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Critical current densities of high-Tc superconductors (Cu,Cr)Sr2Can−1CunO2n+3 (n=2,3) prepared under high pressure
Physica C 316 Ž1999. 69–76
Critical current densities of high-Tc superconductors žCu,Cr /Sr2 Ca ny1Cu nO 2 nq3 žn s 2,3 / prepared under high pressure S. Yu, S.M. Loureiro 1, E. Takayama-Muromachi
)
National Institute for Research in Inorganic Materials, Namiki 1-1, Tsukuba, Ibaraki, 305-0044 Japan Received 28 January 1999; received in revised form 24 February 1999; accepted 15 March 1999
Abstract Critical current densities Ž Jc . and irreversibility fields Ž Birr . of high-Tc superconductors ŽCu,Cr.Sr2 Ca ny1Cu nO 2 nq3 Ž n s 2,3. prepared under high pressure were determined from the hysteresis in DC magnetization loop. They show higher Jc values than other high-Tc superconductors such as YBa 2 Cu 3 O 7 , Bi-phases, Hg-phases, ŽCu,C.-based phases, etc. for the almost entire range of temperature below Tc . Their Birr values are also very high; at 77 K, for instance, Birr of ŽCu,Cr.-1223 was over 5 T. In addition to the polycrystalline bulk samples, magnetically grain aligned samples of the ŽCu,Cr.-phases were investigated to discuss superconductivity anisotropy. The good Jc-related properties of the ŽCu,Cr.-phases seem to be caused by their less anisotropic electronic structures and sufficient number of effective pinning centers due to the random arrangement of Cu and Cr in the blocking layer. q 1999 Published by Elsevier Science B.V. All rights reserved. Keywords: High-Tc superconductor; Magnetic hysteresis; Critical current densities; Irreversibility field; ŽCu,Cr.Sr2 Ca ny1Cu nO 2 nq3 Ž n s 2,3.
1. Introduction High critical current density Ž Jc . under a magnetic field and high irreversibility field Ž Birr . are required for practical applications of superconductors. In most high-Tc superconductors, however, Jc under a magnetic field decreases rapidly with increasing temperature due to dimensional crossover in vortex structures from three-dimensional Ž3D. to quasi two-dimensional Ž2D. w1,2x. Thus, many efforts have been made to increase Jc and it is indispensable ) Corresponding author. Tel.: q81-298-58-5650; Fax: q81298-52-7449; E-mail: [email protected] 1 Present address: Department of Chemistry, Princeton University, Bowen Hall, 70 Prospect Avenue, Princeton, NJ 08540, USA.
for that purpose to find out Jc-determining factors by comparing various high-Tc superconductors. Recently, we have had many high-Tc superconductors with Tc ) 100 K thanks to application of high-pressure synthesis techniques w3x, and their systematic evaluation seems crucial for the understanding of the Jc-related physics. Nevertheless, only a limited number of reports have been published thus far in this field of research. In the case of ŽCu,C.Ba 2Ca ny 1Cu nO 2 nq3 wŽCu,C.-12Ž n y 1. n; n s 3,4x series synthesized under high pressure, they show much higher Jc and Birr than Bi 2 Sr2 Ca ny1Cu nO 2 nq4 wBi22Ž n y 1. n x and HgBa 2 Ca ny1Cu nO 2 nq2q d wHg12Ž n y 1. n x series w4,5x suggesting less anisotropic electronic structures of the ŽCu,C.-phases. Though their Jc values under magnetic field are substantially lower than that of YBa 2 Cu 3 O 7 ŽY-123., neutron
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irradiation was quite effective to introduce pinning centers and drastically enhanced both Jc and Birr w5x. The evaluation of Jc-related properties was also carried out for high-pressure phases of BSr2Ca ny 1Cu nO 2 nq3 wB-12Ž n y 1. n x w6,7x. It was firstly expected that they had good Jc-related properties because of small separation between the conduction layers due to small B 3q ion size. However, their Jc and Birr values were much lower than those of ŽCu,C.-12Ž n y 1. n and Y-123, implying high electric anisotropy. Recently, a new series of high-Tc superconductors ŽCu,Cr.Sr2 Ca ny1Cu nO 2 nq3 wŽCu,Cr.-12Ž n y 1. n; n s 1–9x was synthesized under a high-pressurer high-temperature condition as polycrystalline form w8x. These phases have layered structures composed of stacking of planes, SrO–MO–SrO–CuO 2 – ŽCa– CuO 2 . ny1 where the M site is shared by Cu and Cr with nearly 1:1 ratio. The occupation of Cu at the M site in these phases is a characteristic common to ŽCu,C.-12Ž n y 1. n and Y-123 both of which have high Jc and Birr . The Cu and Cr atoms are placed randomly in the ŽCu,Cr.O plane of ŽCu,Cr.-12Ž n y 1. n in contrast to Cu–C–Cu–C . . . type ordering in the ŽCu,C.O plane of ŽCu,C.-12Ž n y 1. n w9x. In addition, excess oxygen and copper vacancies were reported to be introduced in the ŽCu,Cr.O plane w8x. These structural defects may work as effective pinning centers. Thus, we expect the ŽCu,Cr.-based system to have higher Jc than that of the ŽCu,C.12Ž n y 1. n series and other known superconductors. In the present study, we have carried out magnetization measurements for bulk form of ŽCu,Cr.-12Ž n y 1. n Ž n s 2,3. samples to evaluate their Jc and Birr and the results are compared with other superconductors such as Y-123, Hg-12Ž n y 1. n, ŽCu,C.-12Ž n y 1., B-12Ž n y 1. n, etc. Anisotropy in magnetization hysteresis is also discussed based on magnetization data of grain aligned samples.
