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Superconductivity of Nb-substituted Bi-2223 superconductor
Physica C 304 Ž1998. 293–306
Superconductivity of Nb-substituted Bi-2223 superconductor D.R. Mishra, P.L. Upadhyay ) , R.G. Sharma National Physical Laboratory, Dr. K.S. Krishnan Road, New Delhi 110012, India Received 7 September 1997; revised 27 February 1998; accepted 20 May 1998
Abstract Superconductivity of Nb-doped samples of ŽBiPb. 2 Sr2 Ca 2 Cu 3 O d has been investigated by substitution of this element at the Bi-site and the Cu-site. While a depression of Tc with increasing dopant concentration is observed for both the series of samples, the effect is much more drastic for the Cu-site samples. Magnetic studies also show a more rapid increase in magnetisation of the Cu-site substituted samples. The results are explained on the possibility of formation of a virtual bound state ŽVBS. of the d-level of Nb close to the E F . q 1998 Elsevier Science B.V. All rights reserved. PACS: 74.25 Ha; 74.25 Fy; 74.60 Ec; 74.62 Dh; 74.72 Hs Keywords: Electrical resistivity; Magnetisation; Substitution effects; Pair-breaking
1. Introduction Dopant studies in HTSC play an important role in the in depth understanding of superconductivity only if the physical origin of Tc suppression or enhancement could be ascertained, but because of the complex nature of these materials these efforts have not been very conclusive. There are too many experimental parameters which determine the quality of the specimens and control their Tc s and an additional difficulty arises in case of multi-component systems such as the Bi 2 Sr2 Ca ny1Cu nO 2 nq4 series where the substituted atoms may also affect the relative stability of different superconducting phases of the system. Elemental addition of Pb Žand also Ag, Sb, etc.. has proven useful in enhancing the stability of the 2223 phase and thus facilitating single-phase fabrica)
Corresponding author. Fax: q91-11-5752678, q91-115764189; Telex: 031-77099 NPL IN, 031-77384 RSD IN
tion of this high Tc phase to further investigate the superconductivity of this system by doping with other elements such as the transition metals with their empty d-bands. This paper reports results of our exploration of the Nb-substituted Bi-2223 superconductor which forms part of a detailed investigation on the substitutional effects of d- and f-band materials. Previous reports on Nb-substituted samples of ŽBiPb. 2 Sr2 Ca 2 Cu 3 O d have mostly pointed out that Nb either remains neutral to superconductivity w1x or affects the phase-stability of different superconducting phases w2,3x in this system. Li et al. w3x have shown that the addition of Nb to the Bi-system is much like the role of Pb in the pure Bi–Sr–Ca–Cu–O samples, i.e., it acts like a catalyst and enhances the formation of high Tc 2223 phase. Our studies on samples of Bi-2223 prepared by substitution of Nb at two different sites: viz., the Bi-site and the Cu-site, which have yielded very different results which lead
0921-4534r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 9 8 . 0 0 2 5 7 - 3
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us to believe the possible role of Nb as a magnetic scattering center affecting the ground state of the Bi-superconductor. Magnetisation measurements also suggest the possibility of Nb behaving as paramagnetic centres in this compound.
oscilloscope screen were plotted using the attached plotter.
3. Results and discussion 3.1. Tc behaÕiour and XRD studies
2. Experimental Two series of Nb-substituted samples were prepared by partial replacement of Nb at two different sites: viz., the Bi-site according to ŽBi 1.6yx Nb xPb 0.4 .Sr2 Ca 2 Cu 3 O d Žwhere x s 0.1 to 0.5 and samples are labelled as NBi-1 to NBi-5. and at the C u-site according to Ž B i 1 .6 Pb 0 .4 . S r 2 C a 2 ŽCu 3y y Nb y .O d with y s 0.1 to 0.4 and samples are referred as NCu-1 to NCu-4. The standard solid state diffusion route was adopted for this purpose in which appropriate quantities of Bi 2 O 3 , PbO, SrCO 3 , CaCO 3 , CuO and Nb 2 O5 all of 4 N purity were thoroughly mixed, ground and calcined at 8108C followed by regrinding, and re-calcination at 8158C for 24 h. It was found for both the series of samples that the melting temperature was considerably reduced when Nb 2 O5 was added to the system. The pure sample Ži.e., when x s y s 0. survived up to temperatures as high as 8408C while samples NBi-5 and NCu-4, having the maximum concentration of Nb Ž x s 0.5 and y s 0.4., melted even at the preliminary stages of calcination at 8158C and the sample NCu-3 Žwith y s 0.3. gets partially melted during sintering. All the surviving samples were ground, pelletised and finally sintered in air at 8308C for 30 h and furnace-cooled to room temperature. To observe the superconducting transitions, d.c. resistivity of the specimens was measured as a function of temperature by the four-probe method and the real part of the a.c. susceptibility Ž x X . was measured as a function of temperature using the mutual inductance bridge. Room temperature XRD spectra were obtained on all the specimens for crystal structure determination on a Siemen’s D-500 diffractometer using Cu K a radiation. Zero field cooled a.c. magnetisation measurements were carried out at 77 K on samples ; 4 mm in width and approximately 16 = 1.8 mm2 in cross-section at a frequency of 317 Hz at fixed applied fields Žshown in Table 2. in the range of 1 to 10 Oe. Hysteresis loops observed on the
C om positions of different sam ples of ŽBiPb. 2 Sr2 Ca 2 Cu 3 O d with Nb substituted at Bi-site and at Cu-site, as described above, the heat-treatments given to them, their Tc values determined by the resistivity and susceptibility methods and lattice parameters, etc., are shown in Table 1. Resistivity behaviour of different samples of ŽBiPb. 2 Sr2 Ca 2 Cu 3 O d in which Bi is replaced with Nb according to ŽBi 1.6y x Nb x Pb 0.4 .Sr 2 Ca 2 Cu 3 O d are shown in Fig. 1Ža. and that of samples in which Cu is replaced by Nb according to Bi 1.6 Pb 0.4 Sr2 Ca 2 ŽCu 3y y Nb y .O d , are shown in Fig. 1Žb.. In case of NBi series, it is seen that Tc decreases with increase in dopant concentration Žup to x s 0.3., but in case of NCu samples, Tc decreases much faster and for y ) 0.2, superconductivity was not observed at T ) 77 K. As generally observed for pure Bi samples, higher normal state resistivity is accompanied with a lower Tc , the same trend is observed here for all the samples. Samples not showing superconductivity above 77 K also follow this behaviour and their normal state resistivity shows an increasing trend with temperature, i.e., there is no indication of a semiconducting behaviour which leads to a metal-insulator transition typically observed in materials doped with other elements such as Y, Tm, etc. w4,5x. Superconducting transitions observed by the a.c. susceptibility method also show similar variations for different dopant concentrations at the two doping sites, as observed in the resistivity measurements. Fig. 2Ža. shows the variation of the real part of a.c. susceptibility Ž x X . with temperature of samples having Nb at the Bi-site, i.e., the NBi series, and Fig. 2Žb. shows the same for Nb-substituted at the Cu-site: viz., NCu series. Tc values obtained from these transitions are slightly lower than those obtained by resistivity measurements, and here also one observes that the reduction in Tc is more prominent in case of NCu samples than in NBi ones. These results suggest that Nb substitution at the Cu-site has a more drastic
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Table 1 Sample compositions, Tc values and summary of XRD results Sample
Tc ŽK.
XRD results: c-axis of
Composition and heat-treatment Ž8C = h.
Label
Res Ž R s 0.
x X Žonset. Žoffset.
2223 phase ˚. ŽA
2212 phase ˚. ŽA
ŽBi 1.6 Pb 0.4 .Sr2 Ca 2 Cu 3 Ž810 = 24 q 815 = 24 q 830 = 30. ŽBi 1.5 Nb 0.1 Pb 0.4 .Sr2 Ca 2 Cu 3 Ž810 = 24 q 815 = 24 q 830 = 30. ŽBi 1.4 Nb 0.2 Pb 0.4 .Sr2 Ca 2 Cu 3 Ž810 = 24 q 815 = 24 q 830 = 30. ŽBi 1.3 Nb 0.3 Pb 0.4 .Sr2 Ca 2 Cu 3 Ž810 = 24 q 815 = 24 q 830 = 30. ŽBi 1.2 Nb 0.4 Pb 0.4 .Sr2 Ca 2 Cu 3 Ž810 = 24 q 815 = 24 q 830 = 30. ŽBi 1.1 Nb 0.5 Pb 0.4 .Sr2 Ca 2 Cu 3 Ž810 = 24 q 815 = 24. ŽBi 1.6 Pb 0.4 .Sr2 Ca 2 Cu 2.9 Nb 0.1 Ž810 = 24 q 815 = 24 q 830 = 30. ŽBi 1.6 Pb 0.4 .Sr2 Ca 2 Cu 2.8 Nb 0.2 Ž810 = 24 q 815 = 24 q 830 = 30. ŽBi 1.6 Pb 0.4 .Sr2 Ca 2 Cu 2.7 Nb 0.3 Ž810 = 24 q 815 = 24 q 830 = 30. ŽBi 1.6 Pb 0.4 .Sr2 Ca 2 Cu 2.6 Nb 0.4 Ž810 = 24 q 815 = 24.
