Flux pinning and the irreversibility lines in the HgBa2CuO4+δ, HgBa2CaCu2O6+δ and HgBa2Ca2Cu3O8+δ compounds

PHYSICA

Physica C 218 (1993) 373-378 North-Holland

Flux pinning and the irreversibility lines in the HgBa2CuO4 HgBa2CaCu206 + and HgBa2Ca2Cu308 + compounds U. Welp, G.W. Crabtree, J.L. Wagner a n d D.G. Hinks Materials Science Division and Science and Technology Centerfor SuperconductiVity, Argonne National Laboratory, Argonne, IL 60439, USA

Received 11 October 1993 Revised manuscript received29 October 1993

Using measurements of the magnetizationhysteresiswe determine the temperature and field dependenceof the critical current density, the pinningforceand the irreversibility lines ofpolycrystallineHgBa2CuO4+6,HgBa2CaCu2Or+6and HgBa2Ca2CuaOs÷6. For all three materials we observe an exponential decay of the critical current with increasing temperature and/or field. The temperature dependenceof the irreversibilityfields followsa powerlaw,H~oc ( 1- T/To)4, and is intermediate to that of YBa2Cu307 and the Bi/Tl-based superconductors.These results are analyzedin a model in which the separation betweenthe superconducting CuO2 blocks is the important parameter. At 77 K the irreversibility field of HgBa2Ca2CuaOa+6is 2 T and the critical current density at 77 K in a field of I T is about 2× 103A/cm2. The plate-like nature of the grains in the double- and triple-layer compounds offersthe possibility that textured materials with goodcouplingbetween the grains can be prepared.

The discovery [ 1 ] of superconductivity in the single CuO2-1ayer compound HgBa2fuO4+,~ (Hg-1201) with Tc=94 K has initiated intensive research on systems with the general composition HgBa2Ca,_ iCu,O2,+2+6. Besides the single-layer compound (n = 1 ), the double-layer (n = 2, Hg- 1212) [2,3 ] with T~= 128 K and the triple-layer compound ( n = 3 , Hg-1223) [4-6] with T¢= 135 K have been synthesized. All three compounds are characterized by blocks of superconducting CuO2 layers separated by one unit of insulating BaO-Hg-BaO resulting in rather short distances between the superconducting blocks: 9.51 A for Hg-1201 [7], 9.52 A for Hg-1212 [3] and 9.38 A for Hg-1223 [6]. For comparison, in the materials most commonly used for the development of large-scale applications, namely Bi2Sr2CaCu2Os (Bi2212) and Bi2Sr2Ca2Cu3Ol0 (Bi2223 ), the corresponding distances are 12.3/i and 11.8 A, respectively. It has been shown [ 8 ] that the performance of the layered CuO2 superconductors in magnetic fields is closely related to the distance between the CuO2 blocks. A large separation causes large anisotropies in the superconducting properties and the formation of quasi-two-dimensional vortices

("pancake" vortices). These, in turn, give rise to reduced irreversibility fields and critical currents. Therefore, the small separation between the superconducting blocks in the Hg-compounds makes these materials a promising alternative in the development of high-T¢ based devices. Here, we present a systematic study of the magnetization hysteresis and the irreversibility line of polycrystalline optimally doped samples of the three Hg-compounds. We find that the magnetization hysteresis AM (i.e. the critical current) in Hg-1201 is smaller by a factor of five to ten than in the doubleand triple-layer compounds. However, for all three materials AM decays rapidly, quasi-exponentially, with increasing temperature. This is an indication for substantial thermally activated flux motion. The characteristic temperature for this decay, around 6 K, is the same for the three compounds. Also, the temperature dependence of the irreversibility field, H i , ( T ) , as determined from the vanishing of AM is very similar for the three compounds. Hir~ follows approximately a power law: Hi~oc ( 1 - T / T c ) m with m ~ 4 . We interpret these similarities between the compounds as a consequence of the fact that they