2. Experimental Polycrystalline samples of ŽCu,Cr.-1212 and 1223 used in the present study were prepared under a high-pressurerhigh-temperature condition of 6 GPa and 12008C. Details of the sample preparation are described elsewhere w8x. The X-ray powder diffrac-
tion patterns showed that the ŽCu,Cr.-1223 sample was single phase while ŽCu,Cr.-1212 sample contained an unknown additional impurity phase. The impurity was estimated ; 5% in content but no correction for the magnetization hysteresis data was applied since the exact content was unknown and ; 5% uncertainty does not affect the discussion below. Onset superconducting transition temperatures ŽTc . determined by magnetic susceptibility data were 81 K and 103 K for ŽCu,Cr.-1212 and 1223, respectively, and are in good agreement with the previous reports w8x. For grain alignment, a well-pulverized powder sample and epoxy Žstycast 1266. were mixed then cured in magnetic field of 5–7 T at room temperature for 12 h w10x. The degree of grain alignment was checked by X-ray diffraction. Magnetization hystereses Ž D M . were measured using a SQUID magnetometer ŽQuantum Design. for the bulk and aligned samples with magnetic field up to 5 T at various temperatures. For the latter samples, magnetic field was applied either parallel or perpendicular to the c-axis. The microstructures of the bulk and aligned samples were observed using a scanning electron microscope ŽSEM. to determine their average grain sizes. The grains were found to be disk-like with an average diameter of ; 3 mm in the bulk samples and ; 1 mm in the aligned samples.
3. Results and discussion 3.1. Bulk samples Many Jc and Birr data based on magnetic measurements were determined for sintered polycrystalline samples, and therefore we discuss firstly the data obtained for the bulk ŽCu,Cr.-samples and compare them with previously reported values. Fig. 1 shows typical examples of magnetic hysteresis loop observed for ŽCu,Cr.-1223. Both ŽCu,Cr.-1212 and ŽCu,Cr.-1223 have quite large magnetization hysteresis D M especially in low temperature region. Since the SEM observations indicated that the average grain diameter for the bulk samples was not so large, the large D M value implies large Jc , and therefore good pinning properties of the present system. Fur-
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Fig. 1. Magnetic hysteresis loops for the bulk ŽCu,Cr.-1223 sample at 5 K, 15 K and 25 K.
thermore, temperature dependences of D M are far less pronounced compared with Bi-2212 w11x, Hg1223 w12x and B-12Ž n y 1. n Ž n s 3,4. w6x, etc. suggesting high Birr values. In order to estimate Jc and Birr from the D M data, Bean’s model w13x was used. Since the intergrain critical current density can be neglected in the present samples due to weak coupling between grains, the intragrain critical current density may be calculated as, Jc s 30 D Mrd,
Ž 1.
where d means the average grain size in diameter which is 3 mm for both ŽCu,Cr.-1212 and 1223 according to the SEM observations. Magnetic field dependencies of Jc thus calculated are shown in Fig. 2Ža. and Žb. for ŽCu,Cr.-1212 and 1223, respectively. In low-temperature region, both phases have extremely high Jc values, and furthermore magnetic field dependence is quite small suggesting presence of enough effective pinning centers in number. This is in contrast to the results for the ŽCu,C.-12Ž n y 1. n phase whose Jc is much smaller and presents large decay with increasing magnetic field even at low temperatures w4x. With increasing temperature, the field dependence of Jc becomes larger in ŽCu,Cr.-1212 and 1223, but its degree is
less pronounced implying once more high Birr values Žsee below.. The Jc at 1 T is shown in Fig. 3 for the ŽCu,Cr.phases as a function of normalized temperature, TrTc . Data previously reported for Bi-2212 w11x, Hg-1223 w12x B-12Ž n y 1. n w6x, Y-123 w4x, ŽCu,C.12Ž n y 1. n w4x are also given for comparison. The Jc values of Hg-1223 in Ref. w12x were calculated by an equation slightly different from Eq. Ž1., and for direct comparison, we recalculated them by Eq. Ž1. using the original D M data in Ref. w12x. Among all the phases shown, ŽCu,Cr.-1212 and 1223 show the largest Jc value in the almost entire range of TrTc . Although crystal structures of ŽCu,Cr.-12Ž n y 1. n series are closely related to those of ŽCu,C.-12Ž n y 1. n series w8,9x, their Jc values are very different as seen in Fig. 3. If we compare ŽCu,Cr.-1223 and ŽCu,C.-1223, for instance, the former phase has one order of magnitude larger Jc at 1 T than the latter. High-resolution electron microscopy together with composition analysis indicated that Cu and C atoms in the ŽCu,C.O charge reservoir plane in ŽCu,C.12Ž n y 1. n are placed in an ordered way along the a-axis as Cu–C–Cu–C . . . w9x. On the other hand, in the case of ŽCu,Cr.-1212 and 1223, the Cu and Cr atoms within the charge reservoir are placed randomly or the ordering occurs, if any, very locally
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contains Cu-vacancies and excess oxygen atoms w8x. These results indicate that the ŽCu,Cr.-12Ž n y 1. n phases contain a higher density of crystal defects than the ŽCu,C.-12Ž n y 1. n ones. These defects are expected to work as effective pinning centers and seem to explain why Jc ’s of ŽCu,Cr.-phases are much larger than those of ŽCu,C.-phases and other superconductors. The Jc values of ŽCu,Cr.-1212 are smaller than those of ŽCu,Cr.-1223 at low temperatures, suggesting a relatively smaller volume pinning force of ŽCu,Cr.-1212 which may be caused by a lower density of crystal defects in the 1212 phase. The Cr-based phases show much less temperature dependence of Jc compared with the Bi-, Hg- and B-based phases. At high temperatures near Tc , however, fairly large decay of Jc is seen in both ŽCu,Cr.1212 and 1223 and the Y-123 phase shows a better property in this region of TrTc . The temperature dependence of Jc is closely related to the anisotropy of electronic structure of a superconductor, and in
Fig. 2. Magnetic field dependence of critical current density, Jc , at various temperatures for the bulk ŽCu,Cr.-1212 Ža. and ŽCu,Cr.1223 Žb. samples.
