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Influence of Re substitution on the flux pinning in (Hg,Re)Ba2Ca2Cu3O8+δ single crystals
Physica C 309 Ž1998. 161–169
Influence of Re substitution on the flux pinning in žHg,Re /Ba 2 Ca 2 Cu 3 O 8qd single crystals R. Puzniak a , J. Karpinski b,) , A. Wisniewski a , R. Szymczak a , M. Angst b, H. Schwer b, R. Molinski b, E.M. Kopnin b a
Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32 r 46, PL 02-668 Warsaw, Poland b Laboratorium fur ETH Honggerberg, CH 8093 Zurich, Switzerland ¨ Festkorperphysik ¨ ¨ Received 22 July 1998; revised 21 September 1998; accepted 7 October 1998
Abstract Flux pinning in Hg 0.77 Re 0.23 Ba 2 Ca 2 Cu 3 O 8q d single crystals ŽTc s 130 K. and in HgBa 2 Ca 2 Cu 3 O 8q d crystals with Tc equal to 130 and 120 K was studied. In the investigated crystals Re atoms substitute at the Hg site of HgBa 2 Ca 2 Cu 3O 8q d and alter the defect structure of the host layer by pulling in four new oxygen atoms at Ž0.34, 0.34, 0. position to form an octahedron around Re. As a result a significant improvement of the irreversibility line position at the H–T phase diagram for Hg 0.77 Re 0.23 Ba 2 Ca 2 Cu 3 O 8q d crystal is observed only at low temperatures Žbelow 80 K.. However, magnetization measurements performed at temperatures above 100 K show insignificant change in the irreversibility line position only, as compared to that one of the parent compound with Tc s 130 K. This indicates insignificant influence of new pinning centers, introduced by Re substitution, in the temperature range, where thermal activation energy becomes significant in comparison with pinning energy. Furthermore, obtained data indicate that no improvement of the coupling between superconducting layers was achieved as a result of the expected increase of electrical conductivity in the blocking layer. Since only a small change in the c-axis lattice constant was observed in Re substituted crystals, we can conclude that a larger shortening of the blocking layer in the material is necessary to improve the irreversibility line position in HgBa 2 Ca 2 Cu 3 O 8q d crystals at high temperatures. q 1998 Elsevier Science B.V. All rights reserved. PACS: 74.60.Ge; 74.25.Ha; 74.72.Gr Keywords: Flux pinning; Irreversibility line; Hg-based single crystals; Re-doping
1. Introduction One of the most important factors limiting possible applications of high-Tc superconductors, at least in the high temperature region, is a relatively low position of the irreversibility line ŽIL. on the H–T )
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phase diagram. At high temperatures and low fields the vortex structure for the magnetic field H 5 c-axis can be well described by three-dimensional Ž3D. flux lines’ behavior. In higher fields and at lower temperatures a crossover to two-dimensional Ž2D. ‘pancake’ vortex structure is observed for layered materials. Using the description of Houghton et al. w1x, based upon a thermodynamic melting of the flux lattice Žin the limit of negligible pinning., the irre-
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 5 8 9 - 9
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versibility line in high temperature range can be described by: n
Hirr Ž T . s H0 Ž 1 y TrTc . , H0 A 1rŽ l4a b Ž0.g 2 ..
Ž 1.
where Here, l a b is the in-plane penetration depth and g s Ž m )c rm )a b .1r2 is the effective mass anisotropy. From the above it is obvious that an improvement of the irreversibility line position at high temperatures cannot be achieved Žassuming negligible pinning. without decreasing the penetration depth or the effective mass anisotropy. A decrease of the penetration depth can be achieved by the increase of superconducting carrier concentration andror the decrease of carrier effective mass. The decrease of effective mass anisotropy leads to the improvement of the coupling between superconducting layers of the material. This coupling is Josephson-like, its strength being exponentially reduced with increasing interlayer distance w2,3x. The interlayer distance d i generally increases from YBa 2 Cu 3O 7y d and CuBa 2 Ca ny1Cu nO 2 nq4y d , across La 2yxSr x CuO4 ŽLa-214. and HgBa 2 Ca ny1Cu nO 2 nq2q dŽHg-12Ž n y 1. n., to Bi 2 Sr2 Ca ny1Cu nO 2 nq4q d ŽBi22Ž n y 1. n., and so the irreversibility line position decreases. The interlayer coupling in the material can be improved by shortening the ‘blocking layer’ distance Žthe distance between groups of n CuO 2 conducting planes. and by the increase of electrical conductivity in the blocking layer, which can eventually lead to proximity-induced weak superconductivity in the blocking layer w4x. The above factors controlling the position of the irreversibility line are related to the intrinsic parameters of the material. There is another, extrinsic possibility of an improvement of the irreversibility line position—the introduction of more effective pinning centers into material. Such centers usually are very effective at relatively low temperatures, where the pinning energy is high in comparison with the thermal activation energy. However, the effectiveness of the pinning centers may decrease rather fast with increasing temperature and pinning may be ineffective close to Tc . Finally, a shift of the irreversibility line to higher fields and temperatures on the H–T phase diagram may originate from the existence of surface barrier making the movement of vortices more difficult. The highest transition temperature among the high-Tc superconductors, equal to 135 K under nor-
mal pressure, was achieved for the Žoptimally doped. mercury-based copper-oxide compound HgBa 2 Ca ny 1Cu nO 2 nq2q d with n s 3 ŽHg-1223. w5,6x. Unfortunately, the intrinsic flux pinning properties of the material are not as good as the ones of some other high-Tc superconductors, e.g., YBa 2 Cu 3 O 7y d w7–9x. Recently, significant improvement in the flux pinning of the mercury based ceramic samples with Hg partially replaced by Re or Cr and with Ba replaced by Sr was reported in the literature for several times w10–13x. Shimoyama et al. w10–14x have argued that at least two features of the chemically substituted compounds may enhance flux pinning: Ž1. the substitution of Sr for Ba significantly ˚ w13x; shortens the blocking layer by about 0.8–0.9 A and Ž2. the chemical substitution at the Hg site may make the blocking layer more metallic w4,14x. Chmaissem et al. explored the possibility that extended defects in a chemically substituted HgSr2CuO4q d compound could act additionally as pinning centers w15x. Studies of Fabrega et al. w16x performed on grain-aligned Hg 1y x Re x Ba 2 Ca 2 Cu 3 O 8q d samples showed that Re substitution enhances bulk pinning at low temperatures. They have observed that surface barriers dominate the irreversible magnetization in their samples at high temperatures. In Re substituted samples they found an increase of the irreversibility line also at high temperatures. However, due to the high anisotropy of HTSC and additional phases in ceramic samples only measurements on single crystals are reliable for the determination of true intrinsic properties of these compounds. Polycrystalline material has randomly oriented grains, what makes the measurements of superconducting parameters of highly anisotropic materials difficult. Grain boundaries limit the macroscopic critical current. The grain boundaries may act as pinning centers. Therefore, to measure ‘intrinsic’ parameters of HTSC with any appreciable accuracy, one has to use single crystals. In order to study the influence of different factors on the irreversibility line position we determined the Hirr ŽT . in a wide temperature range for Re substituted HgBa 2 Ca 2 Cu 3 O 8q d single crystals. The results were compared with the data obtained for underdoped and almost optimally doped HgBa 2Ca 2 Cu 3 O 8q d . The characteristic features of Re substituted crystals in comparison with single crystals of
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the parent compounds are Ž1. planar defects Žfaults. caused by Re substitution, acting as pinning centers, Ž2. a small shortening of the c-axis lattice constant in comparison with that one for the material without substitution ŽRe substitution decreases the blocking ˚ w17x., and Ž3. a layer distance Ba–Ba by about 0.2 A possibly more metallic character of the blocking layer obtained as a result of chemical substitution at the Hg site. In order to study the temperature dependence of well defined components of irreversibility field the measurements were performed on single crystals. In the investigated material, we found, at high temperatures, no significant enhancement of the irreversibility field parallel to the c-axis in comparison with that one of the host material with Tc s 130 K. This means that introduced defects are ineffective as pinning centers controlling the position of the irreversibility line at high temperatures. Furthermore, the data indicate that no improvement of the coupling between superconducting layers can be achieved by Re substitution into HgBa 2 Ca 2 Cu 3 O 8q d only, without larger shortening of the blocking layer in the material.
