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Low field AC susceptibility study of intergranular critical current density in Mg-substituted CuBa2Ca3Cu4O12−y high temperature superconductors
Journal of Physics and Chemistry of Solids 66 (2005) 729–734 www.elsevier.com/locate/jpcs
Low field AC susceptibility study of intergranular critical current density in Mg-substituted CuBa2Ca3Cu4O12Ky high temperature superconductors S.K. Agarwal*, B.V. Kumaraswamy Superconductivity and Cryogenics Division, National Physical Laboratory, Dr K.S. Krishnan Road, New Delhi-110012, India Received 24 February 2004; revised 1 September 2004; accepted 6 October 2004
Abstract Measurements of the a.c.susceptibility (cZc 0 Cic 00 ) have been made on the Mg substituted high TC superconducting system, CuBa2(MgxCa1Kx)3Cu4O12Ky (Cu-1234) with xZ0, 0.10 & 0.20, at different values of the a.c.field amplitude. Estimates of the intergranular critical current density(JC) made from the field dependent c 00 -T curves show an improvement in the Mg-substituted Cu-1234 system. Results have been analysed in the light of the crystal structure and the superconducting anisotropy factor (gZxab/xc) of the Cu-1234 system. Lower superconducting anisotropy emanating from Mg substitution has been found to be significant, resulting in better superconducting properties. q 2004 Elsevier Ltd. All rights reserved. Keywords: A. Oxides; A. Superconductors; D. Magnetic properties; D. Superconductivity.
1. Introduction The emergence of the Hg-based high temperature cuprate superconductor with a TC of w136 K and its further improvement to 156 K using high pressure synthesis, has sparked off hectic activity in synthesizing similar high TC compounds. Since then, many other systems like TlBa2Ca2 Cu3Oy and (Cu, Tl)Ba2Ca3Cu4Oy have been synthesized [1–3] which exhibit TC values of about 120 K. The most notable among these compounds is CuBa2Ca3Cu4Oy which belongs to the family CuBa2CanK1CunOy with nZ4. This compound is synthesized employing high pressures and high temperatures and exhibit superconductivity at ambient pressure with a TC of about 118 K. Besides possessing high TC, Cu-1234 exhibits high values of intragranular critical current density (JC) and irreversibility field (Hirr). Moreover, this system has superconducting anisotropy xab/xcZ1.6, which is the lowest for any of the high TC cuprates synthesized to date [4–11]. To further explore the possibilities of improvement in the superconductivity in Cu-1234 system, Mg (in place of Ca) * Corresponding author. Tel.: C91-11-25742610x2276/2239, fax: C9111-25726938. E-mail address: [email protected] (S.K. Agarwal). 0022-3697/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2004.10.003
substitution was carried out by one of us [11]. Significantly, this has resulted in further reduction in the anisotropy factor of Cu-1234 without any decrease in the TC. While there have been studies of the intragranular critical current density (Jc) in Cu-1234 system through d.c.magnetization techniques [9,10], there has not been any report on intergranular Jc in these samples, which also has important bearing from commercial applications viewpoint involving the polycrystalline material. From the early days of high TC superconductivity, a.c.susceptibility technique has been employed as a probe to study the weak link intergranular region in the high TC oxide superconductors. General features of the complex a.c. susceptibility, (cZc 0 Cic 00 ) in these materials are described in Refs. [12,13]. The complex a.c. susceptibility can give information about several physical properties concerning the weak link intergranular region in the high TC systems, the most important among them being the temperature dependent intergranular critical current density JC(T). To analyze the temperature and the a.c. field amplitude dependent variation of the a.c. susceptibility (c 0 and c 00 ) and to estimate critical current density, critical state models have been developed [13–18]. In this communication, we present the results of the low field a.c. susceptibility studies and the estimation of
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intergranular JC values in the Mg (in place of Ca) substituted Cu-1234 samples.