and imperfectly w8x. Furthermore, electron microprobe analysis suggested that the ŽCu,Cr.O plane
Fig. 3. Variation of critical current density, Jc , under 1 T as a function of T r Tc for the bulk ŽCu,Cr.-1212 and ŽCu,Cr.-1223 samples compared with previous data for Bi-2212 w11x, Hg-1223 Žsee text., B-12Ž ny1. n w6x, Y-123 w4x and ŽCu,C.-12Ž ny1. n w4x.
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Fig. 4. Variation of the irreversibility field, Birr , as a function of T r Tc for the bulk ŽCu,Cr.-1212 and ŽCu,Cr.-1223 samples compared with previous data for Bi-2212 w11x, Hg-1223 Žsee text., B-12Ž ny1. n w6x, Y-123 w4x, ŽCu,C.-12Ž ny1. n w4x.
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order to discuss this more quantitatively, the irreversibility field was calculated according to the Jc s 10 3 Arcm2 criterion. Fig. 4 shows Birr plotted against TrTc for the Cr-based phases and other superconductors. The Birr data of Hg-1223 were originally given by a criterion different from Jc s 10 3 Arcm2 w12x. Therefore, we redetermined them according to the Jc s 10 3 Arcm2 criterion for the new Jc values recalculated by Eq. Ž1.. The Birr values of the Cr-based phases are quite high; at 77 K, for instance, Birr of ŽCu,Cr.-1223 is over 5 T as seen in Fig. 2Žb.. The superconductors in Fig. 4 can be categorized into two groups; the first group includes the ŽCu,Cr.- and ŽCu,C.-phases and Y-123 with Cu-containing blocking layers while the second one does Hg-, B- and Bi-phases with Cu-free blocking layers. As seen clearly in Fig. 4, the former group phases have much higher Birr ’s than the latter. The Birr is related to the dimensional crossover of vortex structure form 3D to 2D and if a system has a highly anisotropic electronic structure, this crossover occurs at a relatively low temperature and low magnetic field causing low Birr . In the M-12Ž n y 1. n structure, the Cu atoms of adjacent conduction layers
Fig. 5. X-ray powder diffraction patterns for the grain aligned ŽCu,Cr.-1212 and ŽCu,Cr.-1223 samples. A peak marked by ‘U ’ belongs to an impurity phase.
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are connected through M and the apical oxygen atoms as –Cu–O–M–O–Cu– along the c-axis. Therefore, the M-site occupation of Cu which is the same metal as the conduction layer contains, will greatly contribute to increase electric conductivity along the c-axis. We believe that the less anisotropic electronic structure is the main reason for the higher Birr values of the former group. In the former group phases with the Cu-containing blocking layers, the irreversibility lines of ŽCu,Cr.-phases are located between those of ŽCu,C.phases and Y-123. In the Y-123 structure, the M-sites are fully occupied by Cu while only 50% in other
phases and this fact accounts well for the highest Birr of Y-123. The Cr-based phases have higher Birr ’s than the ŽCu,C.-phases and the difference looks quite large. As stated before, this result seems to be caused by the high density of crystal defects in the Cr-based phases. It is worth noting here that the Jc-related properties were improved drastically in the ŽCu,C.phases after introduction of pinning centers by neutron irradiation w5x. The ŽCu,Cr.-1212 has slightly larger Birr than ŽCu,Cr.-1223. The former phase has an overdoping state of holes while the latter does the optimum doping state judging from their Tc ’s w8x. The overdoping of carriers diminishes electric ani-
Fig. 6. Magnetic field dependence of the anisotropy ratio, R s D M 5rD M H , for the grain aligned ŽCu,Cr.-1212 Ža. and ŽCu,Cr.-1223 Žb. samples.
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sotropy of a superconductors w14x and this may account for slightly larger Birr of ŽCu,Cr.-1212. 3.2. Grain-aligned samples Fig. 5 shows powder X-ray diffraction patterns of the magnetically grain-aligned samples for ŽCu,Cr.1212 and 1223 which were taken for planes perpendicular to the applied magnetic field. Most of peaks can be indexed as 00 l reflections of the ŽCu,Cr.12Ž n y 1. n-type structures. A small number of h / 0 or k / 0 peaks such as 110, 104, 103, etc. are also observed but their intensities are minimal. A fairly strong peak denoted by ‘U ’ in ŽCu,Cr.-1212 does not belong to the ŽCu,Cr.-phases and is ascribed to an unknown impurity phase. The X-ray patterns indicate that the grains of each phase were aligned almost completely with the c-axis parallel to the applied magnetic field. Magnetization hystereses of the ŽCu,Cr.-1212 and 1223 grain aligned samples were observed for two Fig. 8. Variation of the in-plane critical current density, Jc Ž I H , B 5 ., under 1 T as a function of T r Tc for the grain aligned ŽCu,Cr.-1212 and ŽCu,Cr.-1223 samples compared with previous data for B-12Ž ny1. n Ž ns 3–5. w7x.
