ni, T. Matsushita, K. Yamafcji and Y. Muto, Ciyogenics 29, 263 (1989).
Anomaly in J,-B hysteresis
541
4. M. E. McHemy, M. I? Maley and J. 0. Willes, Phys. Rev. B. 40, (1986). 5. T. Hikata, M. Ueyama, H. Mukai and K. Sata, Cryogenics 30, 924 (1990). 6. J. Kase, K. Togano, H. Kumakura, D. R. Dietderich, N. Irisawa, T. Morimoto and H. Maeda, Jpn. J Appl. Phys. 29, L1096 (1990). 7. J. Kase, T. Morimoto, K. Togano, H. Kumakura, D. R. Dietderich and H. Mae&, Z&G Trans. Magn. 27, 1254 (1991). 8. J. Shimoyama, J. I&e, T. Morimoto, H. Kitaguchi, H. Kumakura, H.Maeda and K. Togano, Jpn. L Appl. Phys. 31, L1167 (1992). 9. J. Shimoyama, N. Tomita, T. Morimoto, H. Kitaguchi, H. Kumakura, K. Togano. H. Maeda, K, Nomura and M. Seido, Jpn. 1 Appl. Phys. 31, L1328 (1992).
Pergamon
SO964-1807(96)00004-X
1995 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0964-1807/95 $9.50 + 0.00
ANOMALY IN J,--B HYSTERESIS FOR Bi-22 12/Ag COMPOSITE TAPE IN HIGH FIELD HITOSHI KITAGUCHI, HIROAKI KUMAKURA and KAZUMASA TOGANO National Research Institute for Metals, 1-2-1 Sengen, Tsukuba 305, Japan (Received 23 November 1995)
Abstract--Transport J, is measured at 4.2 K in high magnetic fields up to 23 T for c-axis oriented Bi2212/Ag composite tape prepared by using doctor blade cast tape and melt-solidification techniques, and we find an anomaly in J=-B hysteresis around 19 T. The magnetic field is applied parallel to the tape surface (along the ab-plane). A typical J&3 hysteresis for polycrystalline oxide superconductor is observed below 18 T. Above 19 T, an inversion in J,-B hysteresis is observed. .J,-B curves in field increasing and field decreasing sequence cross each other at a field around 19 T. Between 18.7 T and 18.6 T in field decreasing sequence, J, increases 20% in value. At stable fields above 19 T after field increasing sequence, J, decreases with time.
INTRODUCTION
Hysteretic behavior of transport critical current density (J,) in magnetic fields is well known for polycrystalline (oxide superconductors. Transport J, in decreasing magnetic field is higher than that in increasing magnetic field at the same intensity of the field (except in a very low field). Many explanations for this J,-B hysteresis have been reported [l-5], which are based on the distribution of trapped flux in grains and boundaries. In this paper, a new kind of J,-B hysteresis in high field of around 19 T for Bi2Sr2CaCu20, (Bi-22 12)/Ag composite tape is reported. We found an anomalous J,-B hysteresis where J,-B curves in field increasing and field decreasing sequence cross each other at a field of around 19 T. We observed time-decay of J, values in stable magnetic fields higher than 19 T even in field increasing sequence while J, increases with time in the same aspect of a typical hysteresis. Moreover, we observed a large J, increases in field decreasing sequence. This anomalous behavior cannot be followed by existing explanations. EXPERIMENTAL
The samples are BL2212lAg composite tapes prepared through the doctor blade casting and melt-solidification technique [6-91. A slurry consisting of the Bi-2212 oxide powder with the nominal composition of Bi:Sr:Ca:Cu = 2.00:2.00:0.96:2.00, organic solvent (trichloroethlene), binder (poly(viny1 butyral)), and dispersant (sorditan trioleate) was cast under a doctor blade into a green tape. The green tape was cut into - 30 mm in length and 2-3 mm in width, and then mounted on a silver substrate of 30-50 ,um in thickness. Melt-solidification technique was employed in order to obtain an excellent J, property. The samples were put on the powder mixture of Bi2AL09 +A1203 and covered with an alumina crucible to suppress the vaporization of bismuth from Bi-2212 phase during the high temperature stage of the heat treatment. Then, the samples were heated to 892°C at a rate of 300”Ck, held for 5 min, cooled to 837°C at 5”C/h, held for 1 h, and rapidly cooled to room temperature (at a rate higher than 6OO”C/h in the temperature range above 600°C to suppress the deformation of Bi-2212 phase to BizSrzCuOX (Bi-2201) phase). The samples prepared in this way have a highly textured microstructure of the oxide layer, where the Bi-22 12 grains are oriented with the c-axes aligned perpendicular to the tape surface. The thickness of the oxide layer after the heat treatment was 25 ,um for each sample. The I-Vprofile was measured in various magnetic fields up to 23 T at 4.2 K (in liquid helium) by using the four-probe resistive method. Current and voltage leads were soldered to the silver substrate of each sample. A schematic illustration for samples is shown in Fig. 1. The criterion for 535