2. Experimental Samples of CuBa2(MgxCa1Kx)3Cu4O12Ky with xZ0, 0.10, 0.20 were synthesized through the solid state reaction at high pressure and high temperature of appropriate precursor compounds. The precursor compounds were prepared by calcining the well ground mixtures of BaCO3, CaCO3, CuO and MgO (purity of more than 99.9%) in flowing oxygen for a total period of 72 h with intermediate grindings to ensure homogeneity of powder mixture. AgO and AgCO3 were added to the precursor as oxidizer and phase stabilizers respectively. The final reaction of the precursor compounds to synthesize Cu-1234 phase was carried out in a gold capsule at about 1000 8C and about 3.5 GPa pressure using a cubic type anvil high pressure machine (Riken CAP-07). The sample was subsequently quenched to ambient temperature before the pressure was released. The samples were examined through an X-ray diffractometer (Rigaku RINT-1000) and were found to be single phased with all the characteristic Cu-1234 phase peaks [11]. While in such cases of substituted materials, there is always a possibility of intergrowth or superstructures being present [10], such peaks however, were not noticed here. Some very minor peaks seen in higher Mg-content samples might correspond to unidentified impurity phase. Lattice parameters, Tc and the ensuing superconducting anisotropy factor for the different samples are already presented elsewhere [11]. It is worthwhile to note that Tc values of the Mg substituted samples closely correspond to that of the pristine Cu-1234 sample. This incidentally also confirms the near invariance of the oxygen content from sample to sample even though no direct attempt was made to estimate these. The complex a.c. susceptibility measurements on the samples were made using Lakeshore AC susceptometer (Model ACS 7000). The samples were placed tightly in the sample holder and ac field was applied along the longest dimension of the samples. The measurements were made in the temperature range 77–150 K at ac field amplitudes of 80, 200, 400, 600 and 800 A/m and frequency 111.1 Hz with a heating rate of 0.5 K/min. The data points were acquired at intervals of 0.8 K. From the acquired data the values of c 0 and c 00 were calculated using the dedicated software. The estimates of the grain size and the grain size distribution in the samples were made through scanning electron micrographs.
3. Results and discussion Fig. 1 shows the ac susceptibility (c 0 and c 00 ) vs. temperature behaviour of the undoped (pure) and Mg-doped
Cu-1234 samples. It is observed that the diamagnetic onset temperatures of the Mg-doped samples are essentially the same as that of pure Cu-1234 sample. But more importantly Mg-substitution shifts the c 0 -T and c 00 -T curves to higher temperature side, decreases the transition width in c 0 -T curves and also improves the shielding fraction of the superconducting phase in the samples. This is a clear indication of the improvement in the superconducting properties brought about by Mg substitution in the Cu1234 system. c 00 -T curves of the samples are shown in Fig. 2(a–c). While the c 0 -T curve of a high Tc sample indicates the diamagnetic onset temperature and the shielding fraction of the superconducting phases, it is the c 00 -T curve which is of prime importance in understanding the dynamics of flux penetration into superconductor while its temperature is being increased in the presence of magnetic field. The variation c 00 (which represents the losses in the sample) with temperature and mechanism of formation of peaks in the c 00 vs T curves of the high Tc samples (hereafter referred to as c 00 peak) has been nicely described in the literature [12,13]. Accordingly, in a high Tc sample the broad peak appearing well below the diamagnetic onset temperature is due to the gradual penetration of flux into the center of the intergranular regions (i.e. center of the sample). As the temperature is increased from a sufficiently low value at which the sample is in the Meissner state (c 0 ZK1 and c 00 Z 0), c 00 starts becoming positive due to gradual penetration of the field into the grain boundary regions. At a temperature Tp, when the flux penetrates into the center of the sample, a peak in c 00 develops. Above Tp the extent of flux penetration decreases as the sample is already almost fully penetrated, this leads to a drop in c 00 which decreases gradually again to zero. The second peak in c 00 , as observed in our samples Fig. 2(a–c) which is just below the diamagnetic onset temperature is less prominent but quite sharp. This peak is due to the flux penetration into the grains of the sample. The mechanism of this peak formation is similar to that of intergranular c 00 peak formation. The grains being better superconductor than the intergranular region, the flux penetration into them occurs at a higher temperature than into the intergranular regions. It is pointed out here that only intergranular peaks in c 00 are normally observed in the case of fine grained (less than 10 mm size) high Tc polycrystalline samples which are common nowadays and was only in the early period of High Tc superconductivity, when the samples used to be coarse grained (more than 20 mm in size) that two peaks in c 00 could be observed [19–21]. Possible reason for the non-occurrence of the intragranular peaks in a high Tc sample has been explained by Goldfarb et al. [12]. The most likely course for the size being of the order of penetration depth, in which case the grains are already in the penetrated state to start with, excluding any further flux penetration which is needed for intragranular c 00 peak formation. But the grain size of the our Cu-1234 samples (w6 mm) can by no
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Fig. 1. (a) Variation of c 0 and c 00 with temperature for the pristine Cu-1234 Superconductor. (b) Variation of c 0 and c 00 with temperature for 10% Mg-doped Cu1234 Superconductor. (c) Variation of c 0 and c 00 with temperature for 20% Mg-doped Cu-1234 Superconductor.