Pure
102.6
102.0 82.0 98.0 82.0 92.0 ; 77 K –
36.47
29.74
36.56
30.01
36.60
30.74
NBi-1
96.5
NBi-2
92.0
NBi-3
79.0
NBi-4 NBi-5
Not superconducting above 77 K melted –
NCu-1
96.0
NCu-2
82.0
NCu-3 NCu-4
effect than at the Bi-site, and to investigate into its causes we now examine the XRD results, as discussed below. The effect of substitution on the formation of various phases in the Bi-series can be seen from the XRD patterns of the pure and Nb-substituted samples of ŽBiPb. 2 Sr2 Ca 2 Cu 3 O d , shown in Fig. 3Ža–d., for NBi, and in Fig. 4Ža–d. for NCu series. The XRD spectra of the un-doped Bi-2223 sample shows mainly the dominant lines of the 3-layer 2223 phase but most of the substituted samples of NBi series show peaks corresponding to 2223 and 2212 phases for low dopant concentrations Ž x - 0.2. and those corresponding to 2212 and 2201 phases for the higher concentration of x s 0.3. This is true for the NCu samples also except that the 2212 peaks start dominating at a lower concentration of y s 0.2 only. Most of these reflections correspond to a psuedo-tetragonal unit cell having lattice parameters a f b f ˚ and values of c-parameter for different 5.408 A samples are shown in Table 1. Deviation of these values from the ideal structure values reported in the literature suggests the possible formation of a dis-
94.0 82.0 80.0 74.0
Not superconducting above 77 K melted
–
29.72
–
–
–
–
36.15
30.50
–
30.48
–
30.34
–
–
torted crystal structure. Such deviations are encountered even in pure ŽBi,Pb.-2223 superconductor, where this can be caused by different cation ratios and varying oxygen contents. In our doped samples, the possibility of a distorted lattice is further enhanced considering the off-stoichiometry of the compound caused by the replacement of Bi and Cu by Nb or by its inclusion at various substitutional or interstitial sites, whatever the case may be. Since the quantity of Nb added is very small, the possibility of its detection, if involved in any compound-formation in XRD spectra is negligible, and consequently no such lines could be identified in the spectra. It is known w6x that the formation of 2223 phase in the pure BiSrCaCuO specimen is extremely slow and its rapid growth in the ŽBiPb. 2 Sr2 Ca 2 Cu 3 O d system is assisted by the formation of a liquid phase ŽCa 2 PbO4 , Sr2 PbO4 , or a eutectic of 2201 and Ca 2 PbO4, w7x. thus improving the mobility of ions. Partial melting of our Nb substituted samples for low dopant concentrations and melting of these at higher dopant concentrations Ž x ) 0.4. at only 8308C suggests that the addition of Nb 2 O5 further extends the
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Fig. 1. Resistance vs. temperature curves for Nb-substituted samples of Bi-2223. Ž a. Nb-substitution at Bi-site, i.e., ŽBi 1.6yx Nb x Pb 0.4 .Sr2 Ca 2 Cu 3 O d ŽNBi. series. Žb. Nb-substitution at Cu-site, i.e., ŽBi 1.6 Pb 0.4 .Sr2 Ca 2 ŽCu 3yy Nb y .O d ŽNCu. series.
range of the liquid phase, thus increasing the possibility of the substituents occupying the positions Žinterstitial or substitutional sites. in the crystal structure, which may get well precipitated out if subjected
to prolonged annealing. Resistivity behaviour of our samples, however, indicates some possibility of Nbatoms being retained in the lattice because of the decrease in Tc . Similar indications are given by the
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297
X Fig. 2. A.c. susceptibility Ž x . as a function of temperature for Nb-substituted samples of Bi-2223. Ža. Nb-substitution at Bi-site. Žb. Nb-substitution at Cu-site.
XRD results which show mainly the lines of the 2223 phase for small dopant concentrations ŽFigs. 3Žb. and 4Žb.. which means that the 2223 does get
rapidly formed and the lattice parameter values ŽTable 1. suggest a small distortion of the crystal structure either due to the presence of the substituted
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atoms or due to the off-stoichiometry of the compound. We notice from XRD results that as the dopant concentration increases, formation of the 2223 is suppressed and 2212 becomes the dominant phase. Tc values also correspond to the typical values of the 2212 phase Ž; 80 K. and for y s 0.3 for the NCu and x s 0.4 for the NBi samples superconductivity above 77 K was not observed at all. XRD of such a sample ŽFig. 4Žd.. shows some reflections corresponding to the 2212 phase, thereby implying that Nb-substitution may also be causing a decrease in the Tc of the 2212 phase as well. The degradation of Tc is particularly drastic for the NCu samples whereas the NBi samples show a less rapid decrease of Tc with Nb concentration. An immediate implication of this appears to be that in the NCu samples Nb gets incorporated in the CuO 2 layers in which presence of any impurity element has a dramatic effect on the Tc of the superconductor. The details of how this takes place are described later in Section 3.3 on the role of Nb. Another possibility for such a lowering of Tc , particularly for the NCu-series comes from the phase-formations affected by the deficiency of Cu when partly substituted by Nb. Cu-deficient compositions of 2223 may result in enhanced formation of the 2212 Žand also 2201. phases which in turn affects the Tc , particularly the Tc Žzero. of these samples. This may also explain why superconductivity could not be observed above 77 K when the dopant concentration was increased beyond y s 0.3. In case of NBi samples, the initial drop of Tc is similar to the NCu samples Žup to x s 0.1., but with higher concentrations, it decreases less rapidly. At x s 0.3, only the 2212 phase appears to be present and its Tc value of 79 K does not reflect any significant effect of the dopant. This relatively less sensitivity of the Tc of NBi samples to the dopant concentration suggests the possibility of Nb occupying an out-of-CuO2 plane position in the crystal structure or just getting segregated in the grainboundaries. The deviations observed in the 2 u positions of the important Ž001. reflections in the XRD pattern ŽFig. 3Žb–d.., however, indicate that some
Fig. 3. XRD patterns of ŽBi 1.6yx Nb x Pb 0.4 .Sr2 Ca 2 Cu 3 O d series, where Ža–d. x s 0 to 0.3 as shown in the figure. wŽ'. 2223, Žv . 2212 and Žq. 2201 phasesx.
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lattice-sites could possibly be occupied by the dopant ions Žor possibly due to different cation ratios and varying oxygen contents, as described above.. For cuprate superconductors, it is generally observed that substitution in the planes other than CuO 2 which create a charge imbalance and are expected to modify the formal valence of Cu through charge-transfer can result in the lowering of their Tc . The presence of Nb in the q5 state in the Bi–O layer could be causing such a degradation effect because of the replacement of Biq3 . There is, however, some experimental evidence w8x that Bi is not always present as Biq3 , and oxygen non-stoichiometry of the Bi–O layers can alter its valence state to Biq5 , and thus induces a variation of Tc even in the undoped sam˚ . and ples. Since the ionic radii of Biq5 Ž0.74 A ˚ . are close to each other, there is a Nbq5 Ž0.70 A good possibility of Nb occupying positions in Bi–O layers, i.e., a substitution which would not affect the charge-balance if Bi originally existed in the Biq5 state and, therefore the Tc of the material. If, however, Bi existed in the Biq3 state and gets replaced by Nbq5 after doping, the Tc variation expected would be similar to that of undoped sample in which Biq3 gets converted to Biq5 because of variation of oxygen content. Thus, we notice that in NBi samples, Tc variations affected by Nb-doping are similar to variations observed in un-doped ŽBiPb.SrCaCuO, which is in general agreement to what is generally reported w1–3x, and the effect is not as prominent as in the case of NCu samples where there is a possibility of Tc getting degraded because of pair-breaking by magnetic scattering. This is also supported by our magnetisation measurements at low magnetic fields. 3.2. Magnetisation and low field hysteresis An interesting feature of the HTSC is the observation of low field hysteresis loop when these materials are placed in an a.c. magnetic field which means that penetration of flux and formation of a vortex lattice becomes possible at a field as low as 1 Oe. This is mainly due to the granular nature of these ceramics
Fig. 4. XRD patterns of ŽBi 1.6 Pb 0.4 .Sr2 Ca 2 ŽCu 3yy Nb y .O d series, where Ža–d. y s 0 to 0.3 as shown in the figure. wŽ'. 2223, Žv . 2212 and Žq. 2201 phasesx.