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U. Welp et al. ~.Flux pinning and IL's in Hg compounds

374

have almost the same separation between the C u O 2 blocks. The samples used in this study were synthesized from appropriate mixtures of the corresponding oxides as described previously [3,6,7]. For Hg-1201 the sample is a 80% dense ceramic whereas for the double- and triple-layer materials the samples are very porous and less than 50% dense. While the double- and triple-layer compounds contain plate-like grains of about 10 ~tm size, the grains in Hg-1201 are more three-dimensional but of roughly the same size as determined from SEM studies on these samples. Figure l shows the temperature dependence of the (zero-field cooled) susceptibility for the three compounds. For this measurement the bulk samples have been ground up into fine powders in order to avoid problems due to intergrain coupling. The single-layer compound shows a sharp transition at 95 K and reaches 100% shielding near 85 K. The double- and triple-layer samples are characterized by sharp superconducting onsets at 128 K and 135 K, respectively, and tails on the low-temperature side of the transition. However, both materials have nearly 100% shielding fraction at low temperatures and do not show second superconducting phases. The magnetization of the three compounds has been measured in a Quantum Design SQUID magnetometer at various temperatures as a function of the applied field up to 7 T. For all measurements the

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same measurement sequence has been used in order to ensure comparability of the results. After each field increment the system pauses for 2 min in order to avoid drifts caused by the relaxation of the superconducting magnet. Each data point is the average of four individual scans. The results for the field dependence of the magnetization hysteresis at various temperatures for the three materials are shown in fig. 2. The magnetization hysteresis of the single-layer compound is about a factor of five to ten smaller than that of the double- and triple-layer compounds. This low value of AM is in agreement with earlier reports [9] and suggests that this material is very clean (i.e.,

U. Welp et al. ~Flux pinning and IL's in Hg compounds

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it has few defects that could serve as pinning centers). However, it has been observed that neutron irradiation causes an enhancement by a factor of five of the critical currents and a shift of the irreversibility line to higher fields [ 10]. At temperatures above 20 K (above 40 K for Hg-1212 and above 50 K for Hg-1223) the magnetization hysteresis drops precipitously with increasing field up to about 1 T and then decays almost exponentially in higher fields. This behavior has been observed previously for Hg1201 [ 1 I, 12 ]. The behavior at low fields suggests breaking of the weak links between the grains. With increasing temperature AM for Hg- 1212 and Hg-1223 displays a very systematic evolution sug-

375

gesting a scaling property of the data. This is shown in fig. 3 plotting the normalized volume pinning force versus reduced field H/H*. The pinning force is given by Fp=JcXB, where the critical current density, Jc, is calculated from the magnetization hysteresis using Bean's model, J¢=2OAM/D. A grain size of D = 10 ~tm was used for all three materials. H* is determined from the location of the maximum in the Fp(H) dependence. The data for the pinning force of Hg-1223 show almost perfect scaling behavior. This kind of scaling has commonly been used to analyze pinning forces [13] where Fp is written in the form Fpoc ( H / n i r r ) P ( 1-H/nirr) q where nirr is the irreversibility field. For Hg- 1201 values ofp = 0.5 and q= 1.15 have been reported [12]. Our data do not follow this form for two reasons: the above expression for Fp predicts a sharp vanishing of Fp at H ~ whereas our data for all three materials are characterized by long tails extending to very high fields. These long tails are probably associated with the distribution of grain orientations when measuring polycrystalline samples. Secondly, the scaling field is usually identified with the irreversibility field Hip. Since our field range of up to 7 T does not allow us to make the direct determination of the irreversibility field at low temperatures ( T < 50 K) we chose the peak position in Fp(H) as scaling field. The parameters used to obtain the scaling in fig. 3 are listed in table 1. Although the choice of H* might appear somewhat arbitrary we note that any set of fields having the ratios of 6.8:4:2.3:1 (for the data at 30, Table 1 Parameters H* and Fp,m~,used in the scaling of the pinning force of rig-1201, Hg-1212 and Hg-1223 Hg-1201