means be taken as coarse. The occurrence of the intragranular c 00 peaks in our samples is probably due to the smaller values of the penetration depth (in comparison to the grain size) observed in Cu-1234 system which in turn is the result of lower anisotropy of Cu-1234 crystal structure [22]. From Fig. 2(c) (20%Mg doped sample), the intragranular peaks in c 00 are almost masked by the intergranular c 00 peaks. This probably is due to well coupled nature of the grains in this sample (possibly brought about by
the improved superconductivity due to Mg substitution) in which case a small measuring field will cause the coupling peak (i.e. intergranular peak) to obscure the intragranular peak [12]. From Fig. 2(a–c) it is seen that the increase in ac field amplitude shifts the intergranular c 00 peaks towards left (decrease superconductivity in grain boundaries) whereas intragranular c 00 peaks remain unaffected by the increase in ac field amplitude. This is because in the amplitude range shown (which is the maximum available in the ACS 7000
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Fig. 2. (a) Variation of c 00 with temperature for the pristine Cu-1234 superconductor. (b) Variation of c 00 with temperature for 10% Mg-doped Cu-1234 superconductor. (c) Variation of c 00 with temperature for 20% Mg-doped Cu-1234 superconductor.
susceptometer), the ac field does not have any effect on the superconductivity of the grains at all. From the c 00 -T curves of the Mg substituted Cu-1234 samples it is seen that Mg doping not only shifts the curves towards higher temperatures but also reduces the transition width of c 00 -T curves. As is well established the superconducting properties of a high Tc sample, as revealed by the ac susceptibility are very much dependent upon the sample specific parameters such as sample size, its density, grain size and grain size distribution etc [23,24]. The present samples have been found to have similar sizes (w3–4!2!1.5 mm3). They have similar grain sizes (w6 mm) and grain distributions as revealed by the scanning electron microscopy. It is worthwhile to include here that all the samples, pristine as well as substituted ones have been synthesized under identical conditions and the average grain sizes obtained here confirm to those reported earlier [9,10,22]. Therefore, the variations in c 00 -T behaviour in Fig. 2 can be exclusively attributed to the substitutional effects of Mg in Cu-1234 system. The broadened peaks prominent in c 0 -T curves being the sure signatures of intergranular properties in a high Tc sample, we can use critical state models [13–17] to estimate the temperature dependent intergranular critical current density Jc(T) in the present Cu-1234 samples. We employ Bean’s model [12,15,18] to calculate the intergranular
critical current density in the present samples using the relation Jc(Tp)ZHa/a for a sample having cross section 2a!2b where a!b. Jc(Tp) is the intergranular critical current density at Tp—the temperature of the c 00 peak, Ha is the amplitude of the applied ac field. The calculated values of Jc(Tp) for the pure as well as 10 and 20% Mg substituted (at Ca- site) sample are plotted against temperature (Tp) in Fig. 3.
Fig. 3. JC versus TP behavior for the (a) pristine, (b) 10% Mg-doped and (c) 20% Mg-doped Cu-1234 superconductors. Solid lines drawn are only guide to the eye.
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From Fig. 3 it is seen that the Jc values estimated are for the temperatures just in the neighbourhood of the Tc value of the sample. In the available field range in the susceptometer (maximum ac field amplitude of 800 A/m), it was not possible to shift the c 00 peaks (at which the Jc values could be calculated using Bean’s model) further down to the lower temperatures. It is also seen from Fig. 3 that Mg doping brings about an improvement in the intergranular critical current density of Cu-1234 samples. This can be understood on the basis of the improvement (reduction) in the superconducting anisotropy factor brought about by the Mg substitution in Cu-1234 system, studied in detail in Ref. [11]. It has been found by XRD that Mg substitution decreases the lattice parameters a and c. It is inferred that Mg2C may substitute both in the superconducting blocks (SLB) Ca3Cu4O8 (in place of Ca2C) and in the charge reservoir blocks (CRB), CuBa2O4Ky in place of Cu-site vacancies. In both cases there is a decrease in the c-parameter. In the first case, the decrease is due to smaller ionic size of Mg2C as compared to that of Ca2C and in