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Preparation of 110 K BiPbSrCaCuO thin films by d.c. magnetron sputtering and their transport properties
Preparation of 110 K BiPbSrCaCuO thin films by d.c. magnetron sputtering and their transport properties Y.H. Wang, L. Li, Y.Z. Zhang, Y.Y. Zhao and P. Xu. Institute of Physics, Chinese Academy of Sciences, Beijing, China
Received 10 September 1990 Superconducting BiPbSrCaCuO thin films with a zero resistivity temperature of 110 K and a critical current density of 2733 A c m - 2 (at 77.02 K) were deposited on MgO(lO0) single crystal substrates by d.c. magnetron sputtering. After deposition at 3 5 0 - 4 0 0 ° C and post annealing at 8 4 0 ° C for 1 - 5 h in air, with different cooling rates, thin films were obtained which were highly c-axis orientated, with zero resistivity temperatures, Too, of 1 0 3 - 1 1 0 K. The thin film samples with a mixture of high (2223) and low (2212) To phases show a low T c onset and sharp transition in the resistivity-temperature ( R - T ) curves with relatively high current densities. The transport properties of the 1O0 K sample were studied. The temperature dependence of the critical current density exhibits weaklink characteristics. The mechanism of 2223 phase formation is also discussed.
Keywords: high Tc superconductivity; thin films; BiPbSrCaCuO
The superconducting BiPbSrCaCuO system has been found to consist of a series of phases. They can be expressed as Bi2Sr2Ca~,_l)Cu, O x (where x = 2n + 4), i.e. Bi2Sr2Ca2Cu30 x (2223) high Tc phase (110 K) for n = 3, Bi2Sr2Ca1Cu20~ (2212) low Tc phase (80 K) for n = 2 and Bi2Sr2CuOx (2201) phase (20 K) for n = 1. It has proved very difficult to obtain material with a high proportion of 2223 phase I. Since Takano e t a l . 2 obtained > 9 0 % of 2223 phase in a Pb-doped BiSrCaCuO system, many scientists have become interested in the preparation and properties of the Pbdoped Bi-based superconducting system. The roles of Pb are likely to be: 1, to decrease the melting point of the Bi compound and to widen the range of temperature for annealing; and 2, to enhance the formation of 2223 phase from 2212 phase. For the latter role, a slight excess of Ca and Cu from the ideal 2223 composition was reported to be effective. Laser ablation 3, ion beam sputtering 4, coevaporation 5, magnetron sputtering 6'7 and thermal decomposition 8 methods have all been employed to prepare the BiPbSrCaCuO films. Among these techniques, magnetron sputtering is considered to be the simplest method for preparation of the high Tc oxide thin films, as it can be carried out rather easily. In this paper, the d.c. magnetron sputtering of BiPbSrCaCuO films using a single planar target is described. A BiPbSrCaCuO film with Tc0 = 110 K has successfully been prepared. Alongside the various conditions used for preparation, characterization of the films by X-ray diffraction and scanning electron microscopy of the films is presented, together with the
superconductivity results. Laboratory manufactured d.c. magnetron sputtering apparatus was used. The substrates were polished MgO(100) single crystals. The target was a sintered disc of the BiPbSrCaCuO complex oxide, starting from the raw materials PBO(99%), Bi203(99.9%), CACO3(99.99%) and SRCO3(99%). A solid state reaction was used and the resulting nominal composition was Bi2 7PbxSr2Ca2.25Cu3.750~.6. It was found that the sintering conditions of the target are important for obtaining high quality films. The process involved calcination of the well ground and mixed powder in air at 800°C for 10 h, followed by pressing at a pressure of 3.5 x 10 4 N c m - : in a steel mould. The as pressed disc was then sintered at 830°C for > 10 h. Sputtering was carried out in an atmosphere of argon and oxygen ( A r - O 2 ratio =9:1) and the pressure was maintained at 0.01 torr* during film deposition. A sputtering voltage of 160 V and current of 0.2 A were used to obtain a plasma ring 40 mm in diameter on a 60 mm diameter disc target. The distance between the target and the substrate was 27 mm. The substrate temperature was varied from 250 to 500°C. The deposition rate was = 2 0 0 #, + min -1 and the film thickness varied from 8000 A to 1.5 #m. After deposition, the film was cooled to room temperature in 02 gas at 1 tort or an 02 and Ar mixed gas with an O z - A r ratio of 9:1 at 1 torr. The as sputtered film was annealed in a tube furnace at 8 3 0 - 8 6 0 ° C . After annealing the film was highly c-axis orientated. "1 t o r r = 1 3 3 . 3 2 2
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0011 2275/91/060439 -05 c~, 1 9 9 1 B u t t e r w o r t h H e i n e m a n n Ltd
Cryogenics 1991 Vol 31 June
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BiPbSrCaCuO thin films: Y.H. Wang et al.