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2. Single crystals preparation and structural characterization One of the main difficulties with the synthesis of the mercury based compounds Hg-12Ž n y 1. n is their low thermal stability. The compounds melt peritectically, but at ambient pressure they decompose before melting and the volatile components evaporate. Since the total pressure of volatile components PO 2 , P Hg , and P HgO exceeds 100 bar at temperatures above the melting point, encapsulation of the sample with a high hydrostatic inert gas pressure was used to prevent decomposition. Crystal growth experiments have been already described in several publications w18– 21x. All crystal growth experiments have been performed at Ar pressures of 10 kbar. Due to the high density of Ar gas at this pressure Ž) 1 grcm3 ., evaporation of Hg was strongly suppressed. Single crystals were grown from a flux or from a stoichiometric melt. As a flux, BaCuO 2 –CuO–Ag 2 O was used. The reasons of using Ag 2 O were: Ži. decreasing of the melting temperature of the BaCuO 2 –CuO eutecic w22x, Žii. increasing of the partial oxygen
Fig. 1. Model of ŽHg,Re.O layer in Hg 1y x Re x Ba 2 Ca ny1Cu nO 2 nq2q d single crystals.
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pressure during synthesis. The crystallization temperature was 10008C- T - 10708C. Crystals of Hg12Ž n y 1. n for n s 1–7 were obtained in such a way and crystals of Hg-1223 were selected for further studies. In some experiments ReO 2 was added to precursor in order to substitute Hg. We measured different Re substituted 12Ž n y 1. n crystals on an X-ray four circle single crystal diffractometer w17,23x. Structure analysis showed that rhenium is introduced at the Hg site and coordinated octahedrally by oxygen atoms. These results, in particular the coordination and bondlengths of Re–O are in agreement with studies of Chmaissem et al. w24x. Re–O bond lengths ˚ . in the basal z s 0 plane are much shorter Ž1.86 A ˚ .. than the corresponding Hg–O distances Ž2.725 A Hg and Re atoms cannot be distinguished with Xrays, but oxygen atoms bonded to Re can easily be located in the Fourier maps at Ž0.344, 0.344, 0. position. The amount of this oxygen is four times as high as that of rhenium. The occupation of the HgrRe site is almost full. Excess oxygen atoms connected to mercury are located at Ž1r2,1r2,0. and are only partially occupied. One unit cell cannot carry more than one oxygen atom in HgrRe–O plane, which is either connected to Hg or to Re. It is impossible that two adjacent rhenium atoms exist with an octahedral oxygen coordination, because in this case two oxygen atoms will be placed in one unit cell ŽFig. 1.. Therefore the maximum Re substitution is 25%. From the EDX analysis Re content in investigated crystals was 23 " 2%. According to our X-ray single crystal investigations w25x the Hg site occupation in crystals without rhenium was refined to 80–85%, implying that some carbonate might be incorporated there. A further hint for the presence of CO 32y in these crystals is the short c-axis parameter; CO 32y impurities could also be responsible for the lower Tc in some crystals.
3. Experimental details For magnetic measurements a crystal of ŽHg,Re.Ba 2 Ca 2 Cu 3 O 8q d , further denoted HgRe-1223, with a well resolved transition temperature of about 130 K, defined as the onset of the diamagnetic signal, was chosen. We have studied also two unsubstituted reference crystals of Hg-1223; one with Tc s 130 K
and second one with Tc s 120 K. It is rather difficult to grow HgBa 2 Ca 2 Cu 3 O 8q d crystals with a high carrier concentration, close to that of optimally doped materials. Our reference crystal HgBa 2 Ca 2 Cu 3 O 8q d with Tc s 120 K was underdoped while both Re substituted and unsubstituted crystals with Tc s 130 K were almost optimally doped. In view of earlier crystal growth experiments with Hg-1223, it is very likely that the unsubstituted crystal with Tc s 130 K is slightly underdoped w18,25x. The Re substituted crystal may well be slightly overdoped, as Re substituted material was usually found to be overdoped by other authors Žsee, e.g., Refs. w13,26x., but we do not have any evidence of this. However, as will be seen later, this possibility cannot change our conclusions. It is worth to notice that the Re substitution at Hg site of mercury based crystals makes much easier growth of single crystals with the number of Cu–O planes higher than three. The selected crystal of Hg 1y x Re x Ba 2 Ca 2 Cu 3O 8q d with x s 0.23 " 0.02 Žas determined by EDX-measurements. had an approximately parallelepiped shape with dimensions 0.7 = 0.7 mm2 in the a–b plane and the thickness of about 27 mm along the c-axis. Magnetization measurements with accuracy better than 10y7 emu were performed on a home made SQUID magnetometer with a Quantum Design SQUID sensor and on a commercial Quantum Design MPMS 5. All measurements were performed in the magnetic field geometry H 5 c. The irreversibility temperature Tirr , defined as a temperature of the appearance of the first resolved difference between zero field cooled and field cooled magnetization, was determined from M ŽT . dependencies measured in fixed magnetic field. The irreversibility field Hirr , defined as a field where the two branches of the hysteresis loop M Ž H . measured at fixed temperature meet within the resolution of magnetic measurements was determined as well. The criterion of 3 = 10y7 emu in the determination of detectable magnetic moment difference between two branches of hysteresis loop was applied. For the given sample —dimensions of 0.7 = 0.7 = 0.027 mm3 —this corresponds to the magnetization criterion of 2.3 = 10y2 Gs and for the magnetic field applied along the c-axis of the sample to the criterion of 10 Arcm2 for the critical current density. It was confirmed that both Hirr and Tirr data placed in the H–T phase
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diagram coincide rather well and form one well defined irreversibility line.