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but some researchers believe that Hc1 of the superconducting grain itself is as low as a few gauss w9,10x and flux penetrates not only in the inter-granular space but also within the grains. Our results on a.c. magnetisation measurements on pure and Nb-substituted Žboth at Bi- and Cu-site. carried out at fields between 1.15 and 9.7 Oe, have shown the opening of the hysteresis loop at 77 K which remains elliptical up to a certain field and then distorts from the elliptical shape but remains a closed loop up to the fields available in our experiment Ž9.7 Oe.. The field at which distortion sets in Žrepresenting irreversibility. varies for different samples and so does the area of the loop and the hysteretic magnetisation D M. The magnetisation parameters for various applied
a.c. fields for different samples of pure and Nb-substituted Žat Bi- and Cu-site. specimens are given in Table 2. As in the case of Tc behaviour, the magnetisation results of samples of Nb-substituted at the Cu-site are more pronounced than that of Bi-site substitutions. Highest values of D M are observed for the sample NCu-1, i.e., when Nb is substituted at Cu-site with concentration y s 0.1. D M values increase with increasing applied fields and the deviation from the elliptical shape of the hysteresis loop takes place at Ha.c.s 6.15 Oe Ž500 mA. for both y s 0.1 and y s 0.2. For the un-doped sample and the Bi-site substituted samples, the distortion of the ellipse takes place at a higher field, 9.7 Oe Ž800 mA. for pure and for
Table 2 Magnetisation data Sample
Pure
NBI-1
NBI-2
NBI-3
NCu-1
NCu-2
Applied field coll-curr., Ha.c. ŽmA.
ŽOe.
75 150 300 500 800 75 150 300 500 800 75 150 300 500 800 75 150 300 500 800 75 150 300 500 800 75 150 300 500 800
1.15 1.95 3.70 6.15 9.70 1.15 1.95 3.70 6.15 9.70 1.15 1.95 3.70 6.15 9.70 1.15 1.95 3.70 6.15 9.70 1.15 1.95 3.70 6.15 9.70 1.15 1.95 3.70 6.15 9.70
Loopshape
D M Žper unit thickness. ŽOe.
Loop-arear volume Žemurcc.
Jc s 2D Mrd ŽArcm2 .
ellipse ellipse ellipse ellipse distorted ellipse ellipse ellipse ellipse ellipse ellipse ellipse ellipse ellipse distorted ellipse ellipse ellipse ellipse ellipse ellipse ellipse ellipse distorted distorted ellipse ellipse ellipse distorted distorted
11.89 13.25 15.33 16.98 18.65 1.67 2.20 3.26 4.96 7.02 8.16 9.45 9.87 9.98 10.16 1.28 2.13 3.95 6.55 7.87 12.49 14.15 15.99 16.22 17.52 4.88 7.48 10.14 11.65 12.74
0.035 0.082 0.208 0.354 0.754 0.006 0.017 0.056 0.142 0.291 0.021 0.058 0.117 0.134 0.304 0.005 0.018 0.069 0.175 0.332 0.038 0.112 0.235 0.413 0.559 0.018 0.061 0.154 0.297 0.416
95.1 106.0 122.6 135.8 149.2 13.4 17.8 26.8 39.7 56.0 65.2 75.6 79.0 80.0 81.0 10.3 17.0 31.6 52.4 63.0 100.0 113.2 128.0 130.0 140.0 39.2 59.2 81.0 93.2 101.2
D.R. Mishra et al.r Physica C 304 (1998) 293–306
NBi with x s 0.2; the loop remained elliptical up to the maximum field available in our experiment, i.e., 9.7 Oe for NBi having x s 0.1 and x s 0.3. The shape of these hysteresis loops, elliptical as well as distorted, for the pure and NCu-1 Ž y s 0.1. sample for fields of 1.15 to 9.7 Oe are shown in Figs. 5Ža–e. and 6Ža–e.. These distortions of the ellipses at different fields can be understood on the following picture of grains in these ceramic samples. For the pure, undoped sample we expect the grains as mainly composing of the 2223 phase linked with each other with a Josephson-like coupling. As the magnetic field is applied, flux penetrates first the inter-granular region and then at higher Ha.c. s, it enters into the intra-granular region. Since the experiment is carried out at 77 K, at a temperature well below the Tc of the grains, an increase in the applied field Ha.c. gives rise to more and more of magnetisation accompanied with an increase of the area of the loop. Elliptical shape of the loop is retained up to a field of 6.15 Oe Ž500 mA., which means that B is not appreciably out of equilibrium with H even though a large amount of flux is cycling the sample. To give a description of the flux-distribution and associated internal critical currents, Bean’s critical state model appears to be the best choice, as will be discussed later. For the doped samples, when Nb is substituted at Cu-site, we notice that even though the Tc drops to 92 K when the dopant concentration is only y s 0.1, we suppose that the grains still consist mainly of the 2223 phase as is shown in the XRD pattern ŽFig. 4Žb.. and their Tc gets degraded because of the replacement of Cu by Nb atoms. When these samples are placed in the a.c. magnetic field at 77 K, D M values are higher Ževen though only slightly, see Table 2., than the pure sample and the deformation of the elliptical loop takes place at a lower field of 6.15 Oe Ž500 mA.. One possible reason for this may be that the grain-coupling gets weakened at a lower field Žthe intrinsic lower Tc of the grains could also be one of the factors responsible for this., and flux-penetration results in trapping at the inter-granular regions giving rise to comparatively larger values
Fig. 5. A.c. magnetization hysteresis loops of ŽBi 1.6 Pb 0.4 . Sr2 Ca 2 Cu 3 O d at 77 K in different Hmax at 317 Hz.
301
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of D M. One would expect B to be even more out of equilibrium with H than in case of pure specimen while the flux-cycle still remains large. For the NCu-2 sample Ž y s 0.2., we notice from XRD patterns ŽFig. 4Žc.. that the dominant phase appears to be 2212 and the Tc also gets degraded to 82 K. It is quite possible that this 82 K Tc may still be corresponding to some 2223 material as this phase gets formed by a reaction between a liquid phase ŽCa 2 PbO4 , etc.. and the 2212 or the 2201 phase w6x and is, therefore, likely to form a thin layer at the surface while the interior of the grain is still the 2212 or 2201 phase, the dominance of which is reflected in the XRD pattern. Since only a small fraction of 2223 with Nb incorporated in it is present as the superconducting phase, part of the Nb remains segregated and possibly affects the magnetisation of the samples. In case of Bi-site substituted samples, it is seen that not only the Tc degradation effect is less pronounced but the magnetisation behaviour is also not affected as drastically as in the case of Cu-site substituted samples. The NBi samples with x s 0.1 and x s 0.3 show an elliptical loop up to the maximum field applied in these experiments and for the x s 0.2 sample NBi-2 the distortion takes place at 9.7 Oe, same as the pure, undoped specimen. The Cu-site substituted samples on the other hand show a distortion at a lower field of ; 6.15 Oe. Not only the deviation from ellipticity occurs at a higher field for the NBi samples but the area of the hysteresis loops and the value of D M are also much lower for these samples. This suggests that the defects which interact with flux-threads and hinder their motion Žtrapping of flux. are less dominant in NBi samples and the major superconducting phase in these samples appears to be more homogeneous than in NCu samples. Observation of the hysteresis loops for all our samples, pure as well as doped, at an applied field of only 1.15 Oe show that these samples have a small penetration field H ), the minimum field required for the flux to enter and reach the centre of the
Fig. 6. A.c. magnetization hysteresis loops of ŽBi 1.6 Pb 0.4 . Sr2 Ca 2 Cu 2.9 Nb 0.1O d at 77 K in different Hmax at 317 Hz.
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specimen. This penetrated flux establishes a gradient and gives rise to the internal current flow and as the applied field increases, the spatial distribution of magnetic flux within the sample changes and consequently the internal patterns of current-flow will change. In order to establish a relationship between the magnetisation and critical currents to estimate Jc of the superconductor from magnetisation values several models have been given, the basis of all of which is the existence of a critical state, i.e., when the interaction between the flux-lines and the defects in the material is balanced by the magnetic driving force resulting from the gradient in the flux density. The original critical state picture introduced by Bean w11x was based on the assumption of critical current density Jc being independent of B. It is, however, often found that Jc is a strong function of B and this dependence must be taken into consideration for predicting the actual critical currents. A number of authors Žsee, for example, w12x. have provided a detailed analysis of the critical state and its applicability to the granular high Tc superconductors, considering various field-dependencies of Jc , two of them have been discussed in detail by Ji et al. w13x. These are: Ž1. that based on Jc being constant with B ŽBean., and Ž2. in which Jc varies as the inverse of B, i.e., the Kim Anderson model. We have used their analysis to explain the shapes of our experimental hysteresis loops for both Ž1. and Ž2. and find that most of the loops can be matched with the theoretical loops obtained using the field-independent Jc picture of Bean if suitable values of different parameters in the following equation can be assigned. 4p M s 4p M Bean y aH.