Different annealing temperatures, annealing times and cooling rates were applied to study the phase formation in the films. The film structure was studied by X-ray diffraction (XRD), and scanning electron microscopy (SEM). The superconducting transition temperature was determined resistively by the standard four-probe method with silver paste contacts. A d.c. current of 10 #A was used to plot the R - T c u r v e . The a.c. susceptibility was also measured to determine the quality of the films. Figure 1 shows the temperature dependence of resistivity for the samples (A, B, C, D, E, F) annealed at different temperatures and cooled with different cooling rates. Curve A is for a sample with T~o = 80 K which was annealed at a relatively higher temperature of 860°C. Samples B, D and E, corresponding to curves B, D and E, were annealed at 840°C. Sample B was cooled rapidly from the annealing temperature to room temperature in 2 - 3 s. Sample D was annealed at 840°C for 3 h and quenched to room temperature in a quartz tube in air. Sample C (not referred to in Figure 1) was prepared under the same conditions as sample D, except that it was only annealed for 2 h. Sample E was annealed at 840°C for 3 h and cooled to room temperature at a rate of 50°C h-L Sample F was annealed at a lower temperature of 835 °C. Figure 2 shows the a.c. susceptibility of samples D and E. The curves show that the film with the higher T~ onset has a higher transitional temperature and relatively higher percentage of 2223 phase. This is consistent with the XRD result. Figure 3 shows the X-ray diffraction patterns of the films shown in Figure 1. It can be seen that with increase of the height of the 2223 peaks (i.e. with an increased percentage of 2223 phase), the T~ onset increases. All these samples contained both low Tc phase (2212) and high T~ phase (2223). Calculations from the 002L and 002H peaks showed that the volume fraction of 2223 phase in the sample obtained using the optimum anneallng temperature and time, with a low cooling rate of 50°C h -~, reached almost 90%. The lowest volume
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fraction of 2223 phase in the samples obtained was 35.4%, with a corresponding high T~0 of 107 K (samples C). There are large proportions of unknown phases in samples obtained using higher annealing temperatures. It seems that a slow cooling rate is beneficial for forming the high Tc phase. However, an unknown phase demarcated by a 20 peak at 21.9 ° on the XRD pattern was found. This unknown peak existed in sample E but was absent in the quenched samples C and D. Figure 4 shows the SEM micrographs of samples D and F, respectively. From these micrographs it can be seen that the sample with higher Tc onset and higher percentage of 2223 phase has clear grain boundaries with relatively large grains. The XRD patterns and the R - T transition curves indicated that there is an optimum annealing temperature for forming 2223 phase and that the relevant temperature range in the present experiment is only 3 - 4 ° C . Going beyond this temperature range tends to favour formation of 2212 and other oxide phases. From the experimental results, it can be seen that the multiphased Bi-based system is rather complicated. It is difficult to obtain a single 2223 phase film because of its sensitivity to composition and preparation conditions. Even at optimum composition it is difficult to obtain a film with a pure 2223 phase and high T~o and critical current density, Jc. The characteristics of sample E indicate that the nearly pure 2223 phase film with a higher T~ onset has a tail and that its current density is low ( < 102A cm-2). The XRD pattern shows
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the existence of an unknown peak of 20 at 21.9 ° . In contrast, sample D has a lower Tc onset, with a higher current density ( > 103A cm -2) and the film has no tail, with the absence of the unknown phase. The experimental results suggest that the 2223 phase may be formed in two steps. First, 2212 phase is formed because of the relative ease with which it can nucleate from the amorphous Bi-compound, compared to 2223 phase. As the annealing time becomes prolonged, the 2212 phase decomposes into 2223 phase due to the effect of Pb (compare samples C and D). The effect of Pb in decomposing 2212 phase into 2223 phase become rather prominent as the temperature decreases from 840 to 600°C, with slow cooling rates (see sample E). This is obvious from comparing the XRD pattern with the R - T curve and a.c. susceptibility. The R - T curves and a.c. susceptibility show only one transition stage, whereas the XRD patterns show the existence of both 2212 and 2223 phases in samples D and E. The authors propose