536
H. KITAGUCHI et al.
(mm) .--.-.-._.____.... _._._.._____ k
Voltage Tap
2-3 5-6 .---.-~_.._._._.___________. d Solder
C .urent Lead
Fig. 1. Schematic illustration of sample configuration for J, measurement.
critical current determination was 1 pV/cm and the cross-sectional area of the oxide layer was used to calculate the J, value. Magnetic field was applied parallel to the tape surface (thus, along with &-plane and perpendicular to the c-axis of Bi-22 12 grains) by using a water-cooled resistive magnet system or a hybrid magnet system. In the measurement for sample A by using a resistive magnet system in Laboratoire des Champs Magnetiques Intenses (LCMI), CNRS/MPI, Grenoble, France, the magnet system was charged or discharged at the rate of 3 T/mm, and the J, measurement was performed at stable magnetic fields. Another measurement for sample B was performed with a hybrid magnet system in High Field Laboratory for Superconducting Materials, Institute for Materials Research, Tohoku University, Japan.
RESULTS
AND
DISCUSSION
Figure 2 shows J,-B hysteresis for the sample A in (OT-)23 T-10 T-23 T cycle. After cooling the sample to 4.2 K in zero field, magnetic field was generated up to 23 T, and the I-Vprofile was measured as soon as the field became stable. Z-V measurement was repeated at various stable fields in field decreasing sequence (from 23 to 10 T) and subsequent field increasing sequence (from 10 to 23 T). At each magnetic field, the I-Vmeasurement was performed as soon as the field became stable. A large drop in J, value is observed between 21 and 19 T in the J,-B curve for the field decreasing sequence. At 23 T, the sample carries 107 000 A/cm2. J, values decrease with decreasing the field to 21 T, and the J, values are 92 000 and 83 000 A/cm2 at 22 and 21 T, respectively. However, below 19 T, J, values increase with decreasing magnetic field. It is remarkable that J, jumps from 92 000 to 112 000 A/cm2 with decreasing the field over the narrow range from 18.7 to 18.6 T. In the field increasing sequence, J, value decreases with increasing the field. As in the case of a typical hysteresis, the J,-B curve for the field increasing sequence stays lower than that for the field decreasing sequence below 18 T. Above 19 T, J, decreases monotonically with increasing the field. However, J, value decreases steeply between 20 and 22 T: 106 000 A/cm2 at 20 T to 84 000 A/cm2 at 22 T. The relationship between J, and holding duration at 22 and 20 T for sample A is shown in Fig. 3. In this measurement, magnetic field was increased directly from 10 T (to 22 T) and from 14 T (to 20 T) without J, measurement at intermediate field. As in the case of 23 T (from 0 T) shown in Fig. 2, high J, values of 109000 and 108 000A/cm2 at 22 and 2OT, respectively, were obtained when we measured as soon as the magnetic field became stable after charging the magnet. In contrast with a typical hysteresis where J, increases with time in the field increasing sequence, J, value decreases with time both at 22 and 20 T in the present case. After holding for 3 min, the sample carries only about 75% of its initial J, value at each field. The steep slope in J,-B curves shown in Fig. 2 (from 23 to 2 1 T in the field decreasing sequence and from 20 to 22 T in the field increasing sequence) can be explained in terms of this time-decay of J, value. The decrease of J, value in the field decreasing sequence from 23 T (107 000 A/cm’) to 2 1 T (83 000 A/cm2) (see
Anomaly I
-
I
537
in ~~43 hysteresis -
I
*
I
-
I
-
I
-
I
-
Bi-22WAg Tape : 4.2K B//Tape Surface :
1.4 1
0.8 : :
Sample #A I.