Ž 1.
Here, the aH term accounts for the diamagnetism of the grains w14x, and: 4p M Bean s B y H ,
where B s
EH Ž x . d x Ed x
.
We give below the selection of our parameters and their justification. It is seen that the theoretical curves shown in Fig. 7 closely resemble the distorted ellipses experimentally observed in our specimen Žexcept for the small
303
asymmetry in the shape of the loops, the cause of which will be discussed later.. These curves were obtained when in Eq. Ž1. the factor, a, representing the contribution due to the grains in the Meissner state is taken to be negligibly small Žequal to 0.02 or 0. which indicates the dominance of the magnetisation induced by the inter-granular flux distribution represented by the first term. This means that at applied fields of only 6.3 Oe for NCu Žand 9.7 Oe for pure and NBi-2. series flux penetration in the sample has substantially affected not only the intergranular region but also the diamagnetism of the grains. Even otherwise, because of the intrinsic anisotropy of the high Tc superconductors, the magnetisation does not remain parallel to B unless the field is directed along one of the principal crystal directions, its magnetic response is, therefore, no longer purely diamagnetic. Thus, we see that samples in which distortion of the ellipse occurs at a lower field are likely to have a contribution to the magnetisation from the intra-granular region also, particularly at higher Ha.c. s. Since the use of Bean’s model gives a reasonable description of flux-distribution in our samples, Jc values were evaluated from the magnetisation data ŽTable 2. using the formula: Jc s 2D Mrd, where d is the thickness of the sample. Although these values are quite low, a large variation is observed for different samples and the Cu-site substituted samples give the highest value of 100 Arcm2 at a field of 1.15 Oe. For the NBi samples, the values range from 10 to 60 Arcm2 for dopant concentration of x s 0.1 and 0.3, and from 65 to 81 Arcm2 for x s 0.2. Since for x s 0.1 and 0.2 samples, the dominant phase appears to be the 2223 phase, the marginal increase observed in x s 0.2 sample can be attributed to the increased inhomogeneity and therefore more defects in the superconducting grains Žof 2223.. For x s 0.3, the grains are expected to be of the 2212 phase, which could be relatively pure and homogeneous, thus giving low magnetisation and Jc values. For the NCu samples, despite a lower Tc , highest Jc values are estimated because of their large magnetisation values. Since the dominant phase appears to be the 2223 phase, at least for x s 0.1 sample, these results indicate that the different magnetisation and Jc behaviour could be arising from the Nb-addition to these samples.
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Fig. 7. Hysteresis loops predicted by the formulae Žafter Ref. w13x.: Ža. 4p M s 4p M Bean y Ž0.02. H; Žb. 4p M s 4p M Bean .
Another useful information coming from the magnetisation loops ŽFigs. 5 and 6. is the small asymmetry in their shapes after distortion from the ellipse; the flatness appears to be longer in the lower region than in the upper half. This is true for the pure sample also but is more noticeable in the NCu specimens because for these samples the distortion begins at a lower field and the applied field could be further increased up to the maximum available experimentally to see it expanding. Such an anisotropy is believed to be due to the presence of Bean– Livingston w15x surface barriers, i.e., defects are present at the surface which hinder the entry and exit of fluxoides in the sample. Defects at the surface are expected in our samples because, as discussed before along with XRD results, the formation of Bi-2223 phase takes place through a surface reaction in which its formation begins as a thin layer at the surface which increases as the diffusion time increases. This also indicates that the magnetisation behaviour is
influenced by not only the inter-granular region but also by flux-penetration within the superconducting grains with a contribution from the surface defects also. 3.3. Role of Nb It is generally observed in cuprate superconductors that cationic substitution at the Cu-site degrades the Tc irrespective of whether the substituted element is magnetic or nonmagnetic. This is because of the strong hybridization of the Cu-3d states with O-2p levels, whereby electrons collectively give rise to the metallic conduction. Any disturbance caused by the substituted element in this CuO 2 layer would modify the overlap of orbitals and hence the density of states near the Fermi level or below it, thus affecting the Tc . Alternatively, if the added impurity has its own magnetic moment, Tc degradation takes place by pair-breaking effects. In our Cu-site Nb-substituted
D.R. Mishra et al.r Physica C 304 (1998) 293–306
samples, reduction in the Tc Žonset. values Žas suggested by the resistivity measurements and confirmed by the susceptibility measurements. can be explained by such a pair-breaking effect. Here, it may be possible for Nb, with its partially filled d-band having free spins, to retain its paramagnetism and form a level below E F when present at the Cu-site of the Bi-2223 material and destroy its Tc because of the pair-breaking effect due to the spinpolarisation at E F . Evidence of Nb forming a level close to the EF or contributing to the VBS Žvirtual bound state. function has also been suggested by Asthana et al. w16x in their studies on Nb-doped Y1 Ba 2 Cu 3 O 7yd superconductor. Lowering of Tc Žoffset. in samples where two transitions are clearly seen could also be an effect of the above phenomenon but there is also a possibility that the general broadening of transition width DTc , and the reduction in Tc Žzero. in resistivity measurements are due to the increased relative volume fractions of the lower Tc 2212 and 2201 phases. A further support to our contention above of Nb forming a level close to the E F and contributing to the VBS function comes from the magnetisation results. If Nb is indeed forming a level below the E F and retaining its paramagnetism, as suggested by our Tc degradation effect, it should have its impact on the magnetisation induced in the sample when immersed in a magnetic field. Our magnetisation experiments have shown that the maximum value of D M is observed in samples containing Nb at the Cu-site ŽTable 2.. Even though the same amount of Nb is present in the samples in which Bi is replaced by Nb, the magnetisation effects are not so pronounced in this case. The hysteresis loops remain elliptical up to the same applied fields as for the pure, undoped specimen Žor even at higher fields.. This indicates that Nb is more or less neutral to superconductivity in these samples, as also suggested by w1x, while in the samples where Nb is substituted at the Cu-site, the Nb ion is influential as a scattering centre responsible for the reduction in Tc . The role of Nb, when incorporated at the Bi-site is to affect the charge-balance in the Bi–O layers and influence the formal valence of copper through transfer of charge. This induces deviation from the optimum hole concentration and causes a reduction in the Tc values.
305
4. Conclusions The above discussion suggests that the decrease in Tc of the Bi-2223 phase is dominated by the substitution effect of Nb to the ŽBiPb. 2 Sr2 Ca 2 Cu 3 O d superconductor and there is also a possibility of contribution from other effects such as the increased relative fractions of lower Tc 2212 and 2201 phases. The effect is more rapid and increases with the dopant concentration when Nb is substituted at the Cu-site than at the Bi-site and is also accompanied with an increase in magnetisation of the samples. This indicates the possibility of the d-level of Nb lying close to the E F and contributing to the pair-breaking effects responsible for the lowering of Tc .
Acknowledgements We gratefully acknowledge the support and encouragement of our Director Prof. E.S.R. Gopal in carrying out this research. Dr. D.K. Suri and other colleagues of our X-Ray section have been extremely helpful in conducting XRD measurements and analysis. One of us ŽPLU. would like to thank Dr. S.N. Ekbote for fruitful discussions, and ŽDRM. is grateful to CSIR for providing a research fellowship.
References w1x T. Kanai, T. Kamo, S.-P. Matsuda, Jpn. J. Appl. Phys. 28 Ž1989. 1551. w2x H. Nasn, N. Kuriyama, K. Kamiya, Jpn. J. Appl. Phys. 29 Ž1990. 1415. w3x Y. Li, B. Yang, J. Mater. Sci. Lett. 13 Ž1994. 594. w4x R. Yoshizaki, Y. Saito, Y. Abe, H. Ikeda, Physica C 152 Ž1988. 408. w5x J. Clayhold, S.J. Hagen, N.P. Ong, J.M. Tarascon, P. Barboax, Phys. Rev. B 39 Ž1989. 7320. w6x S. Nhien, G. Desgardin, Physica C 272 Ž1996. 309. w7x Jun Takada, Y. Ikada, M. Takamo, in: Hiroshi Maeda, Kazumasa Togano ŽEds.., Bismuth-based High-Temperature Superconductors, Chap. 5, Marcel Dekker, USA, 1996, p. 93. w8x F. Abbattista, C. Brisi, D. Mazza, M. Vallino, Mater. Res. Bull. 26 Ž1991. 107. w9x A.K. Grover, C. Radhakrishnamurty, P. Chaddah, G. Ravikumar, G.V. Subharao, Pramana-J. Phys. 30 Ž1988. L167.