the second case, the decrease is because of the increased Cu2C site occupancy leading to the decrease in the CRB size. The second phenomenon has been observed in other substitution cases also in Cu-1234 system such as CuxCe1KxBa2CanK1CunOy and CuxBa2CanK1CunOy [25]. The Mg substitution at Cu-site vacancies no doubt renders the CRB less conducting but by reducing the size of CRB, it also brings the two adjoining SLB closer. Assuming that SLB-CRB-SLB form a superconductor—normal—superconductor junction, the superconducting wave functions from the adjacent SLBs will have a better overlap (which more than compensates the reduced conductivity of the resulting CRBs), because of the reduction in the size of the intervening CRB, in the case of Mg doped samples. This eventually leads to the reduced superconducting anisotropy along the c-axis and improved intragranular superconductivity in Mg-substituted Cu-1234 system. Such an improvement in the intragranular superconducting properties will ultimately result in the intergranular superconducting properties also viz. intergranular J c in Mg-substituted Cu-1234 samples. The polycrystalline Cu-1234 samples can be modeled as superconductor— insulator—superconductor (SIS) junction consisting of superconducting grains separated by insulating grain boundaries. Any improvement in the intragranular properties will lead to the improvement in the overlapping of the superconducting wave functions across the SIS junction, which results in the improvement of the intergranular Jc as observed in the case of the Mg substituted Cu-1234 samples. A detailed mathematical description of the SIS junction and the intragranular phenomenon affecting the intergranular Jc in a polycrystalline Er-123 HTSC system is provided in Ref. [26]. The intragranular c 00 peaks in pure and 10% Mg doped Cu-1234 samples occur at about 115 K (just below the diamagnetic onset temperature of 117 K) and these are amplitude invariant also. Again using the Bean’s model we
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can calculate the intragranular Jc at 115 K for the samples using the relation Jc (115 K)ZHa/g here Ha is the applied ac field amplitude and 2 g is the grain thickness (which is 4 mm in our samples). A intragranular Jc of 4!104 A/cm2 at 115 K is obtained with HaZ800 A/m. This compares favourably with the intragranular Jc values for the Cu-1234 samples [9,10].
4. Conclusions Field dependent ac susceptibility studies were carried out in pure and Mg-substituted Cu-1234 samples. The c 00 -T curves at different ac field amplitudes exhibit both inter- and intra-granular peaks. The values of the intergranular critical current densities as estimated from the c 00 -T curves are higher for Mg-substituted samples as compared to the pure Cu-1234 sample. Such an improvement in the intergranular Jc of the substituted samples is attributed to the reduction in the superconducting anisotropy of the Cu-1234 crystal structure brought about by Mg2C incorporation, which ultimately results in better inter- as well as intra-granular superconducting properties.
Acknowledgements The authors are grateful to the Director of their laboratory for providing the facilities to carry out the present work. Computational help from Ajay, Neeraj and Vikram is gratefully acknowledged. One of us (SKA) would like to thank the superconducting materials section of Professor H. Ihara, of the Electro-technical Laboratory, Tsukuba, Japan and Professor T. Watanabe of the Science University of Tokyo, Chiba for utilizing their excellent high pressure synthesis facilities.
References [1] H. Ihara, K. Tokiwa, H. Ozawa, M. Hirabayashi, H. Matsuhata, A. Nagishi, Y.S. Song, Jpn J. Appl. Phys. 33 (1994) L300. [2] M.A. Alario-Franco, C. Chaillout, J.J. Capponi, J.-L. Tholence, B. Souletie, Physica C 222 (1994) 52. [3] C.Q. Jin, X.-J. Wu, S. Adachi, H. Yamauchi, S. Tanaka, Physica C 223 (1994) 238. [4] H. Ihara, K. Tokiwa, A. Iyo, M. Hirabayashi, N. Terada, M. Tokumoto, Y.S. Song, Physica C 235–240 (1994) 981. [5] H. Ihara, K. Tokiwa, A. Iyo, N. Terada, M. Tokumoto, M. Umeda, Advances in Superconductivity VIII, Springer-Verlag, Tokyo, 1996, p. 247. [6] H. Ihara, R. Sugise, M. Hirabayashi, N. Terada, M. Jo, K. Hayashi, A. Negishi, M. Tokumoto, Y. Kimura, T. Shimomura, Nature 334 (1988) 510. [7] H. Ihara, R. Sugise, M. Hirabayashi, N. Terada, M. Jo, K. Hayashi, A. Negishi, M. Tokumoto, T. Shimomura, S. Ohashi, H. Oyanagi, A. Atoda, Phys. Rev. B 38 (1988) 11952. [8] H. Ihara, K. Tagana, K. Kawashima, E. Takayama-Muromachi, Physica C 226 (1994) 222.