4. Results and discussion Fig. 2 presents zero field cooled and field cooled magnetization data for HgRe-1223 single crystal recorded in the vicinity of Tc in magnetic field of 100 Oe applied along the c-axis of the crystal. A well defined irreversibility temperature, of 123 K in the particular case of 100 Oe external magnetic field, is clearly seen on the plot. The inset presents the data obtained in the full temperature range, in the field of 10 Oe. The results indicate the absence of any additional magnetically detectable phase in the crystal. The relatively small ratio of the field cooled magnetization to the zero field cooled magnetization, equal to about 0.2, suggests the existence of a large number of pinning centers in the material. Such centers may be related to the existence of planar defects Žfaults. separating domains formed by the Re–O6 octahedra and domains ordered in different ways. These objects were clearly identified in the crystallographic structure studies of Re substituted Hg 1y x Re x Ba 2 Ca 2 Cu 3 O 8q d samples w13x. Fig. 3 presents the field dependence of the magnetization measured for the Hg 0.77 Re 0.23 Ba 2 Ca 2 Cu 3-
Fig. 2. Zero field cooled and field cooled magnetization in the vicinity of Tc measured for almost optimally doped Hg 0.77 Re 0.23 Ba 2 Ca 2 Cu 3 O 8q d in a magnetic field of 100 Oe applied along the c-axis of the crystal. The inset presents the data obtained in the full temperature range.
Fig. 3. Field dependence of the magnetization measured for a Hg 0.77 Re 0.23 Ba 2 Ca 2 Cu 3 O 8q d single crystal in a magnetic field parallel to the c-axis at temperatures of 40, 50, 60, and 70 K. The inset shows an estimation of the irreversibility field at a temperature of 60 K Ž Hirr s 31 kOe..
O 8q d single crystal in the magnetic field parallel to the c-axis at several temperatures: 40, 50, 60, and 70 K. The inset shows the estimation of the irreversibility field at a temperature of 60 K Ž Hirr s 31 kOe in particular case.. Hysteresis loops for all investigated temperatures seem to be symmetric around reversible magnetization, i.e., the M–H dependence expected for the material without pinning. Both lower and upper branches of the loop are strongly field dependent at all temperatures. There is no flat part in the M–H dependence, indicating that surface barrier effects are not important in this temperature range. The width of the hysteresis loops is determined by the bulk pinning effect. The same was found for unsubstituted Hg-1223 single crystals. One can see, especially at the lower measured temperatures, a second maximum in the width of hysteresis loop in all presented temperatures indicating a rather high critical current density in the moderate field range. We did not observe a ‘fishtail effect’ at temperatures higher than 80 K. The temperature dependence of the irreversibility field for HgRe-1223 in the magnetic field applied along the c-axis of the crystal is presented in Fig. 4. In Fig. 4a the values of Hirr plotted as a function of reduced temperature Ž1 y TrTc . are compared with the data obtained for underdoped HgBa 2 Ca 2 Cu 3 O 8q d crystal with Tc of about 120 K w27x and for almost optimally doped Hg-1223 crystal with Tc f 130 K.
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Fig. 4. Ža. Irreversibility field of Hg 0.77 Re 0.23 Ba 2 Ca 2 Cu 3 O 8q d , underdoped HgBa 2 Ca 2 Cu 3 O 8q d with Tc s120 K w27x, almost optimally doped Hg-1223 with Tc s130 K single crystals determined in a magnetic field parallel to the c-axis are presented as a function of Ž1yT r Tc .. The inset presents irreversibility field data as a function of T, in higher temperature region. Žb. The data for Hg 0.77 Re 0.23 Ba 2 Ca 2 Cu 3 O 8q d are compared with the data for Bi 2 Sr2 CaCu 2 O 8q d w30x and La 1.86 Sr0.14 CuO4 w31x.