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w10x P.L. Gammel, D.J. Bishop, G.J. Dolan, J.R. Kwo, L.A. Murray, L.F. Schneemeyer, J.V. Waszczak, Phys. Rev. Lett. 59 Ž1987. 2592. w11x C.P. Bean, Rev. Mod. Phys. 36 Ž1964. 31. w12x K.H. Muller, in: R.A. Hein, T.L. Francavilla, D.H. Licbenberg ŽEds.., Magnetic Susceptibility of Superconductors and Other Spin Systems, Plenum, New York, 1991, p. 229, Žsee also A. Sanchez, D.X. Chen, p. 251..
w13x L. Ji, R.H. Sohn, G.C. Spalding, C.J. Lobb, M. Tinkham, Phys. Rev. B. 40 Ž1989. 10936. w14x J.R. Clem, Physica C 153–155 Ž1988. 50. w15x C.P. Bean, J.D. Livingston, Phys. Rev. Lett. 12 Ž1964. 14. w16x P. Asthana, S.N. Ekbote, S. Chandra, Proc. of DAE Symp. on Solid State Phys. ŽBARC, India., 33C, 1990, p. 236.
Superconductivity of Nb-substituted Bi-2223 superconductor D.R. Mishra, P.L. Upadhyay ) , R.G. Sharma National Physical Laboratory, Dr. K.S. Krishnan Road, New Delhi 110012, India Received 7 September 1997; revised 27 February 1998; accepted 20 May 1998
Abstract Superconductivity of Nb-doped samples of ŽBiPb. 2 Sr2 Ca 2 Cu 3 O d has been investigated by substitution of this element at the Bi-site and the Cu-site. While a depression of Tc with increasing dopant concentration is observed for both the series of samples, the effect is much more drastic for the Cu-site samples. Magnetic studies also show a more rapid increase in magnetisation of the Cu-site substituted samples. The results are explained on the possibility of formation of a virtual bound state ŽVBS. of the d-level of Nb close to the E F . q 1998 Elsevier Science B.V. All rights reserved. PACS: 74.25 Ha; 74.25 Fy; 74.60 Ec; 74.62 Dh; 74.72 Hs Keywords: Electrical resistivity; Magnetisation; Substitution effects; Pair-breaking
1. Introduction Dopant studies in HTSC play an important role in the in depth understanding of superconductivity only if the physical origin of Tc suppression or enhancement could be ascertained, but because of the complex nature of these materials these efforts have not been very conclusive. There are too many experimental parameters which determine the quality of the specimens and control their Tc s and an additional difficulty arises in case of multi-component systems such as the Bi 2 Sr2 Ca ny1Cu nO 2 nq4 series where the substituted atoms may also affect the relative stability of different superconducting phases of the system. Elemental addition of Pb Žand also Ag, Sb, etc.. has proven useful in enhancing the stability of the 2223 phase and thus facilitating single-phase fabrica)
Corresponding author. Fax: q91-11-5752678, q91-115764189; Telex: 031-77099 NPL IN, 031-77384 RSD IN
tion of this high Tc phase to further investigate the superconductivity of this system by doping with other elements such as the transition metals with their empty d-bands. This paper reports results of our exploration of the Nb-substituted Bi-2223 superconductor which forms part of a detailed investigation on the substitutional effects of d- and f-band materials. Previous reports on Nb-substituted samples of ŽBiPb. 2 Sr2 Ca 2 Cu 3 O d have mostly pointed out that Nb either remains neutral to superconductivity w1x or affects the phase-stability of different superconducting phases w2,3x in this system. Li et al. w3x have shown that the addition of Nb to the Bi-system is much like the role of Pb in the pure Bi–Sr–Ca–Cu–O samples, i.e., it acts like a catalyst and enhances the formation of high Tc 2223 phase. Our studies on samples of Bi-2223 prepared by substitution of Nb at two different sites: viz., the Bi-site and the Cu-site, which have yielded very different results which lead
0921-4534r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 9 8 . 0 0 2 5 7 - 3
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us to believe the possible role of Nb as a magnetic scattering center affecting the ground state of the Bi-superconductor. Magnetisation measurements also suggest the possibility of Nb behaving as paramagnetic centres in this compound.
oscilloscope screen were plotted using the attached plotter.
3. Results and discussion 3.1. Tc behaÕiour and XRD studies
2. Experimental Two series of Nb-substituted samples were prepared by partial replacement of Nb at two different sites: viz., the Bi-site according to ŽBi 1.6yx Nb xPb 0.4 .Sr2 Ca 2 Cu 3 O d Žwhere x s 0.1 to 0.5 and samples are labelled as NBi-1 to NBi-5. and at the C u-site according to Ž B i 1 .6 Pb 0 .4 . S r 2 C a 2 ŽCu 3y y Nb y .O d with y s 0.1 to 0.4 and samples are referred as NCu-1 to NCu-4. The standard solid state diffusion route was adopted for this purpose in which appropriate quantities of Bi 2 O 3 , PbO, SrCO 3 , CaCO 3 , CuO and Nb 2 O5 all of 4 N purity were thoroughly mixed, ground and calcined at 8108C followed by regrinding, and re-calcination at 8158C for 24 h. It was found for both the series of samples that the melting temperature was considerably reduced when Nb 2 O5 was added to the system. The pure sample Ži.e., when x s y s 0. survived up to temperatures as high as 8408C while samples NBi-5 and NCu-4, having the maximum concentration of Nb Ž x s 0.5 and y s 0.4., melted even at the preliminary stages of calcination at 8158C and the sample NCu-3 Žwith y s 0.3. gets partially melted during sintering. All the surviving samples were ground, pelletised and finally sintered in air at 8308C for 30 h and furnace-cooled to room temperature. To observe the superconducting transitions, d.c. resistivity of the specimens was measured as a function of temperature by the four-probe method and the real part of the a.c. susceptibility Ž x X . was measured as a function of temperature using the mutual inductance bridge. Room temperature XRD spectra were obtained on all the specimens for crystal structure determination on a Siemen’s D-500 diffractometer using Cu K a radiation. Zero field cooled a.c. magnetisation measurements were carried out at 77 K on samples ; 4 mm in width and approximately 16 = 1.8 mm2 in cross-section at a frequency of 317 Hz at fixed applied fields Žshown in Table 2. in the range of 1 to 10 Oe. Hysteresis loops observed on the
C om positions of different sam ples of ŽBiPb. 2 Sr2 Ca 2 Cu 3 O d with Nb substituted at Bi-site and at Cu-site, as described above, the heat-treatments given to them, their Tc values determined by the resistivity and susceptibility methods and lattice parameters, etc., are shown in Table 1. Resistivity behaviour of different samples of ŽBiPb. 2 Sr2 Ca 2 Cu 3 O d in which Bi is replaced with Nb according to ŽBi 1.6y x Nb x Pb 0.4 .Sr 2 Ca 2 Cu 3 O d are shown in Fig. 1Ža. and that of samples in which Cu is replaced by Nb according to Bi 1.6 Pb 0.4 Sr2 Ca 2 ŽCu 3y y Nb y .O d , are shown in Fig. 1Žb.. In case of NBi series, it is seen that Tc decreases with increase in dopant concentration Žup to x s 0.3., but in case of NCu samples, Tc decreases much faster and for y ) 0.2, superconductivity was not observed at T ) 77 K. As generally observed for pure Bi samples, higher normal state resistivity is accompanied with a lower Tc , the same trend is observed here for all the samples. Samples not showing superconductivity above 77 K also follow this behaviour and their normal state resistivity shows an increasing trend with temperature, i.e., there is no indication of a semiconducting behaviour which leads to a metal-insulator transition typically observed in materials doped with other elements such as Y, Tm, etc. w4,5x. Superconducting transitions observed by the a.c. susceptibility method also show similar variations for different dopant concentrations at the two doping sites, as observed in the resistivity measurements. Fig. 2Ža. shows the variation of the real part of a.c. susceptibility Ž x X . with temperature of samples having Nb at the Bi-site, i.e., the NBi series, and Fig. 2Žb. shows the same for Nb-substituted at the Cu-site: viz., NCu series. Tc values obtained from these transitions are slightly lower than those obtained by resistivity measurements, and here also one observes that the reduction in Tc is more prominent in case of NCu samples than in NBi ones. These results suggest that Nb substitution at the Cu-site has a more drastic
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295
Table 1 Sample compositions, Tc values and summary of XRD results Sample
Tc ŽK.
XRD results: c-axis of
Composition and heat-treatment Ž8C = h.