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[9] A. Iyo, K. Tokiwa, T. Kanehira, H. Ozawa, N. Kobayashi, N. Terada, M. Tokumoto, M. Hirabayashi, H. Ihara, Advances in Superconductivity VII, Springer-Verlag, Tokyo, 1995. p. 825. [10] A. Iyo, K. Tokiwa, N. Terada, M. Tokumoto, M. Hirabayashi, H. Ihara, ICMC-95. [11] S.K. Agarwal, A. Iyo, K. Tokiwa, Y. Tanaka, K. Tanaka, M. Tokumoto, N. Tereda, T. Saya, M. Umeda, H. Ihara, M. Hamao, T. Watanabe, A.V. Narlikar, Phys. Rev. B 58 (1998) 9504. [12] R.B. Goldfarb, M. Leirental, C.A. Thomson in Magnetic Susceptibility of superconductors and other spin systems. in Eds. R.A. Hein,T.L. Francavilla, D.H. Liebenberg. [13] D.X. Chen, J. Nogues, K.V. Rao, Cryogenics 29 (1989) 800. [14] F. Gomory, P. Lobotka, Solid State Commun. 66 (1988) 645. [15] C.P. Bean, Rev. Mod. Phys. 36 (1964) 31. [16] J.R. Clem, Physica C 153–155 (1988) 50. [17] K.H. Muller, Physica C 159 (1989) 777. [18] Literature of AC Susceptometer Model ACS 7000 Lakeshore Cryotronics Inc. USA.
[19] H. Kupfer, I. Aptelstedt, R. Flukinger, R. Meir-Hirmer, W. Schauer, T. Wolf, H. Watel in ‘International Discussion meeting on high Tc superconductors, Feb 7–11 1988, Mauterndorf, Austria. [20] H. Kupfer, I. Aptelstedt, R. Flukinger, R. Meir-Hirmer, C. Kellor, B. Runtsch, A. Turowski, U. Weich, T. Wolf, Cryogenics 28 (1988) 650. [21] H. Kupfer, S.M. Green, C. Jiang, Yu. Mei, H. Luo, R. Meier-Hirmer, C. Politis, Z. Phys. B-conden. Mater. 71 (1995) 255. [22] H. Ihara, Advances in Superconductivity VII, Springer-Verlag, Tokyo, 1995. p. 255. [23] D.X. Chen, A. Sanchez, T. Puig, F. Martinez, J.S. Monoz, Physica C 168 (1990) 652. [24] V. Skumaryev, M.R. Koblischka, H. Kronmuller, Physica C 184 (1991) 332. [25] J. Akimoto, K. Tokiwa, A. Iyo, H. Ihara, H. Hayakawa, Y. Gotoh, Y. Oosawa, Physica C 279 (1997) 181. [26] B.V. Kumarswamy, R. Lal, A.V. Narlikar, Phys. Rev. B 52 (1995) 1320.
Low field AC susceptibility study of intergranular critical current density in Mg-substituted CuBa2Ca3Cu4O12Ky high temperature superconductors S.K. Agarwal*, B.V. Kumaraswamy Superconductivity and Cryogenics Division, National Physical Laboratory, Dr K.S. Krishnan Road, New Delhi-110012, India Received 24 February 2004; revised 1 September 2004; accepted 6 October 2004
Abstract Measurements of the a.c.susceptibility (cZc 0 Cic 00 ) have been made on the Mg substituted high TC superconducting system, CuBa2(MgxCa1Kx)3Cu4O12Ky (Cu-1234) with xZ0, 0.10 & 0.20, at different values of the a.c.field amplitude. Estimates of the intergranular critical current density(JC) made from the field dependent c 00 -T curves show an improvement in the Mg-substituted Cu-1234 system. Results have been analysed in the light of the crystal structure and the superconducting anisotropy factor (gZxab/xc) of the Cu-1234 system. Lower superconducting anisotropy emanating from Mg substitution has been found to be significant, resulting in better superconducting properties. q 2004 Elsevier Ltd. All rights reserved. Keywords: A. Oxides; A. Superconductors; D. Magnetic properties; D. Superconductivity.