One can see that the irreversibility line for Re substituted crystals is located in fields slightly higher than related fields for the irreversibility line of the host underdoped material with Tc of 120 K. At high temperatures ŽT ) 90 K. the IL for HgRe-1223 is essentially the same as the IL for the parent compound with Tc s 130 K. In this temperature region presumably the pinning energy of defects caused by Re substitution is low in comparison with the thermal activation energy. Hence, the description of vortex behavior in the limit of negligible pinning may be used. The differences in the irreversibility field for the same reduced temperature among all of
the investigated crystals are small—a factor of approximately two to three. One should take into account that the studied crystals have different anisotropy. It is well known that the anisotropy of the effective mass in cuprates decreases with increasing oxygen concentration. Such a tendency was reported for YBa 2 Cu 3 O 7y d w28x as well as for under- and overdoped HgBa 2 Ca ny1Cu nO 2 nq2q d compounds Žfor example, for Hg-1201, see Hofer et al. w29x.. The underdoped crystal with Tc s 120 K has the highest anisotropy among studied samples. It is expected that the anisotropy of the unsubstituted crystal with Tc s 130 K is about 40% lower than the anisotropy of our reference crystal with Tc s 120 K Žsee, estimation of the effective mass anisotropy for Hg1223 with Tc s 120 K and for Hg-1223 with higher Tc w18x.. The 40% difference in the effective mass anisotropy, according to Eq. Ž1., may change the value of Hirr by a factor of about 2. In the case of HgRe-1223 we cannot exclude that this crystal is overdoped, i.e., that its anisotropy is lower than that of Hg-1223 with Tc s 130 K. If the Re substituted crystal was in fact overdoped, the irreversibility line position for HgRe-1223 would be shifted toward higher fields and temperatures due to the lower anisotropy. The comparison of Hirr data obtained for Re doped HgRe-1223 and for undoped parent compound of Hg-1223 with the same transition temperature of 130 K excludes any significant direct influence of Re substitution on Hg site on the position of the irreversibility line at high temperatures. The irreversibility field data for temperatures above 100 K are essentially the same. Hence, no improvement of the coupling between superconducting layers was achieved by making the blocking layer more metallic as a result of Re substitution into HgBa 2 Ca 2 Cu 3O 8q d . Additionally, planar defects separating domains formatted by the Re–O6 octahedra do not improve the position of the irreversibility line of HgRe-1223 crystals at high temperatures. Only a significant shortening of the blocking layer may be effective to improve position of the Hirr in H–T phase diagram. At lower temperatures the situation seems to be more complicated. For a Hg-1223 crystal with Tc s 120 K, there is clearly visible a crossover in the temperature dependence of the irreversibility field in a field of approximately 2 kOe. A more rapid change
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of Hirr with temperature is observed at lower temperatures. This indicates that there the pinning has a rather two-dimensional Ž2D. character. The appearance of a crossover, in our opinion, is a result of the transition from three-dimensional Ž3D. vortex structure at high temperatures to 2D ‘pancake’ vortex structure at lower temperatures. Assuming negligible pinning, the crossover field is roughly proportional to the inverse effective mass anisotropy of the system, 1rg 2 . For the parent compound with higher Tc Ž130 K., i.e., lower anisotropy, the crossover becomes much less pronounced and is shifted to higher fields of about 5 kOe. This change of the value of the crossover field corresponds to a decrease of the anisotropy g by factor of about 1r3 from g f 60 to g f 40. For the HgRe-1223 crystal with Tc of 130 K the crossover should be expected at roughly the same field as that for optimally doped Hg-1223. However, it is observed at smaller fields, i.e., at higher temperature. Also the position of the IL in the HgRe-1223 crystal, at lower temperatures, is shifted to the region of higher fields and temperatures. Such a behavior can be explained by taking into account the larger number of pinning centers in the sample. These centers are ineffective at high temperatures where the thermal activation energy is high in comparison with the pinning energy. With decreasing temperature pinning becomes more and more effective and deviation from the Hirr ŽT . dependence given by Eq. Ž1. appears before the 3D–2D crossover expected for the material without pinning. Fig. 4b presents a comparison of the irreversibility lines for HgRe-1223, Bi 2 Sr2 CaCu 2 O 8q d w30x, and La 1.86 Sr0.14 CuO4 w31x. All the ILs were determined from the magnetization measurements and correspond to the critical current density criterion 10 Arcm2 . The IL of HgRe-1223 is apparently located in higher magnetic fields and at higher temperatures than the IL of Bi-2212—the compound with much higher anisotropy and greater interlayer spacing d i . On the other hand, the comparison of HgRe-1223 and La-214 Žsystem with lower anisotropy than HgRe-1223 and much better intercell coupling strength. shows a difference in the position of the IL only at high temperatures. At low temperatures, the effectiveness of pinning centers introduced by Re substitution leads to similar values of Hirr for both compounds.
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Fig. 5 presents the width of hysteresis loops determined in magnetic fields of 6, 8, and 10 kOe at temperatures where the critical state inside the superconductor is well established. The width of the loop is proportional to the critical current density. The temperature dependence of the critical current density Jc is often described approximately by the formulae: Jc ŽT . s Jc Ž0.expŽyTrT0 . over a certain magnetic field range w32,33x. The fitting parameter T0 characterizes the decrease of Jc with increasing temperature. The magnitude of T0 changes slightly over the field Žsee for example, behavior of Jc ŽT . found for CuBa 2 Ca 3 Cu 4 O 10q d ŽTc s 117 K. with T0 of about 12 K w34x.. The magnitude of T0 may be used to estimate the capability of the HTSC to be operated at high temperature, and definitely a higher value of T0 would be preferred for application. According to the value of T0 , the Jc behavior can be classified into various groups. The YB 2 Cu 3 O 7y d and CuBa 2 Ca 3 Cu 4 O 10q d compounds characterized by an average T0 ; 14 K rank in the most steady ones, Bi-2212 system characterizes the lowest value of T0 ; 4 K w35x. As one can see in Fig. 5, for HgRe-1223 crystal we found T0 ; 10 K in the temperature range 50–80 K. This confirms that from the point of view of applications, Re substitution of Hg-1223 does not help.
Fig. 5. The width of hysteresis loops determined in magnetic fields of 6, 8, and 10 kOe at temperatures where the critical state inside the superconductor is well established. The width of the loop is proportional to the critical current density.
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5. Conclusions We compared flux pinning in a Hg 0.77 Re 0.23Ba 2 Ca 2 Cu 3 O 8q d single crystal ŽTc s 130 K. and in HgBa 2 Ca 2 Cu 3 O 8q d crystals with Tc equal to 130 and 120 K. Our studies showed that Re atoms substituted at the Hg site enhance flux pinning in Hg-1223 superconductor only at low temperatures. Magnetization measurements performed at high temperatures show insignificant enhancement of the irreversibility field parallel to the c-axis, in comparison with that one for host material with Tc s 130 K. This indicates that no improvement in the coupling between superconducting layers is achieved by making the blocking layer more metallic as a result of Re substitution into HgBa 2 Ca 2 Cu 3 O 8q d . Despite the existence of new pinning centers, the improvement of the irreversibility line position on the H–T phase diagram may be achieved in the higher temperature region by shortening the blocking layer thickness in the material only. This may be achieved by substitution into the Ba sites of Sr atoms or other elements with small ionic radius.
Acknowledgements This work was partially supported by the Polish Government Agency KBN under contract No. 722P039509 and by the Swiss National Science Foundation.
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R. Puzniak et al.r Physica C 309 (1998) 161–169 w27x A. Wisniewski, R. Szymczak, M. Baran, R. Puzniak, J. Karpinski, R. Molinski, H. Schwer, K. Conder, G.I. Meijer, Czech. J. Phys. 46 ŽS3. Ž1996. 1649, Suppl. w28x T.R. Chien, W.R. Datars, B.W. Veal, A.P. Paulikas, P. Kostic, Gu Chun, Y. Jiang, Physica C 229 Ž1994. 273. w29x J. Hofer, J. Karpinski, M. Willemin, G.I. Meyer, E. Kopnin, R. Molinski, H. Schwer, C. Rossel, H. Keller, Physica C 297 Ž1998. 103. w30x J. Ricketts, R. Puzniak, C.-J. Liu, G.D. Gu, N. Koshizuka, H. Yamauchi, Appl. Phys. Lett. 65 Ž1994. 3284.