Label
Res Ž R s 0.
x X Žonset. Žoffset.
2223 phase ˚. ŽA
2212 phase ˚. ŽA
ŽBi 1.6 Pb 0.4 .Sr2 Ca 2 Cu 3 Ž810 = 24 q 815 = 24 q 830 = 30. ŽBi 1.5 Nb 0.1 Pb 0.4 .Sr2 Ca 2 Cu 3 Ž810 = 24 q 815 = 24 q 830 = 30. ŽBi 1.4 Nb 0.2 Pb 0.4 .Sr2 Ca 2 Cu 3 Ž810 = 24 q 815 = 24 q 830 = 30. ŽBi 1.3 Nb 0.3 Pb 0.4 .Sr2 Ca 2 Cu 3 Ž810 = 24 q 815 = 24 q 830 = 30. ŽBi 1.2 Nb 0.4 Pb 0.4 .Sr2 Ca 2 Cu 3 Ž810 = 24 q 815 = 24 q 830 = 30. ŽBi 1.1 Nb 0.5 Pb 0.4 .Sr2 Ca 2 Cu 3 Ž810 = 24 q 815 = 24. ŽBi 1.6 Pb 0.4 .Sr2 Ca 2 Cu 2.9 Nb 0.1 Ž810 = 24 q 815 = 24 q 830 = 30. ŽBi 1.6 Pb 0.4 .Sr2 Ca 2 Cu 2.8 Nb 0.2 Ž810 = 24 q 815 = 24 q 830 = 30. ŽBi 1.6 Pb 0.4 .Sr2 Ca 2 Cu 2.7 Nb 0.3 Ž810 = 24 q 815 = 24 q 830 = 30. ŽBi 1.6 Pb 0.4 .Sr2 Ca 2 Cu 2.6 Nb 0.4 Ž810 = 24 q 815 = 24.
Pure
102.6
102.0 82.0 98.0 82.0 92.0 ; 77 K –
36.47
29.74
36.56
30.01
36.60
30.74
NBi-1
96.5
NBi-2
92.0
NBi-3
79.0
NBi-4 NBi-5
Not superconducting above 77 K melted –
NCu-1
96.0
NCu-2
82.0
NCu-3 NCu-4
effect than at the Bi-site, and to investigate into its causes we now examine the XRD results, as discussed below. The effect of substitution on the formation of various phases in the Bi-series can be seen from the XRD patterns of the pure and Nb-substituted samples of ŽBiPb. 2 Sr2 Ca 2 Cu 3 O d , shown in Fig. 3Ža–d., for NBi, and in Fig. 4Ža–d. for NCu series. The XRD spectra of the un-doped Bi-2223 sample shows mainly the dominant lines of the 3-layer 2223 phase but most of the substituted samples of NBi series show peaks corresponding to 2223 and 2212 phases for low dopant concentrations Ž x - 0.2. and those corresponding to 2212 and 2201 phases for the higher concentration of x s 0.3. This is true for the NCu samples also except that the 2212 peaks start dominating at a lower concentration of y s 0.2 only. Most of these reflections correspond to a psuedo-tetragonal unit cell having lattice parameters a f b f ˚ and values of c-parameter for different 5.408 A samples are shown in Table 1. Deviation of these values from the ideal structure values reported in the literature suggests the possible formation of a dis-
94.0 82.0 80.0 74.0
Not superconducting above 77 K melted
–
29.72
–
–
–
–
36.15
30.50
–
30.48
–
30.34
–
–
torted crystal structure. Such deviations are encountered even in pure ŽBi,Pb.-2223 superconductor, where this can be caused by different cation ratios and varying oxygen contents. In our doped samples, the possibility of a distorted lattice is further enhanced considering the off-stoichiometry of the compound caused by the replacement of Bi and Cu by Nb or by its inclusion at various substitutional or interstitial sites, whatever the case may be. Since the quantity of Nb added is very small, the possibility of its detection, if involved in any compound-formation in XRD spectra is negligible, and consequently no such lines could be identified in the spectra. It is known w6x that the formation of 2223 phase in the pure BiSrCaCuO specimen is extremely slow and its rapid growth in the ŽBiPb. 2 Sr2 Ca 2 Cu 3 O d system is assisted by the formation of a liquid phase ŽCa 2 PbO4 , Sr2 PbO4 , or a eutectic of 2201 and Ca 2 PbO4, w7x. thus improving the mobility of ions. Partial melting of our Nb substituted samples for low dopant concentrations and melting of these at higher dopant concentrations Ž x ) 0.4. at only 8308C suggests that the addition of Nb 2 O5 further extends the
296
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Fig. 1. Resistance vs. temperature curves for Nb-substituted samples of Bi-2223. Ž a. Nb-substitution at Bi-site, i.e., ŽBi 1.6yx Nb x Pb 0.4 .Sr2 Ca 2 Cu 3 O d ŽNBi. series. Žb. Nb-substitution at Cu-site, i.e., ŽBi 1.6 Pb 0.4 .Sr2 Ca 2 ŽCu 3yy Nb y .O d ŽNCu. series.
range of the liquid phase, thus increasing the possibility of the substituents occupying the positions Žinterstitial or substitutional sites. in the crystal structure, which may get well precipitated out if subjected
to prolonged annealing. Resistivity behaviour of our samples, however, indicates some possibility of Nbatoms being retained in the lattice because of the decrease in Tc . Similar indications are given by the
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297
X Fig. 2. A.c. susceptibility Ž x . as a function of temperature for Nb-substituted samples of Bi-2223. Ža. Nb-substitution at Bi-site. Žb. Nb-substitution at Cu-site.
XRD results which show mainly the lines of the 2223 phase for small dopant concentrations ŽFigs. 3Žb. and 4Žb.. which means that the 2223 does get
rapidly formed and the lattice parameter values ŽTable 1. suggest a small distortion of the crystal structure either due to the presence of the substituted
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atoms or due to the off-stoichiometry of the compound. We notice from XRD results that as the dopant concentration increases, formation of the 2223 is suppressed and 2212 becomes the dominant phase. Tc values also correspond to the typical values of the 2212 phase Ž; 80 K. and for y s 0.3 for the NCu and x s 0.4 for the NBi samples superconductivity above 77 K was not observed at all. XRD of such a sample ŽFig. 4Žd.. shows some reflections corresponding to the 2212 phase, thereby implying that Nb-substitution may also be causing a decrease in the Tc of the 2212 phase as well. The degradation of Tc is particularly drastic for the NCu samples whereas the NBi samples show a less rapid decrease of Tc with Nb concentration. An immediate implication of this appears to be that in the NCu samples Nb gets incorporated in the CuO 2 layers in which presence of any impurity element has a dramatic effect on the Tc of the superconductor. The details of how this takes place are described later in Section 3.3 on the role of Nb. Another possibility for such a lowering of Tc , particularly for the NCu-series comes from the phase-formations affected by the deficiency of Cu when partly substituted by Nb. Cu-deficient compositions of 2223 may result in enhanced formation of the 2212 Žand also 2201. phases which in turn affects the Tc , particularly the Tc Žzero. of these samples. This may also explain why superconductivity could not be observed above 77 K when the dopant concentration was increased beyond y s 0.3. In case of NBi samples, the initial drop of Tc is similar to the NCu samples Žup to x s 0.1., but with higher concentrations, it decreases less rapidly. At x s 0.3, only the 2212 phase appears to be present and its Tc value of 79 K does not reflect any significant effect of the dopant. This relatively less sensitivity of the Tc of NBi samples to the dopant concentration suggests the possibility of Nb occupying an out-of-CuO2 plane position in the crystal structure or just getting segregated in the grainboundaries. The deviations observed in the 2 u positions of the important Ž001. reflections in the XRD pattern ŽFig. 3Žb–d.., however, indicate that some
Fig. 3. XRD patterns of ŽBi 1.6yx Nb x Pb 0.4 .Sr2 Ca 2 Cu 3 O d series, where Ža–d. x s 0 to 0.3 as shown in the figure. wŽ'. 2223, Žv . 2212 and Žq. 2201 phasesx.