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376

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40, 50 and 60 K of Hg-1223, figs. 2 and 3) will produce scaling similar to that in the lower panel of fig. 3. By choosing, for example, the criterion A M = 1.8 e m u / c m 3 - 3.6 × 104 A / c m 2 a different set of scaling fields can be directly read off the bottom panel in fig. 2. These fields have the same ratios as the peak fields, and, therefore, would also collapse all the data into a single curve as in fig. 3. As an added benefit, this criterion allows the scaling field for the 70 K data to be determined as half of those for 60 K. Using the peak-field criterion, no scaling field for T = 70 K can be found, because the peak occurred below the lowest measuring field. The scaling of the 70 K data in fig. 3 was achieved using half the scaling field for the 60 K data. The occurrence of scaling shows that the flux-line dynamics in the entire temperature range (up to 70 K) is governed by the same pinning mechanism. For the Hg-1212 sample we observe deviations from scaling which we attribute to traces of "fish-tail" behavior seen at temperatures above 40 K. We note, however, that the scaled 30 K data of Hg-1212 superimpose perfectly onto those of Hg-1223 indicating that the underlying pinning mechanism for both materials is the same. The flux pinning in the Hg1201 sample is characterized by the "fish-tail" behavior. As a consequence, there are deviations from scaling at low and high fields, and the peak in Fp/ Fp . . . . is much narrower than for Hg- 1223. Since flux pinning in the Hg-1201 sample is weak the "fish-tail" behavior is a dominant contribution, whereas in Hg1212 and Hg- 1223 pinning is much stronger and the "fish-tail" behavior results only in small distortions of the AM versus//curveFigure 4 shows a comparison of the temperature dependence of the critical current in fields of 1 T and 3 T of the three materials. The behavior at temperatures above 40 K is characterized by an exponential decay of the form J ~ o c e x p ( - T / T o ) . At lower temperatures grain-coupling or "fish-tail" behavior causes deviations from the exponential dependence. The characteristic temperature, To, has the same value of about 6 K for all three materials. This is another indication that the nature of the flux dynamics in the three materials is very similar. For Hg-1201 a value of To about 7 K has been reported previously [11 ]. The exponential decay of the critical current with increasing temperature is an indication for

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and 3 T of rig-1201, Hg-1212 and Hg-1223. strong thermally activated flux motion. The critical currents are highest for the triple-layer compound. This is partly due to the higher value of To. However, fig. 5 shows that also on a reduced temperature scale Hg-1223 has the highest critical currents, which, in the absence of any study of the defect structure in these as-grown materials, would imply that the triple-layer compound is intrinsically capable of carrying higher critical currents. In a qualitative way this can be understood since with an increasing number of CuO2 planes the effective fraction of high-current carrying material per unit cell increases. The irreversibility lines for the three materials are shown in fig. 6. The presence of the long tails in

U. Welp et aL ~Flux pinning and IL's in Hg compounds

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Fig. 6. Temperature dependence of the irreversibilityfield of the three materials. Solid symbolsand solid lines refer to the low current criterion (300 A/cm 2), open symbolsand broken lines refer to the high current criterion ( 1× 104 A/cm2). A M ( H ) requires the choice of a criterion for the irreversibility line. Figure 6 shows the results for two choices for this criterion: A M = 1.5 × 10 -2 e m u / c m 3 which corresponds to a current of 300 A / c m 2 on a 10 ~tm-grain (solid symbols and solid lines) and, second, A M = 0.5 e m u / c m ~ corresponding to 1 × 104 A / c m 2 (open symbols and broken lines). The first choice is the lowest value of Jc that can be reliably measured in the presence of drifts in the baseline and relaxation of the superconducting magnet. The second criterion marks the region where the material might be of technological interest. The temperature dependence for these lines is very similar, if plotted as a function of reduced temperature they are well described by Hi,r(T) oc ( 1 - T I T s ) 4. This tempera-