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w31x A. Schilling, R. Jin, J.D. Guo, H.R. Ott, I. Tanaka, H. Kojima, Physica B 194–196 Ž1994. 1555. w32x S. Senoussi, M. Oussena, ´ G. Collin, I.A. Campbell, Phys. Rev. B 37 Ž1988. 9792. w33x D.K. Christen, J.R. Thompson, Nature ŽLondon. 364 Ž1993. 98. w34x C.-Q. Jin, S. Adachi, X.-J. Wu, H. Yamauchi, S. Tanaka, Advances in Superconductivity—VII, in: K. Yamafuji, T. Morishita ŽEds.., Springer, Tokyo, 1995, p. 249. w35x C.-Q. Jin et al., to be published.
Influence of Re substitution on the flux pinning in žHg,Re /Ba 2 Ca 2 Cu 3 O 8qd single crystals R. Puzniak a , J. Karpinski b,) , A. Wisniewski a , R. Szymczak a , M. Angst b, H. Schwer b, R. Molinski b, E.M. Kopnin b a
Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32 r 46, PL 02-668 Warsaw, Poland b Laboratorium fur ETH Honggerberg, CH 8093 Zurich, Switzerland ¨ Festkorperphysik ¨ ¨ Received 22 July 1998; revised 21 September 1998; accepted 7 October 1998
Abstract Flux pinning in Hg 0.77 Re 0.23 Ba 2 Ca 2 Cu 3 O 8q d single crystals ŽTc s 130 K. and in HgBa 2 Ca 2 Cu 3 O 8q d crystals with Tc equal to 130 and 120 K was studied. In the investigated crystals Re atoms substitute at the Hg site of HgBa 2 Ca 2 Cu 3O 8q d and alter the defect structure of the host layer by pulling in four new oxygen atoms at Ž0.34, 0.34, 0. position to form an octahedron around Re. As a result a significant improvement of the irreversibility line position at the H–T phase diagram for Hg 0.77 Re 0.23 Ba 2 Ca 2 Cu 3 O 8q d crystal is observed only at low temperatures Žbelow 80 K.. However, magnetization measurements performed at temperatures above 100 K show insignificant change in the irreversibility line position only, as compared to that one of the parent compound with Tc s 130 K. This indicates insignificant influence of new pinning centers, introduced by Re substitution, in the temperature range, where thermal activation energy becomes significant in comparison with pinning energy. Furthermore, obtained data indicate that no improvement of the coupling between superconducting layers was achieved as a result of the expected increase of electrical conductivity in the blocking layer. Since only a small change in the c-axis lattice constant was observed in Re substituted crystals, we can conclude that a larger shortening of the blocking layer in the material is necessary to improve the irreversibility line position in HgBa 2 Ca 2 Cu 3 O 8q d crystals at high temperatures. q 1998 Elsevier Science B.V. All rights reserved. PACS: 74.60.Ge; 74.25.Ha; 74.72.Gr Keywords: Flux pinning; Irreversibility line; Hg-based single crystals; Re-doping
1. Introduction One of the most important factors limiting possible applications of high-Tc superconductors, at least in the high temperature region, is a relatively low position of the irreversibility line ŽIL. on the H–T )
Corresponding author. Tel.: q41-1-633-22-54; Fax: q41-1633-10-72; E-mail: [email protected]
phase diagram. At high temperatures and low fields the vortex structure for the magnetic field H 5 c-axis can be well described by three-dimensional Ž3D. flux lines’ behavior. In higher fields and at lower temperatures a crossover to two-dimensional Ž2D. ‘pancake’ vortex structure is observed for layered materials. Using the description of Houghton et al. w1x, based upon a thermodynamic melting of the flux lattice Žin the limit of negligible pinning., the irre-
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 5 8 9 - 9
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versibility line in high temperature range can be described by: n
Hirr Ž T . s H0 Ž 1 y TrTc . , H0 A 1rŽ l4a b Ž0.g 2 ..
Ž 1.
where Here, l a b is the in-plane penetration depth and g s Ž m )c rm )a b .1r2 is the effective mass anisotropy. From the above it is obvious that an improvement of the irreversibility line position at high temperatures cannot be achieved Žassuming negligible pinning. without decreasing the penetration depth or the effective mass anisotropy. A decrease of the penetration depth can be achieved by the increase of superconducting carrier concentration andror the decrease of carrier effective mass. The decrease of effective mass anisotropy leads to the improvement of the coupling between superconducting layers of the material. This coupling is Josephson-like, its strength being exponentially reduced with increasing interlayer distance w2,3x. The interlayer distance d i generally increases from YBa 2 Cu 3O 7y d and CuBa 2 Ca ny1Cu nO 2 nq4y d , across La 2yxSr x CuO4 ŽLa-214. and HgBa 2 Ca ny1Cu nO 2 nq2q dŽHg-12Ž n y 1. n., to Bi 2 Sr2 Ca ny1Cu nO 2 nq4q d ŽBi22Ž n y 1. n., and so the irreversibility line position decreases. The interlayer coupling in the material can be improved by shortening the ‘blocking layer’ distance Žthe distance between groups of n CuO 2 conducting planes. and by the increase of electrical conductivity in the blocking layer, which can eventually lead to proximity-induced weak superconductivity in the blocking layer w4x. The above factors controlling the position of the irreversibility line are related to the intrinsic parameters of the material. There is another, extrinsic possibility of an improvement of the irreversibility line position—the introduction of more effective pinning centers into material. Such centers usually are very effective at relatively low temperatures, where the pinning energy is high in comparison with the thermal activation energy. However, the effectiveness of the pinning centers may decrease rather fast with increasing temperature and pinning may be ineffective close to Tc . Finally, a shift of the irreversibility line to higher fields and temperatures on the H–T phase diagram may originate from the existence of surface barrier making the movement of vortices more difficult. The highest transition temperature among the high-Tc superconductors, equal to 135 K under nor-