Fig. 7. Variation of the crossover magnetic field, B0 , as a function of T r Tc for the grain aligned ŽCu,Cr.-1212 and ŽCu,Cr.-1223 samples compared with previous data for B-12Ž ny1. n Ž ns 3–5. w7x.
different directions of magnetic field, parallel Ž B 5 . and perpendicular Ž B H . to the c-axis. In order to discuss superconductivity anisotropy, an anisotropy factor R was calculated as R s D M 5rD M H w7x for the magnetic hystereses D M 5 and D M H corresponding to B 5 and B H , respectively. The R-value thus obtained is plotted against magnetic field in Fig. 6Ža. and Žb.. At low temperatures ŽT F 35 K for ŽCu,Cr.-1212 and T F 55 K for ŽCu,Cr.-1223., the R-value is larger than 2 for the entire range of magnetic field up to 5 T, showing the relation D M 5 ) D M H . At higher temperatures, the R vs. B curve crosses the R s 1 line at a certain magnetic field Ž B0 ., i.e., the relation between D M 5 and D M H is reversed to D M 5 - D M H for B ) B0 . Recently, we obtained similar results for B-12Ž n y 1. n Ž n s 3–5. aligned samples and suggested that the crossover in D M 5 and D M H is caused by different vortex structures corresponding to magnetic field parallel and perpendicular to the c-axis. The D M 5 term is related to only one kind of critical
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current density, Jc Ž I H , B 5 ., with electric current perpendicular to Ž I H . and magnetic field parallel to the c-axis. On the other hand, the D M H term is governed by two kinds of critical current densities, Jc Ž I 5 , B H . and Jc Ž I H , B H . which are in-plane Ž I 5 . and c-axis direction components of Jc Ž B H ., respectively. We suggested that serious decay of Jc Ž I H , B 5 . due to the formation of a 2D pancake vortex structure causes the crossover in D M 5 and D M H in a high temperature region w7x. It was also suggested that there is a correlation between the B0-value and the electric anisotropy of a system, i.e., a less anisotropic system shows a larger B0 value with the crossover occurring at a higher temperature and a higher magnetic field w7x. The crossover field B0 is plotted against TrTc in Fig. 7 for ŽCu,Cr.-1212 and 1223 together with B-12Ž n y 1. n Ž n s 3–5.. The B0 vs. TrTc lines of the ŽCu,Cr.-phases shift largely to the right compared with those of the B-phases. The Jc Ž I H , B 5 . value at 1 T was calculated using D M 5 and d s 1 mm based on the Bean’s model for the ŽCu,Cr.-phases and is shown in Fig. 8 compared with the B-system. It is seen from this figure that the Jc Ž I H , B 5 . values of the B-phases show much steeper decrease with increasing temperature in a high temperature region than those of the ŽCu,Cr.-phases, causing the small B0 values of the B-phases. All these results indicate that the present ŽCu,Cr.-phases have less anisotropic electronic structures and decay of Jc Ž I H , B 5 . due to decoupling of the pancakes of vortices is far less pronounced.
4. Conclusion We have carried out DC magnetization measurements for ŽCu,Cr.-12Ž n y 1. n Ž n s 2,3. in order to evaluate Jc , Birr and superconductivity anisotropy. The present ŽCu,Cr.-based phases have much higher Jc ’s under magnetic field for the almost entire range of temperature compared with other superconductors such as Bi-2212, Hg-1223, B-12Ž n y 1. n Ž n s 3–5., ŽCu,C.-12Ž n y 1. n Ž n s 3,4., Y-123, etc. The ŽCu,Cr.-phases also present high Birr values with the Birr vs. TrTc lines located between those of the Y-123 and ŽCu,C.-phases. The crossover between D M 5 and D M H was observed and it occurs at much
higher temperature and higher magnetic field compared with the B-12Ž n y 1. n system. These results indicated that ŽCu,Cr.-based oxides have less anisotropic electronic structures and good pinning properties due to enough effective pinning centers in number introduced by the irregular arrangement of the Cu and Cr atoms in the blocking layer.
Acknowledgements The authors express their sincere thanks to Drs. M. Akaishi and S. Yamaoka of NIRIM for helpful suggestions on high-pressure synthesis of the ŽCu,Cr.-12Ž n y 1. n samples. This work was supported by the Multi-core Project, the COE project and Special Coordination Funds of the Science and Technology Agency of the Japanese Government.