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299
lattice-sites could possibly be occupied by the dopant ions Žor possibly due to different cation ratios and varying oxygen contents, as described above.. For cuprate superconductors, it is generally observed that substitution in the planes other than CuO 2 which create a charge imbalance and are expected to modify the formal valence of Cu through charge-transfer can result in the lowering of their Tc . The presence of Nb in the q5 state in the Bi–O layer could be causing such a degradation effect because of the replacement of Biq3 . There is, however, some experimental evidence w8x that Bi is not always present as Biq3 , and oxygen non-stoichiometry of the Bi–O layers can alter its valence state to Biq5 , and thus induces a variation of Tc even in the undoped sam˚ . and ples. Since the ionic radii of Biq5 Ž0.74 A ˚ . are close to each other, there is a Nbq5 Ž0.70 A good possibility of Nb occupying positions in Bi–O layers, i.e., a substitution which would not affect the charge-balance if Bi originally existed in the Biq5 state and, therefore the Tc of the material. If, however, Bi existed in the Biq3 state and gets replaced by Nbq5 after doping, the Tc variation expected would be similar to that of undoped sample in which Biq3 gets converted to Biq5 because of variation of oxygen content. Thus, we notice that in NBi samples, Tc variations affected by Nb-doping are similar to variations observed in un-doped ŽBiPb.SrCaCuO, which is in general agreement to what is generally reported w1–3x, and the effect is not as prominent as in the case of NCu samples where there is a possibility of Tc getting degraded because of pair-breaking by magnetic scattering. This is also supported by our magnetisation measurements at low magnetic fields. 3.2. Magnetisation and low field hysteresis An interesting feature of the HTSC is the observation of low field hysteresis loop when these materials are placed in an a.c. magnetic field which means that penetration of flux and formation of a vortex lattice becomes possible at a field as low as 1 Oe. This is mainly due to the granular nature of these ceramics
Fig. 4. XRD patterns of ŽBi 1.6 Pb 0.4 .Sr2 Ca 2 ŽCu 3yy Nb y .O d series, where Ža–d. y s 0 to 0.3 as shown in the figure. wŽ'. 2223, Žv . 2212 and Žq. 2201 phasesx.
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300
but some researchers believe that Hc1 of the superconducting grain itself is as low as a few gauss w9,10x and flux penetrates not only in the inter-granular space but also within the grains. Our results on a.c. magnetisation measurements on pure and Nb-substituted Žboth at Bi- and Cu-site. carried out at fields between 1.15 and 9.7 Oe, have shown the opening of the hysteresis loop at 77 K which remains elliptical up to a certain field and then distorts from the elliptical shape but remains a closed loop up to the fields available in our experiment Ž9.7 Oe.. The field at which distortion sets in Žrepresenting irreversibility. varies for different samples and so does the area of the loop and the hysteretic magnetisation D M. The magnetisation parameters for various applied
a.c. fields for different samples of pure and Nb-substituted Žat Bi- and Cu-site. specimens are given in Table 2. As in the case of Tc behaviour, the magnetisation results of samples of Nb-substituted at the Cu-site are more pronounced than that of Bi-site substitutions. Highest values of D M are observed for the sample NCu-1, i.e., when Nb is substituted at Cu-site with concentration y s 0.1. D M values increase with increasing applied fields and the deviation from the elliptical shape of the hysteresis loop takes place at Ha.c.s 6.15 Oe Ž500 mA. for both y s 0.1 and y s 0.2. For the un-doped sample and the Bi-site substituted samples, the distortion of the ellipse takes place at a higher field, 9.7 Oe Ž800 mA. for pure and for
Table 2 Magnetisation data Sample
Pure
NBI-1
NBI-2
NBI-3
NCu-1
NCu-2
Applied field coll-curr., Ha.c. ŽmA.
ŽOe.
75 150 300 500 800 75 150 300 500 800 75 150 300 500 800 75 150 300 500 800 75 150 300 500 800 75 150 300 500 800
1.15 1.95 3.70 6.15 9.70 1.15 1.95 3.70 6.15 9.70 1.15 1.95 3.70 6.15 9.70 1.15 1.95 3.70 6.15 9.70 1.15 1.95 3.70 6.15 9.70 1.15 1.95 3.70 6.15 9.70
Loopshape
D M Žper unit thickness. ŽOe.
Loop-arear volume Žemurcc.
Jc s 2D Mrd ŽArcm2 .
ellipse ellipse ellipse ellipse distorted ellipse ellipse ellipse ellipse ellipse ellipse ellipse ellipse ellipse distorted ellipse ellipse ellipse ellipse ellipse ellipse ellipse ellipse distorted distorted ellipse ellipse ellipse distorted distorted
11.89 13.25 15.33 16.98 18.65 1.67 2.20 3.26 4.96 7.02 8.16 9.45 9.87 9.98 10.16 1.28 2.13 3.95 6.55 7.87 12.49 14.15 15.99 16.22 17.52 4.88 7.48 10.14 11.65 12.74
0.035 0.082 0.208 0.354 0.754 0.006 0.017 0.056 0.142 0.291 0.021 0.058 0.117 0.134 0.304 0.005 0.018 0.069 0.175 0.332 0.038 0.112 0.235 0.413 0.559 0.018 0.061 0.154 0.297 0.416
95.1 106.0 122.6 135.8 149.2 13.4 17.8 26.8 39.7 56.0 65.2 75.6 79.0 80.0 81.0 10.3 17.0 31.6 52.4 63.0 100.0 113.2 128.0 130.0 140.0 39.2 59.2 81.0 93.2 101.2
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NBi with x s 0.2; the loop remained elliptical up to the maximum field available in our experiment, i.e., 9.7 Oe for NBi having x s 0.1 and x s 0.3. The shape of these hysteresis loops, elliptical as well as distorted, for the pure and NCu-1 Ž y s 0.1. sample for fields of 1.15 to 9.7 Oe are shown in Figs. 5Ža–e. and 6Ža–e.. These distortions of the ellipses at different fields can be understood on the following picture of grains in these ceramic samples. For the pure, undoped sample we expect the grains as mainly composing of the 2223 phase linked with each other with a Josephson-like coupling. As the magnetic field is applied, flux penetrates first the inter-granular region and then at higher Ha.c. s, it enters into the intra-granular region. Since the experiment is carried out at 77 K, at a temperature well below the Tc of the grains, an increase in the applied field Ha.c. gives rise to more and more of magnetisation accompanied with an increase of the area of the loop. Elliptical shape of the loop is retained up to a field of 6.15 Oe Ž500 mA., which means that B is not appreciably out of equilibrium with H even though a large amount of flux is cycling the sample. To give a description of the flux-distribution and associated internal critical currents, Bean’s critical state model appears to be the best choice, as will be discussed later. For the doped samples, when Nb is substituted at Cu-site, we notice that even though the Tc drops to 92 K when the dopant concentration is only y s 0.1, we suppose that the grains still consist mainly of the 2223 phase as is shown in the XRD pattern ŽFig. 4Žb.. and their Tc gets degraded because of the replacement of Cu by Nb atoms. When these samples are placed in the a.c. magnetic field at 77 K, D M values are higher Ževen though only slightly, see Table 2., than the pure sample and the deformation of the elliptical loop takes place at a lower field of 6.15 Oe Ž500 mA.. One possible reason for this may be that the grain-coupling gets weakened at a lower field Žthe intrinsic lower Tc of the grains could also be one of the factors responsible for this., and flux-penetration results in trapping at the inter-granular regions giving rise to comparatively larger values
Fig. 5. A.c. magnetization hysteresis loops of ŽBi 1.6 Pb 0.4 . Sr2 Ca 2 Cu 3 O d at 77 K in different Hmax at 317 Hz.
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of D M. One would expect B to be even more out of equilibrium with H than in case of pure specimen while the flux-cycle still remains large. For the NCu-2 sample Ž y s 0.2., we notice from XRD patterns ŽFig. 4Žc.. that the dominant phase appears to be 2212 and the Tc also gets degraded to 82 K. It is quite possible that this 82 K Tc may still be corresponding to some 2223 material as this phase gets formed by a reaction between a liquid phase ŽCa 2 PbO4 , etc.. and the 2212 or the 2201 phase w6x and is, therefore, likely to form a thin layer at the surface while the interior of the grain is still the 2212 or 2201 phase, the dominance of which is reflected in the XRD pattern. Since only a small fraction of 2223 with Nb incorporated in it is present as the superconducting phase, part of the Nb remains segregated and possibly affects the magnetisation of the samples. In case of Bi-site substituted samples, it is seen that not only the Tc degradation effect is less pronounced but the magnetisation behaviour is also not affected as drastically as in the case of Cu-site substituted samples. The NBi samples with x s 0.1 and x s 0.3 show an elliptical loop up to the maximum field applied in these experiments and for the x s 0.2 sample NBi-2 the distortion takes place at 9.7 Oe, same as the pure, undoped specimen. The Cu-site substituted samples on the other hand show a distortion at a lower field of ; 6.15 Oe. Not only the deviation from ellipticity occurs at a higher field for the NBi samples but the area of the hysteresis loops and the value of D M are also much lower for these samples. This suggests that the defects which interact with flux-threads and hinder their motion Žtrapping of flux. are less dominant in NBi samples and the major superconducting phase in these samples appears to be more homogeneous than in NCu samples. Observation of the hysteresis loops for all our samples, pure as well as doped, at an applied field of only 1.15 Oe show that these samples have a small penetration field H ), the minimum field required for the flux to enter and reach the centre of the
Fig. 6. A.c. magnetization hysteresis loops of ŽBi 1.6 Pb 0.4 . Sr2 Ca 2 Cu 2.9 Nb 0.1O d at 77 K in different Hmax at 317 Hz.