mal pressure, was achieved for the Žoptimally doped. mercury-based copper-oxide compound HgBa 2 Ca ny 1Cu nO 2 nq2q d with n s 3 ŽHg-1223. w5,6x. Unfortunately, the intrinsic flux pinning properties of the material are not as good as the ones of some other high-Tc superconductors, e.g., YBa 2 Cu 3 O 7y d w7–9x. Recently, significant improvement in the flux pinning of the mercury based ceramic samples with Hg partially replaced by Re or Cr and with Ba replaced by Sr was reported in the literature for several times w10–13x. Shimoyama et al. w10–14x have argued that at least two features of the chemically substituted compounds may enhance flux pinning: Ž1. the substitution of Sr for Ba significantly ˚ w13x; shortens the blocking layer by about 0.8–0.9 A and Ž2. the chemical substitution at the Hg site may make the blocking layer more metallic w4,14x. Chmaissem et al. explored the possibility that extended defects in a chemically substituted HgSr2CuO4q d compound could act additionally as pinning centers w15x. Studies of Fabrega et al. w16x performed on grain-aligned Hg 1y x Re x Ba 2 Ca 2 Cu 3 O 8q d samples showed that Re substitution enhances bulk pinning at low temperatures. They have observed that surface barriers dominate the irreversible magnetization in their samples at high temperatures. In Re substituted samples they found an increase of the irreversibility line also at high temperatures. However, due to the high anisotropy of HTSC and additional phases in ceramic samples only measurements on single crystals are reliable for the determination of true intrinsic properties of these compounds. Polycrystalline material has randomly oriented grains, what makes the measurements of superconducting parameters of highly anisotropic materials difficult. Grain boundaries limit the macroscopic critical current. The grain boundaries may act as pinning centers. Therefore, to measure ‘intrinsic’ parameters of HTSC with any appreciable accuracy, one has to use single crystals. In order to study the influence of different factors on the irreversibility line position we determined the Hirr ŽT . in a wide temperature range for Re substituted HgBa 2 Ca 2 Cu 3 O 8q d single crystals. The results were compared with the data obtained for underdoped and almost optimally doped HgBa 2Ca 2 Cu 3 O 8q d . The characteristic features of Re substituted crystals in comparison with single crystals of
R. Puzniak et al.r Physica C 309 (1998) 161–169
the parent compounds are Ž1. planar defects Žfaults. caused by Re substitution, acting as pinning centers, Ž2. a small shortening of the c-axis lattice constant in comparison with that one for the material without substitution ŽRe substitution decreases the blocking ˚ w17x., and Ž3. a layer distance Ba–Ba by about 0.2 A possibly more metallic character of the blocking layer obtained as a result of chemical substitution at the Hg site. In order to study the temperature dependence of well defined components of irreversibility field the measurements were performed on single crystals. In the investigated material, we found, at high temperatures, no significant enhancement of the irreversibility field parallel to the c-axis in comparison with that one of the host material with Tc s 130 K. This means that introduced defects are ineffective as pinning centers controlling the position of the irreversibility line at high temperatures. Furthermore, the data indicate that no improvement of the coupling between superconducting layers can be achieved by Re substitution into HgBa 2 Ca 2 Cu 3 O 8q d only, without larger shortening of the blocking layer in the material.
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2. Single crystals preparation and structural characterization One of the main difficulties with the synthesis of the mercury based compounds Hg-12Ž n y 1. n is their low thermal stability. The compounds melt peritectically, but at ambient pressure they decompose before melting and the volatile components evaporate. Since the total pressure of volatile components PO 2 , P Hg , and P HgO exceeds 100 bar at temperatures above the melting point, encapsulation of the sample with a high hydrostatic inert gas pressure was used to prevent decomposition. Crystal growth experiments have been already described in several publications w18– 21x. All crystal growth experiments have been performed at Ar pressures of 10 kbar. Due to the high density of Ar gas at this pressure Ž) 1 grcm3 ., evaporation of Hg was strongly suppressed. Single crystals were grown from a flux or from a stoichiometric melt. As a flux, BaCuO 2 –CuO–Ag 2 O was used. The reasons of using Ag 2 O were: Ži. decreasing of the melting temperature of the BaCuO 2 –CuO eutecic w22x, Žii. increasing of the partial oxygen
Fig. 1. Model of ŽHg,Re.O layer in Hg 1y x Re x Ba 2 Ca ny1Cu nO 2 nq2q d single crystals.
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pressure during synthesis. The crystallization temperature was 10008C- T - 10708C. Crystals of Hg12Ž n y 1. n for n s 1–7 were obtained in such a way and crystals of Hg-1223 were selected for further studies. In some experiments ReO 2 was added to precursor in order to substitute Hg. We measured different Re substituted 12Ž n y 1. n crystals on an X-ray four circle single crystal diffractometer w17,23x. Structure analysis showed that rhenium is introduced at the Hg site and coordinated octahedrally by oxygen atoms. These results, in particular the coordination and bondlengths of Re–O are in agreement with studies of Chmaissem et al. w24x. Re–O bond lengths ˚ . in the basal z s 0 plane are much shorter Ž1.86 A ˚ .. than the corresponding Hg–O distances Ž2.725 A Hg and Re atoms cannot be distinguished with Xrays, but oxygen atoms bonded to Re can easily be located in the Fourier maps at Ž0.344, 0.344, 0. position. The amount of this oxygen is four times as high as that of rhenium. The occupation of the HgrRe site is almost full. Excess oxygen atoms connected to mercury are located at Ž1r2,1r2,0. and are only partially occupied. One unit cell cannot carry more than one oxygen atom in HgrRe–O plane, which is either connected to Hg or to Re. It is impossible that two adjacent rhenium atoms exist with an octahedral oxygen coordination, because in this case two oxygen atoms will be placed in one unit cell ŽFig. 1.. Therefore the maximum Re substitution is 25%. From the EDX analysis Re content in investigated crystals was 23 " 2%. According to our X-ray single crystal investigations w25x the Hg site occupation in crystals without rhenium was refined to 80–85%, implying that some carbonate might be incorporated there. A further hint for the presence of CO 32y in these crystals is the short c-axis parameter; CO 32y impurities could also be responsible for the lower Tc in some crystals.