References w1x J.R. Clem, Rev. B 43 Ž1991. 7837. w2x D.H. Kim, K.E. Gray, R.T. Kampwirth, J.C. Smith, D.S. Richeson, T.J. Marks, J.H. Kang, J. Talvacchio, M. Eddy, Physica C 177 Ž1991. 431. w3x E. Takayama-Muromachi, Chem. Mater. 10 Ž1988. 2686. w4x H. Kumakura, K. Togano, T. Kawashima, E. TakayamaMuromachi, Physica C 226 Ž1994. 222. w5x H. Kumakura, H. Kitaguchi, K. Togano, T. Kawashima, E. Takayama-Muromachi, S. Okayasu, Y. Kazumata, IEEE Trans. Appl. Supercond. 5 Ž1995. 1399. w6x S. Yu, Y. Okuda, T. Kawashima, E. Takayama-Muromachi, Jpn. J. Appl. Phys. 35 Ž1996. 3378. w7x S. Yu, T. Kawashima, Y. Uchida, E. Takayama-Muromachi, Jpn. J. Appl. Phys. 37 Ž1998. 5535. w8x S.M. Loureiro, Y. Matsui, E. Takayama-Muromach, Physica C 302 Ž1998. 244. w9x T. Kawashima, Y. Matsui, E. Takayama-Muromachi, Physica C 224 Ž1994. 69. w10x D.E. Farrell, B.S. Chandrasekhar, M.R. DeGuire, M.M. Fang, V.G. Kogan, J.R. Clem, D.K. Finnemore, Phys. Rev. B 36 Ž1987. 4025. w11x K. Kumakura, H. Kitaguchi, K. Togano, H. Maeda, J. Shimoyama, S. Okayasu, Y. Kazumata, J. Appl. Phys. 74 Ž1993. 451. w12x U. Welp, G.W. Crabtree, J.L. Wagner, D.G. Hinks, Physica C 218 Ž1993. 373. w13x C.P. Bean, Phys. Rev. Lett. 8 Ž1962. 250. w14x T. Kimura, K. Kishio, T. Kobayashi, Y. Nakayama, N. Motohira, K. Kitazawa, K. Yamafuji, Physica C 192 Ž1992. 247.
Critical current densities of high-Tc superconductors žCu,Cr /Sr2 Ca ny1Cu nO 2 nq3 žn s 2,3 / prepared under high pressure S. Yu, S.M. Loureiro 1, E. Takayama-Muromachi
)
National Institute for Research in Inorganic Materials, Namiki 1-1, Tsukuba, Ibaraki, 305-0044 Japan Received 28 January 1999; received in revised form 24 February 1999; accepted 15 March 1999
Abstract Critical current densities Ž Jc . and irreversibility fields Ž Birr . of high-Tc superconductors ŽCu,Cr.Sr2 Ca ny1Cu nO 2 nq3 Ž n s 2,3. prepared under high pressure were determined from the hysteresis in DC magnetization loop. They show higher Jc values than other high-Tc superconductors such as YBa 2 Cu 3 O 7 , Bi-phases, Hg-phases, ŽCu,C.-based phases, etc. for the almost entire range of temperature below Tc . Their Birr values are also very high; at 77 K, for instance, Birr of ŽCu,Cr.-1223 was over 5 T. In addition to the polycrystalline bulk samples, magnetically grain aligned samples of the ŽCu,Cr.-phases were investigated to discuss superconductivity anisotropy. The good Jc-related properties of the ŽCu,Cr.-phases seem to be caused by their less anisotropic electronic structures and sufficient number of effective pinning centers due to the random arrangement of Cu and Cr in the blocking layer. q 1999 Published by Elsevier Science B.V. All rights reserved. Keywords: High-Tc superconductor; Magnetic hysteresis; Critical current densities; Irreversibility field; ŽCu,Cr.Sr2 Ca ny1Cu nO 2 nq3 Ž n s 2,3.
1. Introduction High critical current density Ž Jc . under a magnetic field and high irreversibility field Ž Birr . are required for practical applications of superconductors. In most high-Tc superconductors, however, Jc under a magnetic field decreases rapidly with increasing temperature due to dimensional crossover in vortex structures from three-dimensional Ž3D. to quasi two-dimensional Ž2D. w1,2x. Thus, many efforts have been made to increase Jc and it is indispensable ) Corresponding author. Tel.: q81-298-58-5650; Fax: q81298-52-7449; E-mail: [email protected] 1 Present address: Department of Chemistry, Princeton University, Bowen Hall, 70 Prospect Avenue, Princeton, NJ 08540, USA.
for that purpose to find out Jc-determining factors by comparing various high-Tc superconductors. Recently, we have had many high-Tc superconductors with Tc ) 100 K thanks to application of high-pressure synthesis techniques w3x, and their systematic evaluation seems crucial for the understanding of the Jc-related physics. Nevertheless, only a limited number of reports have been published thus far in this field of research. In the case of ŽCu,C.Ba 2Ca ny 1Cu nO 2 nq3 wŽCu,C.-12Ž n y 1. n; n s 3,4x series synthesized under high pressure, they show much higher Jc and Birr than Bi 2 Sr2 Ca ny1Cu nO 2 nq4 wBi22Ž n y 1. n x and HgBa 2 Ca ny1Cu nO 2 nq2q d wHg12Ž n y 1. n x series w4,5x suggesting less anisotropic electronic structures of the ŽCu,C.-phases. Though their Jc values under magnetic field are substantially lower than that of YBa 2 Cu 3 O 7 ŽY-123., neutron
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irradiation was quite effective to introduce pinning centers and drastically enhanced both Jc and Birr w5x. The evaluation of Jc-related properties was also carried out for high-pressure phases of BSr2Ca ny 1Cu nO 2 nq3 wB-12Ž n y 1. n x w6,7x. It was firstly expected that they had good Jc-related properties because of small separation between the conduction layers due to small B 3q ion size. However, their Jc and Birr values were much lower than those of ŽCu,C.-12Ž n y 1. n and Y-123, implying high electric anisotropy. Recently, a new series of high-Tc superconductors ŽCu,Cr.Sr2 Ca ny1Cu nO 2 nq3 wŽCu,Cr.-12Ž n y 1. n; n s 1–9x was synthesized under a high-pressurer high-temperature condition as polycrystalline form w8x. These phases have layered structures composed of stacking of planes, SrO–MO–SrO–CuO 2 – ŽCa– CuO 2 . ny1 where the M site is shared by Cu and Cr with nearly 1:1 ratio. The occupation of Cu at the M site in these phases is a characteristic common to ŽCu,C.-12Ž n y 1. n and Y-123 both of which have high Jc and Birr . The Cu and Cr atoms are placed randomly in the ŽCu,Cr.O plane of ŽCu,Cr.-12Ž n y 1. n in contrast to Cu–C–Cu–C . . . type ordering in the ŽCu,C.O plane of ŽCu,C.-12Ž n y 1. n w9x. In addition, excess oxygen and copper vacancies were reported to be introduced in the ŽCu,Cr.O plane w8x. These structural defects may work as effective pinning centers. Thus, we expect the ŽCu,Cr.-based system to have higher Jc than that of the ŽCu,C.12Ž n y 1. n series and other known superconductors. In the present study, we have carried out magnetization measurements for bulk form of ŽCu,Cr.-12Ž n y 1. n Ž n s 2,3. samples to evaluate their Jc and Birr and the results are compared with other superconductors such as Y-123, Hg-12Ž n y 1. n, ŽCu,C.-12Ž n y 1., B-12Ž n y 1. n, etc. Anisotropy in magnetization hysteresis is also discussed based on magnetization data of grain aligned samples.