that the decomposition process of 2212 to 2223 phase starts from the outer portion of the 2212 grain and continues to the inner portion of the 2212 grain, eventually leaving small patches of 2212 phase enveloped by a thick layer of 2223 phase, so that the magnetic field cannot penetrate into the 2212 phase. As a whole, the sample exhibits 'pure' 2223 phase characteristics with respect to transport properties. However, as the annealing time is prolonged, the Pb evaporates and its level decreases. Consequently the outer portion of the 2223 phase envelope can be decomposed into other nonsuperconducting oxides, such as that shown by the unknown peak of 20 at 21.9 ° , resulting in dirty grain boundaries with impurity segregations, such as the unknown phase. Thus the thickness of the grain boundaries increases and the barrier height also increases, causing a decrease in the coupling energy of the weaklinks, which leads to the decrease in J~ with increasing volume fraction of 2223 phase. So it is difficult to
Cryogenics
1991
V o l 31 J u n e
441
BiPbSrCaCuO thin films: Y.H. Wang et al.
Figure 4
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prepare a high quality film with a high volume fraction of 2223 phase which can carry large transport current. An experiment was conducted to study the carrying capacity of the transport current of the film and its weaklink behaviour. Figure 5 shows a transition curve under zero field with a measuring current of 20 mA. The R - T curve was determined by a standard four-probe method with good electrical contact 9 to minimize contact heating and so obtain a more accurate result when measuring with a large current. The figure was plotted
using an X - Y recorder with different sensitivities. When the transition curve was close to zero resistance, a higher recording sensitivity was used, until the curve reached zero resistance under the highest sensitivity of the X - Y recorder. The transition can be described in terms of a network of strongly coupled islands of superconducting phase interconnected by grain boundary weak-links, with the islands responsible for tile main transition and the weak-links responsible for the tail. In the low current limit (10 #A, for example), the zero resistance state was achieved near the percolation threshold of the superconducting islands (see sample D in Figure 1). Full transition at higher measuring currents extends to lower temperatures, since the local current density should not exceed the critical current density of the weakest links in reaching file zero resistivity state. The authors consider the strong broadening of the tail with increased current to be due to a decrease in weak-link coupling energy, which is affected by the measuring current. Jc was determined directly from these /'co values with different measuring currents. It should be equal to the value obtained from I - V curves, since both are determined by the onset of finite resistance. Figure 6 shows the temperature dependence of Jc determined from Figure 5. The almost linear dependence on temperature of Jc indicates that the temperature dependence of the grain boundary critical current t° is different from the strong temperature dependence of the intragranular critical current near T~0H. The main transition of the superconducting islands did not shift with the 30 mA measuring current, which indicates the strength of the temperature dependence of the critical current near To0 and the large value of the critical current, i.e. Jc of this Bi-based film can increase markedly as the grain boundary condition improves. So, adding Pb to a bismuth compound enhanced the formation of 2223 phase from 2212 phase. On the other hand, with decreased amounts of Pb, the 2223 phase tends to decompose to other oxides which weaken the coupling between grain boundaries. It is concluded that Pb prohibits the improvement of Jc in the Pb-doped Bi-based system with almost pure 2223 phase.