1. Introduction The emergence of the Hg-based high temperature cuprate superconductor with a TC of w136 K and its further improvement to 156 K using high pressure synthesis, has sparked off hectic activity in synthesizing similar high TC compounds. Since then, many other systems like TlBa2Ca2 Cu3Oy and (Cu, Tl)Ba2Ca3Cu4Oy have been synthesized [1–3] which exhibit TC values of about 120 K. The most notable among these compounds is CuBa2Ca3Cu4Oy which belongs to the family CuBa2CanK1CunOy with nZ4. This compound is synthesized employing high pressures and high temperatures and exhibit superconductivity at ambient pressure with a TC of about 118 K. Besides possessing high TC, Cu-1234 exhibits high values of intragranular critical current density (JC) and irreversibility field (Hirr). Moreover, this system has superconducting anisotropy xab/xcZ1.6, which is the lowest for any of the high TC cuprates synthesized to date [4–11]. To further explore the possibilities of improvement in the superconductivity in Cu-1234 system, Mg (in place of Ca) * Corresponding author. Tel.: C91-11-25742610x2276/2239, fax: C9111-25726938. E-mail address: [email protected] (S.K. Agarwal). 0022-3697/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2004.10.003
substitution was carried out by one of us [11]. Significantly, this has resulted in further reduction in the anisotropy factor of Cu-1234 without any decrease in the TC. While there have been studies of the intragranular critical current density (Jc) in Cu-1234 system through d.c.magnetization techniques [9,10], there has not been any report on intergranular Jc in these samples, which also has important bearing from commercial applications viewpoint involving the polycrystalline material. From the early days of high TC superconductivity, a.c.susceptibility technique has been employed as a probe to study the weak link intergranular region in the high TC oxide superconductors. General features of the complex a.c. susceptibility, (cZc 0 Cic 00 ) in these materials are described in Refs. [12,13]. The complex a.c. susceptibility can give information about several physical properties concerning the weak link intergranular region in the high TC systems, the most important among them being the temperature dependent intergranular critical current density JC(T). To analyze the temperature and the a.c. field amplitude dependent variation of the a.c. susceptibility (c 0 and c 00 ) and to estimate critical current density, critical state models have been developed [13–18]. In this communication, we present the results of the low field a.c. susceptibility studies and the estimation of
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intergranular JC values in the Mg (in place of Ca) substituted Cu-1234 samples.
2. Experimental Samples of CuBa2(MgxCa1Kx)3Cu4O12Ky with xZ0, 0.10, 0.20 were synthesized through the solid state reaction at high pressure and high temperature of appropriate precursor compounds. The precursor compounds were prepared by calcining the well ground mixtures of BaCO3, CaCO3, CuO and MgO (purity of more than 99.9%) in flowing oxygen for a total period of 72 h with intermediate grindings to ensure homogeneity of powder mixture. AgO and AgCO3 were added to the precursor as oxidizer and phase stabilizers respectively. The final reaction of the precursor compounds to synthesize Cu-1234 phase was carried out in a gold capsule at about 1000 8C and about 3.5 GPa pressure using a cubic type anvil high pressure machine (Riken CAP-07). The sample was subsequently quenched to ambient temperature before the pressure was released. The samples were examined through an X-ray diffractometer (Rigaku RINT-1000) and were found to be single phased with all the characteristic Cu-1234 phase peaks [11]. While in such cases of substituted materials, there is always a possibility of intergrowth or superstructures being present [10], such peaks however, were not noticed here. Some very minor peaks seen in higher Mg-content samples might correspond to unidentified impurity phase. Lattice parameters, Tc and the ensuing superconducting anisotropy factor for the different samples are already presented elsewhere [11]. It is worthwhile to note that Tc values of the Mg substituted samples closely correspond to that of the pristine Cu-1234 sample. This incidentally also confirms the near invariance of the oxygen content from sample to sample even though no direct attempt was made to estimate these. The complex a.c. susceptibility measurements on the samples were made using Lakeshore AC susceptometer (Model ACS 7000). The samples were placed tightly in the sample holder and ac field was applied along the longest dimension of the samples. The measurements were made in the temperature range 77–150 K at ac field amplitudes of 80, 200, 400, 600 and 800 A/m and frequency 111.1 Hz with a heating rate of 0.5 K/min. The data points were acquired at intervals of 0.8 K. From the acquired data the values of c 0 and c 00 were calculated using the dedicated software. The estimates of the grain size and the grain size distribution in the samples were made through scanning electron micrographs.