2. Experimental Polycrystalline samples of ŽCu,Cr.-1212 and 1223 used in the present study were prepared under a high-pressurerhigh-temperature condition of 6 GPa and 12008C. Details of the sample preparation are described elsewhere w8x. The X-ray powder diffrac-
tion patterns showed that the ŽCu,Cr.-1223 sample was single phase while ŽCu,Cr.-1212 sample contained an unknown additional impurity phase. The impurity was estimated ; 5% in content but no correction for the magnetization hysteresis data was applied since the exact content was unknown and ; 5% uncertainty does not affect the discussion below. Onset superconducting transition temperatures ŽTc . determined by magnetic susceptibility data were 81 K and 103 K for ŽCu,Cr.-1212 and 1223, respectively, and are in good agreement with the previous reports w8x. For grain alignment, a well-pulverized powder sample and epoxy Žstycast 1266. were mixed then cured in magnetic field of 5–7 T at room temperature for 12 h w10x. The degree of grain alignment was checked by X-ray diffraction. Magnetization hystereses Ž D M . were measured using a SQUID magnetometer ŽQuantum Design. for the bulk and aligned samples with magnetic field up to 5 T at various temperatures. For the latter samples, magnetic field was applied either parallel or perpendicular to the c-axis. The microstructures of the bulk and aligned samples were observed using a scanning electron microscope ŽSEM. to determine their average grain sizes. The grains were found to be disk-like with an average diameter of ; 3 mm in the bulk samples and ; 1 mm in the aligned samples.
3. Results and discussion 3.1. Bulk samples Many Jc and Birr data based on magnetic measurements were determined for sintered polycrystalline samples, and therefore we discuss firstly the data obtained for the bulk ŽCu,Cr.-samples and compare them with previously reported values. Fig. 1 shows typical examples of magnetic hysteresis loop observed for ŽCu,Cr.-1223. Both ŽCu,Cr.-1212 and ŽCu,Cr.-1223 have quite large magnetization hysteresis D M especially in low temperature region. Since the SEM observations indicated that the average grain diameter for the bulk samples was not so large, the large D M value implies large Jc , and therefore good pinning properties of the present system. Fur-
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Fig. 1. Magnetic hysteresis loops for the bulk ŽCu,Cr.-1223 sample at 5 K, 15 K and 25 K.
thermore, temperature dependences of D M are far less pronounced compared with Bi-2212 w11x, Hg1223 w12x and B-12Ž n y 1. n Ž n s 3,4. w6x, etc. suggesting high Birr values. In order to estimate Jc and Birr from the D M data, Bean’s model w13x was used. Since the intergrain critical current density can be neglected in the present samples due to weak coupling between grains, the intragrain critical current density may be calculated as, Jc s 30 D Mrd,
Ž 1.