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442
Cryogenics 1991 Vol 31 June
Figure 6 Temperature dependence of Jc determined directly f r o m a series of Tco values w i t h d i f f e r e n t measuring currents, as plotted in Figure 6
BiPbSrCaCuO thin films: Y.H. Wang et al.
Conclusions In summary, BiPbCaSrCuO films were prepared by d.c. magnetron sputtering using a single planar target. After depositing the films at 3 5 0 - 4 0 0 ° C and annealing at 840°C for 1 - 5 h in air, films were obtained exhibiting Tc onset at 122 K and T~o of 103 K with a tail. Samples exhibiting Tc onset at 115 K with Too of 110 K and Jc of 2733 A cm -2 at 77.02 K showed no tail. The authors conclude that the tailing effect with low critical current density is closely related to the low coupling energy between weak-links of grain boundaries. This arises from the barrier height or thickness of grain boundaries, enhanced by the unknown phase identified by the 20 peak at 21.9 °, and can be improved by quenching the annealed film in air. The critical current of the film shows weak-link behaviour and the coupling energy decreases with increase in the 2223 phase which leads to a drastic decrease in Jc in the film sample.
Acknowledgements This work was sponsored by the National Center for
R & D on Superconductivity and the Chinese National Natural Science Foundation.
References ! Madea, H., Takano, Y., Fukutomi, M. and Ansano, T. Jpn J Appl Phys (1988) 27 L209 2 Takano, M., Takada, J., Oda, K., Kitaguchi, H., Miura, Y., Ikeda, Y., Tomii, Y. and Mazaki, H. Jpn J Appl Phys (1988) 27
L1041 3 Kanai, M., Kawai, T. and Kawai, S. Jpn J Appl Phys (1988) 27 L1293 4 Harker, A.B., Kobrin, P.H., Morgan, P.E.D., DeNatele, J.F., Ratto, J.J., Gergis, I.S. and Howitt, D.G. Appl Phys Left (1988)
52 2186 5 Yoshitake, T., Satoh, T., Kubo, Y. and Igarashi, H. Jpn J Appl Phys (1988) 27 L1262 6 Hakuraku, Y., Arimode, Y., Miyagi, D. and Suresha, N.G. Jpn J Appl Phys (1988) 27 7 Hatta, S., Ichikawa, Y., Hirochi, K., Setsune, K., Adachi, H. and Wasa, K. Jpn J Appl Phys (1988) 27 L855 8 Nasu, H., Kato, T., Makida, S., Imura, T. and Osaka, Y. Jpn J Appl Phys (1988) 27 L2317 9 Wang, Y.H. et al. J Appl Phys in press 10 Goldschmidt, D. Phys Rev B (1989) 39 9139 11 Goldschmidt, D. Phys Rev B (1989) 39 2377
Cryogenics 1991 Vol 31 June
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Received 10 September 1990 Superconducting BiPbSrCaCuO thin films with a zero resistivity temperature of 110 K and a critical current density of 2733 A c m - 2 (at 77.02 K) were deposited on MgO(lO0) single crystal substrates by d.c. magnetron sputtering. After deposition at 3 5 0 - 4 0 0 ° C and post annealing at 8 4 0 ° C for 1 - 5 h in air, with different cooling rates, thin films were obtained which were highly c-axis orientated, with zero resistivity temperatures, Too, of 1 0 3 - 1 1 0 K. The thin film samples with a mixture of high (2223) and low (2212) To phases show a low T c onset and sharp transition in the resistivity-temperature ( R - T ) curves with relatively high current densities. The transport properties of the 1O0 K sample were studied. The temperature dependence of the critical current density exhibits weaklink characteristics. The mechanism of 2223 phase formation is also discussed.