1.
12
8
I.
I.
I.
16
I.
20
I
.
24
Magnetic Field (1) Fig. 2. J&3 hysteresis in 23T-TOT-23T cycle at 4.2 K. After zero field cooling, J, was measured as soon as the field became stable at 23 T. J, measurement was repeated at stable fields in field decreasing sequence to 10 T and subsequent field increasing sequence.
W-221 2/Ag Tape 4.2K B//Tape Surface :
0.8 Time (min) Fig. 3. Relatronship between J, and holding duration at 22 and 20 T. Magnetic field was increased directly from 10 and 14T, respectively, without J, measurement at intermediate field and then kept stable.
538
H. KITAGUCHI et al.
Fig. 2) does not originate in the decrease of magnetic field, but in the fact that the sample was kept in high field for a long period to carry out several I-Y measurements. However, the jump of J, between 18.7 and 18.6 T (see Fig. 2) cannot be explained in terms of this time decay of J,. Below 18 T, time dependence of J, is typical for polycrystalline oxide superconductors. J, decreases with time with keeping a stable field in field decreasing sequence. For example, at 10 T in field decreasing sequence, J,, which is 138200A/cm2 in the initial state, decreases to 1366OOA/cm* with keeping for 8 min. Time decay of J, was confirmed for another sample at 2 1,22, and 23 T in field increasing sequence and at 16 T in field decreasing sequence. Figure 4 shows current-voltage (I-V) curves obtained in the J, measurement for the J,-time plot shown in Fig. 3 and for field decreasing sequences subsequent to holding stages at 22 and 20T. In both holding stages, the slope of Z-V curves corresponding to the superconductingnormal transition become steeper after J, drops than that in the first run of I-Vmeasurement. This fact suggests that current path changes with keeping the sample in high field probably because of a rearrangement of magnetic flux lines around grain boundaries or inside grains. Above 18.7 T in field decreasing sequences, the remarkable increase in J, cannot be confirmed. However, at 18.6 T, notable increase in J, is observed and the slope of the 1-V curve corresponding to the superconducting-normal transition becomes less steep than that above 18.7T. The J, jump between 18.7 and 18.6 T reaches more than 20% of J, at 18.7 T.
Sample #A Bi-2212IAg Tape, t=25pm, w&9mm 4.2K, B//Tape Surface
Magnetic Field (T)
c
18.8 18 + ('? 20 lmin Pmin 3min 4min
18.8 18.7 18.8 18.5 14
e
--A
12 Fig. 4. Current-voltage (Z-V) curves obtained in the .I, measurements for J,-time plot shown in Fig. 3 and for field decreasing sequences subsequent to holding stages at 22 and 20 T.