where d means the average grain size in diameter which is 3 mm for both ŽCu,Cr.-1212 and 1223 according to the SEM observations. Magnetic field dependencies of Jc thus calculated are shown in Fig. 2Ža. and Žb. for ŽCu,Cr.-1212 and 1223, respectively. In low-temperature region, both phases have extremely high Jc values, and furthermore magnetic field dependence is quite small suggesting presence of enough effective pinning centers in number. This is in contrast to the results for the ŽCu,C.-12Ž n y 1. n phase whose Jc is much smaller and presents large decay with increasing magnetic field even at low temperatures w4x. With increasing temperature, the field dependence of Jc becomes larger in ŽCu,Cr.-1212 and 1223, but its degree is
less pronounced implying once more high Birr values Žsee below.. The Jc at 1 T is shown in Fig. 3 for the ŽCu,Cr.phases as a function of normalized temperature, TrTc . Data previously reported for Bi-2212 w11x, Hg-1223 w12x B-12Ž n y 1. n w6x, Y-123 w4x, ŽCu,C.12Ž n y 1. n w4x are also given for comparison. The Jc values of Hg-1223 in Ref. w12x were calculated by an equation slightly different from Eq. Ž1., and for direct comparison, we recalculated them by Eq. Ž1. using the original D M data in Ref. w12x. Among all the phases shown, ŽCu,Cr.-1212 and 1223 show the largest Jc value in the almost entire range of TrTc . Although crystal structures of ŽCu,Cr.-12Ž n y 1. n series are closely related to those of ŽCu,C.-12Ž n y 1. n series w8,9x, their Jc values are very different as seen in Fig. 3. If we compare ŽCu,Cr.-1223 and ŽCu,C.-1223, for instance, the former phase has one order of magnitude larger Jc at 1 T than the latter. High-resolution electron microscopy together with composition analysis indicated that Cu and C atoms in the ŽCu,C.O charge reservoir plane in ŽCu,C.12Ž n y 1. n are placed in an ordered way along the a-axis as Cu–C–Cu–C . . . w9x. On the other hand, in the case of ŽCu,Cr.-1212 and 1223, the Cu and Cr atoms within the charge reservoir are placed randomly or the ordering occurs, if any, very locally
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contains Cu-vacancies and excess oxygen atoms w8x. These results indicate that the ŽCu,Cr.-12Ž n y 1. n phases contain a higher density of crystal defects than the ŽCu,C.-12Ž n y 1. n ones. These defects are expected to work as effective pinning centers and seem to explain why Jc ’s of ŽCu,Cr.-phases are much larger than those of ŽCu,C.-phases and other superconductors. The Jc values of ŽCu,Cr.-1212 are smaller than those of ŽCu,Cr.-1223 at low temperatures, suggesting a relatively smaller volume pinning force of ŽCu,Cr.-1212 which may be caused by a lower density of crystal defects in the 1212 phase. The Cr-based phases show much less temperature dependence of Jc compared with the Bi-, Hg- and B-based phases. At high temperatures near Tc , however, fairly large decay of Jc is seen in both ŽCu,Cr.1212 and 1223 and the Y-123 phase shows a better property in this region of TrTc . The temperature dependence of Jc is closely related to the anisotropy of electronic structure of a superconductor, and in
Fig. 2. Magnetic field dependence of critical current density, Jc , at various temperatures for the bulk ŽCu,Cr.-1212 Ža. and ŽCu,Cr.1223 Žb. samples.
and imperfectly w8x. Furthermore, electron microprobe analysis suggested that the ŽCu,Cr.O plane
Fig. 3. Variation of critical current density, Jc , under 1 T as a function of T r Tc for the bulk ŽCu,Cr.-1212 and ŽCu,Cr.-1223 samples compared with previous data for Bi-2212 w11x, Hg-1223 Žsee text., B-12Ž ny1. n w6x, Y-123 w4x and ŽCu,C.-12Ž ny1. n w4x.
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Fig. 4. Variation of the irreversibility field, Birr , as a function of T r Tc for the bulk ŽCu,Cr.-1212 and ŽCu,Cr.-1223 samples compared with previous data for Bi-2212 w11x, Hg-1223 Žsee text., B-12Ž ny1. n w6x, Y-123 w4x, ŽCu,C.-12Ž ny1. n w4x.
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order to discuss this more quantitatively, the irreversibility field was calculated according to the Jc s 10 3 Arcm2 criterion. Fig. 4 shows Birr plotted against TrTc for the Cr-based phases and other superconductors. The Birr data of Hg-1223 were originally given by a criterion different from Jc s 10 3 Arcm2 w12x. Therefore, we redetermined them according to the Jc s 10 3 Arcm2 criterion for the new Jc values recalculated by Eq. Ž1.. The Birr values of the Cr-based phases are quite high; at 77 K, for instance, Birr of ŽCu,Cr.-1223 is over 5 T as seen in Fig. 2Žb.. The superconductors in Fig. 4 can be categorized into two groups; the first group includes the ŽCu,Cr.- and ŽCu,C.-phases and Y-123 with Cu-containing blocking layers while the second one does Hg-, B- and Bi-phases with Cu-free blocking layers. As seen clearly in Fig. 4, the former group phases have much higher Birr ’s than the latter. The Birr is related to the dimensional crossover of vortex structure form 3D to 2D and if a system has a highly anisotropic electronic structure, this crossover occurs at a relatively low temperature and low magnetic field causing low Birr . In the M-12Ž n y 1. n structure, the Cu atoms of adjacent conduction layers
Fig. 5. X-ray powder diffraction patterns for the grain aligned ŽCu,Cr.-1212 and ŽCu,Cr.-1223 samples. A peak marked by ‘U ’ belongs to an impurity phase.
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are connected through M and the apical oxygen atoms as –Cu–O–M–O–Cu– along the c-axis. Therefore, the M-site occupation of Cu which is the same metal as the conduction layer contains, will greatly contribute to increase electric conductivity along the c-axis. We believe that the less anisotropic electronic structure is the main reason for the higher Birr values of the former group. In the former group phases with the Cu-containing blocking layers, the irreversibility lines of ŽCu,Cr.-phases are located between those of ŽCu,C.phases and Y-123. In the Y-123 structure, the M-sites are fully occupied by Cu while only 50% in other
phases and this fact accounts well for the highest Birr of Y-123. The Cr-based phases have higher Birr ’s than the ŽCu,C.-phases and the difference looks quite large. As stated before, this result seems to be caused by the high density of crystal defects in the Cr-based phases. It is worth noting here that the Jc-related properties were improved drastically in the ŽCu,C.phases after introduction of pinning centers by neutron irradiation w5x. The ŽCu,Cr.-1212 has slightly larger Birr than ŽCu,Cr.-1223. The former phase has an overdoping state of holes while the latter does the optimum doping state judging from their Tc ’s w8x. The overdoping of carriers diminishes electric ani-
Fig. 6. Magnetic field dependence of the anisotropy ratio, R s D M 5rD M H , for the grain aligned ŽCu,Cr.-1212 Ža. and ŽCu,Cr.-1223 Žb. samples.