Keywords: high Tc superconductivity; thin films; BiPbSrCaCuO
The superconducting BiPbSrCaCuO system has been found to consist of a series of phases. They can be expressed as Bi2Sr2Ca~,_l)Cu, O x (where x = 2n + 4), i.e. Bi2Sr2Ca2Cu30 x (2223) high Tc phase (110 K) for n = 3, Bi2Sr2Ca1Cu20~ (2212) low Tc phase (80 K) for n = 2 and Bi2Sr2CuOx (2201) phase (20 K) for n = 1. It has proved very difficult to obtain material with a high proportion of 2223 phase I. Since Takano e t a l . 2 obtained > 9 0 % of 2223 phase in a Pb-doped BiSrCaCuO system, many scientists have become interested in the preparation and properties of the Pbdoped Bi-based superconducting system. The roles of Pb are likely to be: 1, to decrease the melting point of the Bi compound and to widen the range of temperature for annealing; and 2, to enhance the formation of 2223 phase from 2212 phase. For the latter role, a slight excess of Ca and Cu from the ideal 2223 composition was reported to be effective. Laser ablation 3, ion beam sputtering 4, coevaporation 5, magnetron sputtering 6'7 and thermal decomposition 8 methods have all been employed to prepare the BiPbSrCaCuO films. Among these techniques, magnetron sputtering is considered to be the simplest method for preparation of the high Tc oxide thin films, as it can be carried out rather easily. In this paper, the d.c. magnetron sputtering of BiPbSrCaCuO films using a single planar target is described. A BiPbSrCaCuO film with Tc0 = 110 K has successfully been prepared. Alongside the various conditions used for preparation, characterization of the films by X-ray diffraction and scanning electron microscopy of the films is presented, together with the
superconductivity results. Laboratory manufactured d.c. magnetron sputtering apparatus was used. The substrates were polished MgO(100) single crystals. The target was a sintered disc of the BiPbSrCaCuO complex oxide, starting from the raw materials PBO(99%), Bi203(99.9%), CACO3(99.99%) and SRCO3(99%). A solid state reaction was used and the resulting nominal composition was Bi2 7PbxSr2Ca2.25Cu3.750~.6. It was found that the sintering conditions of the target are important for obtaining high quality films. The process involved calcination of the well ground and mixed powder in air at 800°C for 10 h, followed by pressing at a pressure of 3.5 x 10 4 N c m - : in a steel mould. The as pressed disc was then sintered at 830°C for > 10 h. Sputtering was carried out in an atmosphere of argon and oxygen ( A r - O 2 ratio =9:1) and the pressure was maintained at 0.01 torr* during film deposition. A sputtering voltage of 160 V and current of 0.2 A were used to obtain a plasma ring 40 mm in diameter on a 60 mm diameter disc target. The distance between the target and the substrate was 27 mm. The substrate temperature was varied from 250 to 500°C. The deposition rate was = 2 0 0 #, + min -1 and the film thickness varied from 8000 A to 1.5 #m. After deposition, the film was cooled to room temperature in 02 gas at 1 tort or an 02 and Ar mixed gas with an O z - A r ratio of 9:1 at 1 torr. The as sputtered film was annealed in a tube furnace at 8 3 0 - 8 6 0 ° C . After annealing the film was highly c-axis orientated. "1 t o r r = 1 3 3 . 3 2 2
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0011 2275/91/060439 -05 c~, 1 9 9 1 B u t t e r w o r t h H e i n e m a n n Ltd
Cryogenics 1991 Vol 31 June
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BiPbSrCaCuO thin films: Y.H. Wang et al.