Anomaly
in J,-B hysteresis
539
All the results, which are shown in Figs 2-4, are summarized in Fig. 5. J,-B profiles for several sequences show good agreement and the jump of J, in field decreasing sequence between 19 and 18 T is strongly confirmed. This jump is observed in all of four field decreasing sequences. J,-B curves for field increasing sequences and field decreasing ones cross at a field between 18.6 and 18.7 T. Below 18 T, the J,-B curve for field decreasing sequences stays higher than that for field increasing sequences. This J,-B hysteresis shows similar behavior as a typical hysteresis for polycrystalline oxide superconductors containing weak-coupling between grains [l-5]. Above 20 T, initial J, values at each field, which were measured as soon as the field was generated from the lower magnetic field no more than 14 T without intermediate measurement, lie on the extrapolated curve of the J,-B relationship for field increase sequences below 18 T. With repeating the measurement and/or time, J, decreases and reaches the value corresponding to the J,-B curve for field decreasing sequences. Figure 6 shows J,-B hysteresis for sample B measured by using a hybrid magnet system installed in Tohoku University. Anomalous J,-B hysteresis such as time decay of J, at 22 T in the field increasing sequence and a jump of J, in the field decreasing sequence was observed. However, in this case, the jump J, is observed around 19.5 T, while it was observed between 18.6 and 18.7 T for sample A. The inversion and the anomaly of J,-B hysteresis and time decay of J, in high field which we report in this paper cannot be explained simply in terms of the distribution of trapped flux in grains and boundaries [l-5]. The anomaly in J,-B hysteresis suggests that there exists some transformation on pinning and/or electric coupling at the grain boundary. One possible explanation is as follows. A thin layer of secondary superconducting phase whose irreversible field or upper critical field is around 19 T forms at the grain boundary. Experimental results suggest that the magnitude of the magnetic field where the jump of J, appears depends on the geometric configuration of magnetic fields and/or samples. Anomalous J,-B hysteresis is thought
0.8 8
12
16
20
24
Magnetic Field (T) Fig. 5. JC-B profiles for several sequences show good agreement with each other. The jump ofJ, in field decreasing sequence between 19 and 18 T is strongly confirmed. An inversion of J,-B hysteresis can be seen. J,-B curves for field increasing and field decreasing sequence cross at a field between 18.6 and 18.7T.
540
H. KITAGUCHIet al.
I
Bi-2212IAg Tape B//Tape Surface
1
:
Sample#B
0.88 . ’ * ’ . ’ ’ ’ ’ ’ - ’ . ’ 12 16 20 24 Magnetic Field (T) Fig. 6. &-g profile for sample B. J, jump and the inversion of&B hysteresis were observed for another sample measured by using another magnet system.
to be related to magnetic field distribution in the sample. Further study on these points and the relationship between J,-B hysteresis and microstructure should be continued.
CONCLUSION
J,-B hysteresis for Bi-22 12/Ag tape at 4.2 K in high magnetic field applied parallel to the tape surface up to 23 T was studied. An anomaly in J,--B hysteresis was found around 19 T. Below 18 T, the J,-B curve for field decreasing sequences stays higher than that for field increasing sequences as in a typical hysteresis for polycrystalline oxide superconductor containing weak-coupling between each grains. However, J,-B curves for field increasing sequences and field decreasing ones cross at a field between 18.6 and 18.7 T. Above 20 T in field increasing sequence, initial J, values at each field, which were measured as soon as the field was generated from the lower magnetic field of no more than 14 T without intermediate measurement, lie on the extrapolated curve of the J,-B relationship for field increasing sequences below 18 T, however, J, decreases with time in contrast with a typical hysteresis where J, increases with time in the field increasing sequence. A J, jump of about 20% in its value was observed between 18.7 and 18.6 T in the field decreasing sequence. authors wish to express their appreciation to Dr J. C. Vallier, Laboratoire des Champs Magnetiques Intenses (LCMI), CNRSMPI, Grenoble, and Dr K. Watanabe, Institute for Materials Research, Tohoku University, for providing them with frequent opportunities to perform high field measurements. They are grateful also to Dr J. P Senateur, Laboratoire des Mattrlaux et du Genie Physique, INPG-ENSPG-URA, CNRS for tiuitful discussion and his assistance in the sample preparation.
Acknowledgements-The
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