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sotropy of a superconductors w14x and this may account for slightly larger Birr of ŽCu,Cr.-1212. 3.2. Grain-aligned samples Fig. 5 shows powder X-ray diffraction patterns of the magnetically grain-aligned samples for ŽCu,Cr.1212 and 1223 which were taken for planes perpendicular to the applied magnetic field. Most of peaks can be indexed as 00 l reflections of the ŽCu,Cr.12Ž n y 1. n-type structures. A small number of h / 0 or k / 0 peaks such as 110, 104, 103, etc. are also observed but their intensities are minimal. A fairly strong peak denoted by ‘U ’ in ŽCu,Cr.-1212 does not belong to the ŽCu,Cr.-phases and is ascribed to an unknown impurity phase. The X-ray patterns indicate that the grains of each phase were aligned almost completely with the c-axis parallel to the applied magnetic field. Magnetization hystereses of the ŽCu,Cr.-1212 and 1223 grain aligned samples were observed for two Fig. 8. Variation of the in-plane critical current density, Jc Ž I H , B 5 ., under 1 T as a function of T r Tc for the grain aligned ŽCu,Cr.-1212 and ŽCu,Cr.-1223 samples compared with previous data for B-12Ž ny1. n Ž ns 3–5. w7x.
Fig. 7. Variation of the crossover magnetic field, B0 , as a function of T r Tc for the grain aligned ŽCu,Cr.-1212 and ŽCu,Cr.-1223 samples compared with previous data for B-12Ž ny1. n Ž ns 3–5. w7x.
different directions of magnetic field, parallel Ž B 5 . and perpendicular Ž B H . to the c-axis. In order to discuss superconductivity anisotropy, an anisotropy factor R was calculated as R s D M 5rD M H w7x for the magnetic hystereses D M 5 and D M H corresponding to B 5 and B H , respectively. The R-value thus obtained is plotted against magnetic field in Fig. 6Ža. and Žb.. At low temperatures ŽT F 35 K for ŽCu,Cr.-1212 and T F 55 K for ŽCu,Cr.-1223., the R-value is larger than 2 for the entire range of magnetic field up to 5 T, showing the relation D M 5 ) D M H . At higher temperatures, the R vs. B curve crosses the R s 1 line at a certain magnetic field Ž B0 ., i.e., the relation between D M 5 and D M H is reversed to D M 5 - D M H for B ) B0 . Recently, we obtained similar results for B-12Ž n y 1. n Ž n s 3–5. aligned samples and suggested that the crossover in D M 5 and D M H is caused by different vortex structures corresponding to magnetic field parallel and perpendicular to the c-axis. The D M 5 term is related to only one kind of critical
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current density, Jc Ž I H , B 5 ., with electric current perpendicular to Ž I H . and magnetic field parallel to the c-axis. On the other hand, the D M H term is governed by two kinds of critical current densities, Jc Ž I 5 , B H . and Jc Ž I H , B H . which are in-plane Ž I 5 . and c-axis direction components of Jc Ž B H ., respectively. We suggested that serious decay of Jc Ž I H , B 5 . due to the formation of a 2D pancake vortex structure causes the crossover in D M 5 and D M H in a high temperature region w7x. It was also suggested that there is a correlation between the B0-value and the electric anisotropy of a system, i.e., a less anisotropic system shows a larger B0 value with the crossover occurring at a higher temperature and a higher magnetic field w7x. The crossover field B0 is plotted against TrTc in Fig. 7 for ŽCu,Cr.-1212 and 1223 together with B-12Ž n y 1. n Ž n s 3–5.. The B0 vs. TrTc lines of the ŽCu,Cr.-phases shift largely to the right compared with those of the B-phases. The Jc Ž I H , B 5 . value at 1 T was calculated using D M 5 and d s 1 mm based on the Bean’s model for the ŽCu,Cr.-phases and is shown in Fig. 8 compared with the B-system. It is seen from this figure that the Jc Ž I H , B 5 . values of the B-phases show much steeper decrease with increasing temperature in a high temperature region than those of the ŽCu,Cr.-phases, causing the small B0 values of the B-phases. All these results indicate that the present ŽCu,Cr.-phases have less anisotropic electronic structures and decay of Jc Ž I H , B 5 . due to decoupling of the pancakes of vortices is far less pronounced.
4. Conclusion We have carried out DC magnetization measurements for ŽCu,Cr.-12Ž n y 1. n Ž n s 2,3. in order to evaluate Jc , Birr and superconductivity anisotropy. The present ŽCu,Cr.-based phases have much higher Jc ’s under magnetic field for the almost entire range of temperature compared with other superconductors such as Bi-2212, Hg-1223, B-12Ž n y 1. n Ž n s 3–5., ŽCu,C.-12Ž n y 1. n Ž n s 3,4., Y-123, etc. The ŽCu,Cr.-phases also present high Birr values with the Birr vs. TrTc lines located between those of the Y-123 and ŽCu,C.-phases. The crossover between D M 5 and D M H was observed and it occurs at much
higher temperature and higher magnetic field compared with the B-12Ž n y 1. n system. These results indicated that ŽCu,Cr.-based oxides have less anisotropic electronic structures and good pinning properties due to enough effective pinning centers in number introduced by the irregular arrangement of the Cu and Cr atoms in the blocking layer.
Acknowledgements The authors express their sincere thanks to Drs. M. Akaishi and S. Yamaoka of NIRIM for helpful suggestions on high-pressure synthesis of the ŽCu,Cr.-12Ž n y 1. n samples. This work was supported by the Multi-core Project, the COE project and Special Coordination Funds of the Science and Technology Agency of the Japanese Government.
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