3. Results and discussion Fig. 1 shows the ac susceptibility (c 0 and c 00 ) vs. temperature behaviour of the undoped (pure) and Mg-doped
Cu-1234 samples. It is observed that the diamagnetic onset temperatures of the Mg-doped samples are essentially the same as that of pure Cu-1234 sample. But more importantly Mg-substitution shifts the c 0 -T and c 00 -T curves to higher temperature side, decreases the transition width in c 0 -T curves and also improves the shielding fraction of the superconducting phase in the samples. This is a clear indication of the improvement in the superconducting properties brought about by Mg substitution in the Cu1234 system. c 00 -T curves of the samples are shown in Fig. 2(a–c). While the c 0 -T curve of a high Tc sample indicates the diamagnetic onset temperature and the shielding fraction of the superconducting phases, it is the c 00 -T curve which is of prime importance in understanding the dynamics of flux penetration into superconductor while its temperature is being increased in the presence of magnetic field. The variation c 00 (which represents the losses in the sample) with temperature and mechanism of formation of peaks in the c 00 vs T curves of the high Tc samples (hereafter referred to as c 00 peak) has been nicely described in the literature [12,13]. Accordingly, in a high Tc sample the broad peak appearing well below the diamagnetic onset temperature is due to the gradual penetration of flux into the center of the intergranular regions (i.e. center of the sample). As the temperature is increased from a sufficiently low value at which the sample is in the Meissner state (c 0 ZK1 and c 00 Z 0), c 00 starts becoming positive due to gradual penetration of the field into the grain boundary regions. At a temperature Tp, when the flux penetrates into the center of the sample, a peak in c 00 develops. Above Tp the extent of flux penetration decreases as the sample is already almost fully penetrated, this leads to a drop in c 00 which decreases gradually again to zero. The second peak in c 00 , as observed in our samples Fig. 2(a–c) which is just below the diamagnetic onset temperature is less prominent but quite sharp. This peak is due to the flux penetration into the grains of the sample. The mechanism of this peak formation is similar to that of intergranular c 00 peak formation. The grains being better superconductor than the intergranular region, the flux penetration into them occurs at a higher temperature than into the intergranular regions. It is pointed out here that only intergranular peaks in c 00 are normally observed in the case of fine grained (less than 10 mm size) high Tc polycrystalline samples which are common nowadays and was only in the early period of High Tc superconductivity, when the samples used to be coarse grained (more than 20 mm in size) that two peaks in c 00 could be observed [19–21]. Possible reason for the non-occurrence of the intragranular peaks in a high Tc sample has been explained by Goldfarb et al. [12]. The most likely course for the size being of the order of penetration depth, in which case the grains are already in the penetrated state to start with, excluding any further flux penetration which is needed for intragranular c 00 peak formation. But the grain size of the our Cu-1234 samples (w6 mm) can by no
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Fig. 1. (a) Variation of c 0 and c 00 with temperature for the pristine Cu-1234 Superconductor. (b) Variation of c 0 and c 00 with temperature for 10% Mg-doped Cu1234 Superconductor. (c) Variation of c 0 and c 00 with temperature for 20% Mg-doped Cu-1234 Superconductor.
means be taken as coarse. The occurrence of the intragranular c 00 peaks in our samples is probably due to the smaller values of the penetration depth (in comparison to the grain size) observed in Cu-1234 system which in turn is the result of lower anisotropy of Cu-1234 crystal structure [22]. From Fig. 2(c) (20%Mg doped sample), the intragranular peaks in c 00 are almost masked by the intergranular c 00 peaks. This probably is due to well coupled nature of the grains in this sample (possibly brought about by
the improved superconductivity due to Mg substitution) in which case a small measuring field will cause the coupling peak (i.e. intergranular peak) to obscure the intragranular peak [12]. From Fig. 2(a–c) it is seen that the increase in ac field amplitude shifts the intergranular c 00 peaks towards left (decrease superconductivity in grain boundaries) whereas intragranular c 00 peaks remain unaffected by the increase in ac field amplitude. This is because in the amplitude range shown (which is the maximum available in the ACS 7000
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Fig. 2. (a) Variation of c 00 with temperature for the pristine Cu-1234 superconductor. (b) Variation of c 00 with temperature for 10% Mg-doped Cu-1234 superconductor. (c) Variation of c 00 with temperature for 20% Mg-doped Cu-1234 superconductor.
susceptometer), the ac field does not have any effect on the superconductivity of the grains at all. From the c 00 -T curves of the Mg substituted Cu-1234 samples it is seen that Mg doping not only shifts the curves towards higher temperatures but also reduces the transition width of c 00 -T curves. As is well established the superconducting properties of a high Tc sample, as revealed by the ac susceptibility are very much dependent upon the sample specific parameters such as sample size, its density, grain size and grain size distribution etc [23,24]. The present samples have been found to have similar sizes (w3–4!2!1.5 mm3). They have similar grain sizes (w6 mm) and grain distributions as revealed by the scanning electron microscopy. It is worthwhile to include here that all the samples, pristine as well as substituted ones have been synthesized under identical conditions and the average grain sizes obtained here confirm to those reported earlier [9,10,22]. Therefore, the variations in c 00 -T behaviour in Fig. 2 can be exclusively attributed to the substitutional effects of Mg in Cu-1234 system. The broadened peaks prominent in c 0 -T curves being the sure signatures of intergranular properties in a high Tc sample, we can use critical state models [13–17] to estimate the temperature dependent intergranular critical current density Jc(T) in the present Cu-1234 samples. We employ Bean’s model [12,15,18] to calculate the intergranular
critical current density in the present samples using the relation Jc(Tp)ZHa/a for a sample having cross section 2a!2b where a!b. Jc(Tp) is the intergranular critical current density at Tp—the temperature of the c 00 peak, Ha is the amplitude of the applied ac field. The calculated values of Jc(Tp) for the pure as well as 10 and 20% Mg substituted (at Ca- site) sample are plotted against temperature (Tp) in Fig. 3.