Different annealing temperatures, annealing times and cooling rates were applied to study the phase formation in the films. The film structure was studied by X-ray diffraction (XRD), and scanning electron microscopy (SEM). The superconducting transition temperature was determined resistively by the standard four-probe method with silver paste contacts. A d.c. current of 10 #A was used to plot the R - T c u r v e . The a.c. susceptibility was also measured to determine the quality of the films. Figure 1 shows the temperature dependence of resistivity for the samples (A, B, C, D, E, F) annealed at different temperatures and cooled with different cooling rates. Curve A is for a sample with T~o = 80 K which was annealed at a relatively higher temperature of 860°C. Samples B, D and E, corresponding to curves B, D and E, were annealed at 840°C. Sample B was cooled rapidly from the annealing temperature to room temperature in 2 - 3 s. Sample D was annealed at 840°C for 3 h and quenched to room temperature in a quartz tube in air. Sample C (not referred to in Figure 1) was prepared under the same conditions as sample D, except that it was only annealed for 2 h. Sample E was annealed at 840°C for 3 h and cooled to room temperature at a rate of 50°C h-L Sample F was annealed at a lower temperature of 835 °C. Figure 2 shows the a.c. susceptibility of samples D and E. The curves show that the film with the higher T~ onset has a higher transitional temperature and relatively higher percentage of 2223 phase. This is consistent with the XRD result. Figure 3 shows the X-ray diffraction patterns of the films shown in Figure 1. It can be seen that with increase of the height of the 2223 peaks (i.e. with an increased percentage of 2223 phase), the T~ onset increases. All these samples contained both low Tc phase (2212) and high T~ phase (2223). Calculations from the 002L and 002H peaks showed that the volume fraction of 2223 phase in the sample obtained using the optimum anneallng temperature and time, with a low cooling rate of 50°C h -~, reached almost 90%. The lowest volume
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Cryogenics 1991 Vol 31 June
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Figure 3 XRD patterns of samples: (a) A; (b) B; (c) C; (d) D; (e) E; and (f) F, as shown in Figure 1. L denotes the low Tc phase, H the high Tc phase, x the unknown phases and S the substrate, Sample C has the same preparation conditions as D, except that it was annealed for 2 h; it exhibits a Too of 107 K
the existence of an unknown peak of 20 at 21.9 ° . In contrast, sample D has a lower Tc onset, with a higher current density ( > 103A cm -2) and the film has no tail, with the absence of the unknown phase. The experimental results suggest that the 2223 phase may be formed in two steps. First, 2212 phase is formed because of the relative ease with which it can nucleate from the amorphous Bi-compound, compared to 2223 phase. As the annealing time becomes prolonged, the 2212 phase decomposes into 2223 phase due to the effect of Pb (compare samples C and D). The effect of Pb in decomposing 2212 phase into 2223 phase become rather prominent as the temperature decreases from 840 to 600°C, with slow cooling rates (see sample E). This is obvious from comparing the XRD pattern with the R - T curve and a.c. susceptibility. The R - T curves and a.c. susceptibility show only one transition stage, whereas the XRD patterns show the existence of both 2212 and 2223 phases in samples D and E. The authors propose
that the decomposition process of 2212 to 2223 phase starts from the outer portion of the 2212 grain and continues to the inner portion of the 2212 grain, eventually leaving small patches of 2212 phase enveloped by a thick layer of 2223 phase, so that the magnetic field cannot penetrate into the 2212 phase. As a whole, the sample exhibits 'pure' 2223 phase characteristics with respect to transport properties. However, as the annealing time is prolonged, the Pb evaporates and its level decreases. Consequently the outer portion of the 2223 phase envelope can be decomposed into other nonsuperconducting oxides, such as that shown by the unknown peak of 20 at 21.9 ° , resulting in dirty grain boundaries with impurity segregations, such as the unknown phase. Thus the thickness of the grain boundaries increases and the barrier height also increases, causing a decrease in the coupling energy of the weaklinks, which leads to the decrease in J~ with increasing volume fraction of 2223 phase. So it is difficult to
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BiPbSrCaCuO thin films: Y.H. Wang et al.