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specimen. This penetrated flux establishes a gradient and gives rise to the internal current flow and as the applied field increases, the spatial distribution of magnetic flux within the sample changes and consequently the internal patterns of current-flow will change. In order to establish a relationship between the magnetisation and critical currents to estimate Jc of the superconductor from magnetisation values several models have been given, the basis of all of which is the existence of a critical state, i.e., when the interaction between the flux-lines and the defects in the material is balanced by the magnetic driving force resulting from the gradient in the flux density. The original critical state picture introduced by Bean w11x was based on the assumption of critical current density Jc being independent of B. It is, however, often found that Jc is a strong function of B and this dependence must be taken into consideration for predicting the actual critical currents. A number of authors Žsee, for example, w12x. have provided a detailed analysis of the critical state and its applicability to the granular high Tc superconductors, considering various field-dependencies of Jc , two of them have been discussed in detail by Ji et al. w13x. These are: Ž1. that based on Jc being constant with B ŽBean., and Ž2. in which Jc varies as the inverse of B, i.e., the Kim Anderson model. We have used their analysis to explain the shapes of our experimental hysteresis loops for both Ž1. and Ž2. and find that most of the loops can be matched with the theoretical loops obtained using the field-independent Jc picture of Bean if suitable values of different parameters in the following equation can be assigned. 4p M s 4p M Bean y aH.
Ž 1.
Here, the aH term accounts for the diamagnetism of the grains w14x, and: 4p M Bean s B y H ,
where B s
EH Ž x . d x Ed x
.
We give below the selection of our parameters and their justification. It is seen that the theoretical curves shown in Fig. 7 closely resemble the distorted ellipses experimentally observed in our specimen Žexcept for the small
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asymmetry in the shape of the loops, the cause of which will be discussed later.. These curves were obtained when in Eq. Ž1. the factor, a, representing the contribution due to the grains in the Meissner state is taken to be negligibly small Žequal to 0.02 or 0. which indicates the dominance of the magnetisation induced by the inter-granular flux distribution represented by the first term. This means that at applied fields of only 6.3 Oe for NCu Žand 9.7 Oe for pure and NBi-2. series flux penetration in the sample has substantially affected not only the intergranular region but also the diamagnetism of the grains. Even otherwise, because of the intrinsic anisotropy of the high Tc superconductors, the magnetisation does not remain parallel to B unless the field is directed along one of the principal crystal directions, its magnetic response is, therefore, no longer purely diamagnetic. Thus, we see that samples in which distortion of the ellipse occurs at a lower field are likely to have a contribution to the magnetisation from the intra-granular region also, particularly at higher Ha.c. s. Since the use of Bean’s model gives a reasonable description of flux-distribution in our samples, Jc values were evaluated from the magnetisation data ŽTable 2. using the formula: Jc s 2D Mrd, where d is the thickness of the sample. Although these values are quite low, a large variation is observed for different samples and the Cu-site substituted samples give the highest value of 100 Arcm2 at a field of 1.15 Oe. For the NBi samples, the values range from 10 to 60 Arcm2 for dopant concentration of x s 0.1 and 0.3, and from 65 to 81 Arcm2 for x s 0.2. Since for x s 0.1 and 0.2 samples, the dominant phase appears to be the 2223 phase, the marginal increase observed in x s 0.2 sample can be attributed to the increased inhomogeneity and therefore more defects in the superconducting grains Žof 2223.. For x s 0.3, the grains are expected to be of the 2212 phase, which could be relatively pure and homogeneous, thus giving low magnetisation and Jc values. For the NCu samples, despite a lower Tc , highest Jc values are estimated because of their large magnetisation values. Since the dominant phase appears to be the 2223 phase, at least for x s 0.1 sample, these results indicate that the different magnetisation and Jc behaviour could be arising from the Nb-addition to these samples.
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Fig. 7. Hysteresis loops predicted by the formulae Žafter Ref. w13x.: Ža. 4p M s 4p M Bean y Ž0.02. H; Žb. 4p M s 4p M Bean .
Another useful information coming from the magnetisation loops ŽFigs. 5 and 6. is the small asymmetry in their shapes after distortion from the ellipse; the flatness appears to be longer in the lower region than in the upper half. This is true for the pure sample also but is more noticeable in the NCu specimens because for these samples the distortion begins at a lower field and the applied field could be further increased up to the maximum available experimentally to see it expanding. Such an anisotropy is believed to be due to the presence of Bean– Livingston w15x surface barriers, i.e., defects are present at the surface which hinder the entry and exit of fluxoides in the sample. Defects at the surface are expected in our samples because, as discussed before along with XRD results, the formation of Bi-2223 phase takes place through a surface reaction in which its formation begins as a thin layer at the surface which increases as the diffusion time increases. This also indicates that the magnetisation behaviour is
influenced by not only the inter-granular region but also by flux-penetration within the superconducting grains with a contribution from the surface defects also. 3.3. Role of Nb It is generally observed in cuprate superconductors that cationic substitution at the Cu-site degrades the Tc irrespective of whether the substituted element is magnetic or nonmagnetic. This is because of the strong hybridization of the Cu-3d states with O-2p levels, whereby electrons collectively give rise to the metallic conduction. Any disturbance caused by the substituted element in this CuO 2 layer would modify the overlap of orbitals and hence the density of states near the Fermi level or below it, thus affecting the Tc . Alternatively, if the added impurity has its own magnetic moment, Tc degradation takes place by pair-breaking effects. In our Cu-site Nb-substituted
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samples, reduction in the Tc Žonset. values Žas suggested by the resistivity measurements and confirmed by the susceptibility measurements. can be explained by such a pair-breaking effect. Here, it may be possible for Nb, with its partially filled d-band having free spins, to retain its paramagnetism and form a level below E F when present at the Cu-site of the Bi-2223 material and destroy its Tc because of the pair-breaking effect due to the spinpolarisation at E F . Evidence of Nb forming a level close to the EF or contributing to the VBS Žvirtual bound state. function has also been suggested by Asthana et al. w16x in their studies on Nb-doped Y1 Ba 2 Cu 3 O 7yd superconductor. Lowering of Tc Žoffset. in samples where two transitions are clearly seen could also be an effect of the above phenomenon but there is also a possibility that the general broadening of transition width DTc , and the reduction in Tc Žzero. in resistivity measurements are due to the increased relative volume fractions of the lower Tc 2212 and 2201 phases. A further support to our contention above of Nb forming a level close to the E F and contributing to the VBS function comes from the magnetisation results. If Nb is indeed forming a level below the E F and retaining its paramagnetism, as suggested by our Tc degradation effect, it should have its impact on the magnetisation induced in the sample when immersed in a magnetic field. Our magnetisation experiments have shown that the maximum value of D M is observed in samples containing Nb at the Cu-site ŽTable 2.. Even though the same amount of Nb is present in the samples in which Bi is replaced by Nb, the magnetisation effects are not so pronounced in this case. The hysteresis loops remain elliptical up to the same applied fields as for the pure, undoped specimen Žor even at higher fields.. This indicates that Nb is more or less neutral to superconductivity in these samples, as also suggested by w1x, while in the samples where Nb is substituted at the Cu-site, the Nb ion is influential as a scattering centre responsible for the reduction in Tc . The role of Nb, when incorporated at the Bi-site is to affect the charge-balance in the Bi–O layers and influence the formal valence of copper through transfer of charge. This induces deviation from the optimum hole concentration and causes a reduction in the Tc values.
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4. Conclusions The above discussion suggests that the decrease in Tc of the Bi-2223 phase is dominated by the substitution effect of Nb to the ŽBiPb. 2 Sr2 Ca 2 Cu 3 O d superconductor and there is also a possibility of contribution from other effects such as the increased relative fractions of lower Tc 2212 and 2201 phases. The effect is more rapid and increases with the dopant concentration when Nb is substituted at the Cu-site than at the Bi-site and is also accompanied with an increase in magnetisation of the samples. This indicates the possibility of the d-level of Nb lying close to the E F and contributing to the pair-breaking effects responsible for the lowering of Tc .
Acknowledgements We gratefully acknowledge the support and encouragement of our Director Prof. E.S.R. Gopal in carrying out this research. Dr. D.K. Suri and other colleagues of our X-Ray section have been extremely helpful in conducting XRD measurements and analysis. One of us ŽPLU. would like to thank Dr. S.N. Ekbote for fruitful discussions, and ŽDRM. is grateful to CSIR for providing a research fellowship.
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