Fig. 3. JC versus TP behavior for the (a) pristine, (b) 10% Mg-doped and (c) 20% Mg-doped Cu-1234 superconductors. Solid lines drawn are only guide to the eye.
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From Fig. 3 it is seen that the Jc values estimated are for the temperatures just in the neighbourhood of the Tc value of the sample. In the available field range in the susceptometer (maximum ac field amplitude of 800 A/m), it was not possible to shift the c 00 peaks (at which the Jc values could be calculated using Bean’s model) further down to the lower temperatures. It is also seen from Fig. 3 that Mg doping brings about an improvement in the intergranular critical current density of Cu-1234 samples. This can be understood on the basis of the improvement (reduction) in the superconducting anisotropy factor brought about by the Mg substitution in Cu-1234 system, studied in detail in Ref. [11]. It has been found by XRD that Mg substitution decreases the lattice parameters a and c. It is inferred that Mg2C may substitute both in the superconducting blocks (SLB) Ca3Cu4O8 (in place of Ca2C) and in the charge reservoir blocks (CRB), CuBa2O4Ky in place of Cu-site vacancies. In both cases there is a decrease in the c-parameter. In the first case, the decrease is due to smaller ionic size of Mg2C as compared to that of Ca2C and in the second case, the decrease is because of the increased Cu2C site occupancy leading to the decrease in the CRB size. The second phenomenon has been observed in other substitution cases also in Cu-1234 system such as CuxCe1KxBa2CanK1CunOy and CuxBa2CanK1CunOy [25]. The Mg substitution at Cu-site vacancies no doubt renders the CRB less conducting but by reducing the size of CRB, it also brings the two adjoining SLB closer. Assuming that SLB-CRB-SLB form a superconductor—normal—superconductor junction, the superconducting wave functions from the adjacent SLBs will have a better overlap (which more than compensates the reduced conductivity of the resulting CRBs), because of the reduction in the size of the intervening CRB, in the case of Mg doped samples. This eventually leads to the reduced superconducting anisotropy along the c-axis and improved intragranular superconductivity in Mg-substituted Cu-1234 system. Such an improvement in the intragranular superconducting properties will ultimately result in the intergranular superconducting properties also viz. intergranular J c in Mg-substituted Cu-1234 samples. The polycrystalline Cu-1234 samples can be modeled as superconductor— insulator—superconductor (SIS) junction consisting of superconducting grains separated by insulating grain boundaries. Any improvement in the intragranular properties will lead to the improvement in the overlapping of the superconducting wave functions across the SIS junction, which results in the improvement of the intergranular Jc as observed in the case of the Mg substituted Cu-1234 samples. A detailed mathematical description of the SIS junction and the intragranular phenomenon affecting the intergranular Jc in a polycrystalline Er-123 HTSC system is provided in Ref. [26]. The intragranular c 00 peaks in pure and 10% Mg doped Cu-1234 samples occur at about 115 K (just below the diamagnetic onset temperature of 117 K) and these are amplitude invariant also. Again using the Bean’s model we
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can calculate the intragranular Jc at 115 K for the samples using the relation Jc (115 K)ZHa/g here Ha is the applied ac field amplitude and 2 g is the grain thickness (which is 4 mm in our samples). A intragranular Jc of 4!104 A/cm2 at 115 K is obtained with HaZ800 A/m. This compares favourably with the intragranular Jc values for the Cu-1234 samples [9,10].
4. Conclusions Field dependent ac susceptibility studies were carried out in pure and Mg-substituted Cu-1234 samples. The c 00 -T curves at different ac field amplitudes exhibit both inter- and intra-granular peaks. The values of the intergranular critical current densities as estimated from the c 00 -T curves are higher for Mg-substituted samples as compared to the pure Cu-1234 sample. Such an improvement in the intergranular Jc of the substituted samples is attributed to the reduction in the superconducting anisotropy of the Cu-1234 crystal structure brought about by Mg2C incorporation, which ultimately results in better inter- as well as intra-granular superconducting properties.
Acknowledgements The authors are grateful to the Director of their laboratory for providing the facilities to carry out the present work. Computational help from Ajay, Neeraj and Vikram is gratefully acknowledged. One of us (SKA) would like to thank the superconducting materials section of Professor H. Ihara, of the Electro-technical Laboratory, Tsukuba, Japan and Professor T. Watanabe of the Science University of Tokyo, Chiba for utilizing their excellent high pressure synthesis facilities.
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