Figure 4
SEM photographs of (a) sample D and (b) sample E, as
s h o w n in Figure 1
prepare a high quality film with a high volume fraction of 2223 phase which can carry large transport current. An experiment was conducted to study the carrying capacity of the transport current of the film and its weaklink behaviour. Figure 5 shows a transition curve under zero field with a measuring current of 20 mA. The R - T curve was determined by a standard four-probe method with good electrical contact 9 to minimize contact heating and so obtain a more accurate result when measuring with a large current. The figure was plotted
using an X - Y recorder with different sensitivities. When the transition curve was close to zero resistance, a higher recording sensitivity was used, until the curve reached zero resistance under the highest sensitivity of the X - Y recorder. The transition can be described in terms of a network of strongly coupled islands of superconducting phase interconnected by grain boundary weak-links, with the islands responsible for tile main transition and the weak-links responsible for the tail. In the low current limit (10 #A, for example), the zero resistance state was achieved near the percolation threshold of the superconducting islands (see sample D in Figure 1). Full transition at higher measuring currents extends to lower temperatures, since the local current density should not exceed the critical current density of the weakest links in reaching file zero resistivity state. The authors consider the strong broadening of the tail with increased current to be due to a decrease in weak-link coupling energy, which is affected by the measuring current. Jc was determined directly from these /'co values with different measuring currents. It should be equal to the value obtained from I - V curves, since both are determined by the onset of finite resistance. Figure 6 shows the temperature dependence of Jc determined from Figure 5. The almost linear dependence on temperature of Jc indicates that the temperature dependence of the grain boundary critical current t° is different from the strong temperature dependence of the intragranular critical current near T~0H. The main transition of the superconducting islands did not shift with the 30 mA measuring current, which indicates the strength of the temperature dependence of the critical current near To0 and the large value of the critical current, i.e. Jc of this Bi-based film can increase markedly as the grain boundary condition improves. So, adding Pb to a bismuth compound enhanced the formation of 2223 phase from 2212 phase. On the other hand, with decreased amounts of Pb, the 2223 phase tends to decompose to other oxides which weaken the coupling between grain boundaries. It is concluded that Pb prohibits the improvement of Jc in the Pb-doped Bi-based system with almost pure 2223 phase.
4C
c~
3£
E
o o
E
o no I0 o O 0
I 50
I00
150 200 Temperature (K)
250
300
0 70
I 80
I 90
I I00
o
0 I I10
120
T(K)
Figure 5
R - T curve plotted using an X - Y recorder w i t h a measuring current of 2 0 mA. The l o w t e m p e r a t u r e part of the tail is amplified relative t o the main transition part, w i t h a ten-fold increase in sensitivity for each line
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Cryogenics 1991 Vol 31 June
Figure 6 Temperature dependence of Jc determined directly f r o m a series of Tco values w i t h d i f f e r e n t measuring currents, as plotted in Figure 6
BiPbSrCaCuO thin films: Y.H. Wang et al.
Conclusions In summary, BiPbCaSrCuO films were prepared by d.c. magnetron sputtering using a single planar target. After depositing the films at 3 5 0 - 4 0 0 ° C and annealing at 840°C for 1 - 5 h in air, films were obtained exhibiting Tc onset at 122 K and T~o of 103 K with a tail. Samples exhibiting Tc onset at 115 K with Too of 110 K and Jc of 2733 A cm -2 at 77.02 K showed no tail. The authors conclude that the tailing effect with low critical current density is closely related to the low coupling energy between weak-links of grain boundaries. This arises from the barrier height or thickness of grain boundaries, enhanced by the unknown phase identified by the 20 peak at 21.9 °, and can be improved by quenching the annealed film in air. The critical current of the film shows weak-link behaviour and the coupling energy decreases with increase in the 2223 phase which leads to a drastic decrease in Jc in the film sample.
Acknowledgements This work was sponsored by the National Center for
R & D on Superconductivity and the Chinese National Natural Science Foundation.
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52 2186 5 Yoshitake, T., Satoh, T., Kubo, Y. and Igarashi, H. Jpn J Appl Phys (1988) 27 L1262 6 Hakuraku, Y., Arimode, Y., Miyagi, D. and Suresha, N.G. Jpn J Appl Phys (1988) 27 7 Hatta, S., Ichikawa, Y., Hirochi, K., Setsune, K., Adachi, H. and Wasa, K. Jpn J Appl Phys (1988) 27 L855 8 Nasu, H., Kato, T., Makida, S., Imura, T. and Osaka, Y. Jpn J Appl Phys (1988) 27 L2317 9 Wang, Y.H. et al. J Appl Phys in press 10 Goldschmidt, D. Phys Rev B (1989) 39 9139 11 Goldschmidt, D. Phys Rev B (1989) 39 2377
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