3. Experimental details For magnetic measurements a crystal of ŽHg,Re.Ba 2 Ca 2 Cu 3 O 8q d , further denoted HgRe-1223, with a well resolved transition temperature of about 130 K, defined as the onset of the diamagnetic signal, was chosen. We have studied also two unsubstituted reference crystals of Hg-1223; one with Tc s 130 K
and second one with Tc s 120 K. It is rather difficult to grow HgBa 2 Ca 2 Cu 3 O 8q d crystals with a high carrier concentration, close to that of optimally doped materials. Our reference crystal HgBa 2 Ca 2 Cu 3 O 8q d with Tc s 120 K was underdoped while both Re substituted and unsubstituted crystals with Tc s 130 K were almost optimally doped. In view of earlier crystal growth experiments with Hg-1223, it is very likely that the unsubstituted crystal with Tc s 130 K is slightly underdoped w18,25x. The Re substituted crystal may well be slightly overdoped, as Re substituted material was usually found to be overdoped by other authors Žsee, e.g., Refs. w13,26x., but we do not have any evidence of this. However, as will be seen later, this possibility cannot change our conclusions. It is worth to notice that the Re substitution at Hg site of mercury based crystals makes much easier growth of single crystals with the number of Cu–O planes higher than three. The selected crystal of Hg 1y x Re x Ba 2 Ca 2 Cu 3O 8q d with x s 0.23 " 0.02 Žas determined by EDX-measurements. had an approximately parallelepiped shape with dimensions 0.7 = 0.7 mm2 in the a–b plane and the thickness of about 27 mm along the c-axis. Magnetization measurements with accuracy better than 10y7 emu were performed on a home made SQUID magnetometer with a Quantum Design SQUID sensor and on a commercial Quantum Design MPMS 5. All measurements were performed in the magnetic field geometry H 5 c. The irreversibility temperature Tirr , defined as a temperature of the appearance of the first resolved difference between zero field cooled and field cooled magnetization, was determined from M ŽT . dependencies measured in fixed magnetic field. The irreversibility field Hirr , defined as a field where the two branches of the hysteresis loop M Ž H . measured at fixed temperature meet within the resolution of magnetic measurements was determined as well. The criterion of 3 = 10y7 emu in the determination of detectable magnetic moment difference between two branches of hysteresis loop was applied. For the given sample —dimensions of 0.7 = 0.7 = 0.027 mm3 —this corresponds to the magnetization criterion of 2.3 = 10y2 Gs and for the magnetic field applied along the c-axis of the sample to the criterion of 10 Arcm2 for the critical current density. It was confirmed that both Hirr and Tirr data placed in the H–T phase
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diagram coincide rather well and form one well defined irreversibility line.
4. Results and discussion Fig. 2 presents zero field cooled and field cooled magnetization data for HgRe-1223 single crystal recorded in the vicinity of Tc in magnetic field of 100 Oe applied along the c-axis of the crystal. A well defined irreversibility temperature, of 123 K in the particular case of 100 Oe external magnetic field, is clearly seen on the plot. The inset presents the data obtained in the full temperature range, in the field of 10 Oe. The results indicate the absence of any additional magnetically detectable phase in the crystal. The relatively small ratio of the field cooled magnetization to the zero field cooled magnetization, equal to about 0.2, suggests the existence of a large number of pinning centers in the material. Such centers may be related to the existence of planar defects Žfaults. separating domains formed by the Re–O6 octahedra and domains ordered in different ways. These objects were clearly identified in the crystallographic structure studies of Re substituted Hg 1y x Re x Ba 2 Ca 2 Cu 3 O 8q d samples w13x. Fig. 3 presents the field dependence of the magnetization measured for the Hg 0.77 Re 0.23 Ba 2 Ca 2 Cu 3-
Fig. 2. Zero field cooled and field cooled magnetization in the vicinity of Tc measured for almost optimally doped Hg 0.77 Re 0.23 Ba 2 Ca 2 Cu 3 O 8q d in a magnetic field of 100 Oe applied along the c-axis of the crystal. The inset presents the data obtained in the full temperature range.
Fig. 3. Field dependence of the magnetization measured for a Hg 0.77 Re 0.23 Ba 2 Ca 2 Cu 3 O 8q d single crystal in a magnetic field parallel to the c-axis at temperatures of 40, 50, 60, and 70 K. The inset shows an estimation of the irreversibility field at a temperature of 60 K Ž Hirr s 31 kOe..
O 8q d single crystal in the magnetic field parallel to the c-axis at several temperatures: 40, 50, 60, and 70 K. The inset shows the estimation of the irreversibility field at a temperature of 60 K Ž Hirr s 31 kOe in particular case.. Hysteresis loops for all investigated temperatures seem to be symmetric around reversible magnetization, i.e., the M–H dependence expected for the material without pinning. Both lower and upper branches of the loop are strongly field dependent at all temperatures. There is no flat part in the M–H dependence, indicating that surface barrier effects are not important in this temperature range. The width of the hysteresis loops is determined by the bulk pinning effect. The same was found for unsubstituted Hg-1223 single crystals. One can see, especially at the lower measured temperatures, a second maximum in the width of hysteresis loop in all presented temperatures indicating a rather high critical current density in the moderate field range. We did not observe a ‘fishtail effect’ at temperatures higher than 80 K. The temperature dependence of the irreversibility field for HgRe-1223 in the magnetic field applied along the c-axis of the crystal is presented in Fig. 4. In Fig. 4a the values of Hirr plotted as a function of reduced temperature Ž1 y TrTc . are compared with the data obtained for underdoped HgBa 2 Ca 2 Cu 3 O 8q d crystal with Tc of about 120 K w27x and for almost optimally doped Hg-1223 crystal with Tc f 130 K.
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Fig. 4. Ža. Irreversibility field of Hg 0.77 Re 0.23 Ba 2 Ca 2 Cu 3 O 8q d , underdoped HgBa 2 Ca 2 Cu 3 O 8q d with Tc s120 K w27x, almost optimally doped Hg-1223 with Tc s130 K single crystals determined in a magnetic field parallel to the c-axis are presented as a function of Ž1yT r Tc .. The inset presents irreversibility field data as a function of T, in higher temperature region. Žb. The data for Hg 0.77 Re 0.23 Ba 2 Ca 2 Cu 3 O 8q d are compared with the data for Bi 2 Sr2 CaCu 2 O 8q d w30x and La 1.86 Sr0.14 CuO4 w31x.
One can see that the irreversibility line for Re substituted crystals is located in fields slightly higher than related fields for the irreversibility line of the host underdoped material with Tc of 120 K. At high temperatures ŽT ) 90 K. the IL for HgRe-1223 is essentially the same as the IL for the parent compound with Tc s 130 K. In this temperature region presumably the pinning energy of defects caused by Re substitution is low in comparison with the thermal activation energy. Hence, the description of vortex behavior in the limit of negligible pinning may be used. The differences in the irreversibility field for the same reduced temperature among all of
the investigated crystals are small—a factor of approximately two to three. One should take into account that the studied crystals have different anisotropy. It is well known that the anisotropy of the effective mass in cuprates decreases with increasing oxygen concentration. Such a tendency was reported for YBa 2 Cu 3 O 7y d w28x as well as for under- and overdoped HgBa 2 Ca ny1Cu nO 2 nq2q d compounds Žfor example, for Hg-1201, see Hofer et al. w29x.. The underdoped crystal with Tc s 120 K has the highest anisotropy among studied samples. It is expected that the anisotropy of the unsubstituted crystal with Tc s 130 K is about 40% lower than the anisotropy of our reference crystal with Tc s 120 K Žsee, estimation of the effective mass anisotropy for Hg1223 with Tc s 120 K and for Hg-1223 with higher Tc w18x.. The 40% difference in the effective mass anisotropy, according to Eq. Ž1., may change the value of Hirr by a factor of about 2. In the case of HgRe-1223 we cannot exclude that this crystal is overdoped, i.e., that its anisotropy is lower than that of Hg-1223 with Tc s 130 K. If the Re substituted crystal was in fact overdoped, the irreversibility line position for HgRe-1223 would be shifted toward higher fields and temperatures due to the lower anisotropy. The comparison of Hirr data obtained for Re doped HgRe-1223 and for undoped parent compound of Hg-1223 with the same transition temperature of 130 K excludes any significant direct influence of Re substitution on Hg site on the position of the irreversibility line at high temperatures. The irreversibility field data for temperatures above 100 K are essentially the same. Hence, no improvement of the coupling between superconducting layers was achieved by making the blocking layer more metallic as a result of Re substitution into HgBa 2 Ca 2 Cu 3O 8q d . Additionally, planar defects separating domains formatted by the Re–O6 octahedra do not improve the position of the irreversibility line of HgRe-1223 crystals at high temperatures. Only a significant shortening of the blocking layer may be effective to improve position of the Hirr in H–T phase diagram. At lower temperatures the situation seems to be more complicated. For a Hg-1223 crystal with Tc s 120 K, there is clearly visible a crossover in the temperature dependence of the irreversibility field in a field of approximately 2 kOe. A more rapid change
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of Hirr with temperature is observed at lower temperatures. This indicates that there the pinning has a rather two-dimensional Ž2D. character. The appearance of a crossover, in our opinion, is a result of the transition from three-dimensional Ž3D. vortex structure at high temperatures to 2D ‘pancake’ vortex structure at lower temperatures. Assuming negligible pinning, the crossover field is roughly proportional to the inverse effective mass anisotropy of the system, 1rg 2 . For the parent compound with higher Tc Ž130 K., i.e., lower anisotropy, the crossover becomes much less pronounced and is shifted to higher fields of about 5 kOe. This change of the value of the crossover field corresponds to a decrease of the anisotropy g by factor of about 1r3 from g f 60 to g f 40. For the HgRe-1223 crystal with Tc of 130 K the crossover should be expected at roughly the same field as that for optimally doped Hg-1223. However, it is observed at smaller fields, i.e., at higher temperature. Also the position of the IL in the HgRe-1223 crystal, at lower temperatures, is shifted to the region of higher fields and temperatures. Such a behavior can be explained by taking into account the larger number of pinning centers in the sample. These centers are ineffective at high temperatures where the thermal activation energy is high in comparison with the pinning energy. With decreasing temperature pinning becomes more and more effective and deviation from the Hirr ŽT . dependence given by Eq. Ž1. appears before the 3D–2D crossover expected for the material without pinning. Fig. 4b presents a comparison of the irreversibility lines for HgRe-1223, Bi 2 Sr2 CaCu 2 O 8q d w30x, and La 1.86 Sr0.14 CuO4 w31x. All the ILs were determined from the magnetization measurements and correspond to the critical current density criterion 10 Arcm2 . The IL of HgRe-1223 is apparently located in higher magnetic fields and at higher temperatures than the IL of Bi-2212—the compound with much higher anisotropy and greater interlayer spacing d i . On the other hand, the comparison of HgRe-1223 and La-214 Žsystem with lower anisotropy than HgRe-1223 and much better intercell coupling strength. shows a difference in the position of the IL only at high temperatures. At low temperatures, the effectiveness of pinning centers introduced by Re substitution leads to similar values of Hirr for both compounds.
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Fig. 5 presents the width of hysteresis loops determined in magnetic fields of 6, 8, and 10 kOe at temperatures where the critical state inside the superconductor is well established. The width of the loop is proportional to the critical current density. The temperature dependence of the critical current density Jc is often described approximately by the formulae: Jc ŽT . s Jc Ž0.expŽyTrT0 . over a certain magnetic field range w32,33x. The fitting parameter T0 characterizes the decrease of Jc with increasing temperature. The magnitude of T0 changes slightly over the field Žsee for example, behavior of Jc ŽT . found for CuBa 2 Ca 3 Cu 4 O 10q d ŽTc s 117 K. with T0 of about 12 K w34x.. The magnitude of T0 may be used to estimate the capability of the HTSC to be operated at high temperature, and definitely a higher value of T0 would be preferred for application. According to the value of T0 , the Jc behavior can be classified into various groups. The YB 2 Cu 3 O 7y d and CuBa 2 Ca 3 Cu 4 O 10q d compounds characterized by an average T0 ; 14 K rank in the most steady ones, Bi-2212 system characterizes the lowest value of T0 ; 4 K w35x. As one can see in Fig. 5, for HgRe-1223 crystal we found T0 ; 10 K in the temperature range 50–80 K. This confirms that from the point of view of applications, Re substitution of Hg-1223 does not help.
Fig. 5. The width of hysteresis loops determined in magnetic fields of 6, 8, and 10 kOe at temperatures where the critical state inside the superconductor is well established. The width of the loop is proportional to the critical current density.
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5. Conclusions We compared flux pinning in a Hg 0.77 Re 0.23Ba 2 Ca 2 Cu 3 O 8q d single crystal ŽTc s 130 K. and in HgBa 2 Ca 2 Cu 3 O 8q d crystals with Tc equal to 130 and 120 K. Our studies showed that Re atoms substituted at the Hg site enhance flux pinning in Hg-1223 superconductor only at low temperatures. Magnetization measurements performed at high temperatures show insignificant enhancement of the irreversibility field parallel to the c-axis, in comparison with that one for host material with Tc s 130 K. This indicates that no improvement in the coupling between superconducting layers is achieved by making the blocking layer more metallic as a result of Re substitution into HgBa 2 Ca 2 Cu 3 O 8q d . Despite the existence of new pinning centers, the improvement of the irreversibility line position on the H–T phase diagram may be achieved in the higher temperature region by shortening the blocking layer thickness in the material only. This may be achieved by substitution into the Ba sites of Sr atoms or other elements with small ionic radius.
Acknowledgements This work was partially supported by the Polish Government Agency KBN under contract No. 722P039509 and by the Swiss National Science Foundation.
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