377

ture dependence is somewhat stronger than has been reported in a similar study [2 ], but still much weaker than in the most commonly used Bi-2212 and Bi2223 compounds. Correspondingly, the Hg-based materials are capable of carrying substantial (intragrain) currents even at 77 K. In conclusion, we have made a systematic study of the magnetization of polycrystalline Hg-1201, Hg1212 and Hg- 1223 samples, measuring magnetization hysteresis, critical currents, pinning forces and the irreversibility lines. The similar temperature and field dependence of the critical currents, and temperature dependence of the irreversibility field suggest that the flux dynamics in these compounds is very similar. This can be understood in a model [ 8 ] in which the separation between the superconducting CuO2 blocks is the fundamental parameter. Large separations cause large anisotropies, a quasi-2D nature of the vortices and rather low irreversibility fields. In the Hg-compounds these separations are essentially the same, 9.5/k, which explains the striking similarities in their behavior. It also explains that the irreversibility field decays much less drastically with increasing temperature than, for example, in Bi2212 and Bi-2223. A comparison [2,9] of the irreversibility lines of various groups of CuO superconductors indicates that the behavior of the Hg-compounds falls intermediate to YBa2Cu307 and the Bi/ /T1-2212 materials. In the absence of single-crystal samples there is no direct measurement of the anisotropy in the Hg-materials. Measurements of the trapped flux in grain-aligned Hg- 1201 samples [ 14 ] indicate a rather low anisotropy. Among the three Hg compounds the triple-layer material appears to be the most promising. This is caused in part by the higher value of Tc, but, as shown by the critical current as a function of reduced temperature, it also has an "intrinsic" origin. The critical currents determined in this study, especially in fields above 1 T, are intragrain currents. Their exponential decay with increasing temperature suggests that they are subject to considerable thermally activated flux motion. For technological applications the coupling between the grains is as important a property as the intragrain currents. Studies [ 11,12 ] on polycrystalline, untextured Hg-1201 samples indicate that this material is very granular, i.e. has weak coupling between the grains. However, the plate-like nature of the grains

378

u. Welp et al. ~Flux pinning and IL's in Hg compounds

in the d o u b l e - a n d triple-layer c o m p o u n d s suggests that the synthesis o f t e x t u r e d m a t e r i a l w i t h g o o d grain c o u p l i n g m a y be possible.

Acknowledgements T h i s w o r k was s u p p o r t e d by the U S D e p a r t m e n t o f Energy, BES M a t e r i a l s Sciences u n d e r c o n t r a c t # W - 3 1 - 1 0 9 - E N G - 3 8 ( G W C , D G H ) a n d the N S F O f f i c e o f Science a n d T e c h n o l o g y C e n t e r s u n d e r contract # DMR-91-20000 (UW, JLW).

References [ 1 ] S.N. Putilin, E.V. Antipov, O. Chimaissem and M. Marezio, Nature (London) 362 (1993) 226. [2] R.L. Meng, Y.Y. Sun, J. Kulik, Z.K. Huang, F. Chen, Y.Y. Xue and C.W. Chu, Physica C 214 (1993) 307; Z.J. Huang, Y.Y. Xue, R.L. Meng and C.W. Chu, Phys. Rev. B, to be published. [3] P.G. Radaelli, J.L. Wagner, B.A. Hunter, M.A. Beno, G.S. Knapp, J.D. Jorgensen and D.G. Hinks, Physica C 216 (1993) 29.

[4] A. Schilling, M. Cantoni, J.D. Guo and H.R. Ott, Nature (London) 363 (1993) 56. [ 5 ] L. Gao, J.Z. Huang, R.L. Meng, G. Lin, F. Chen, L. Beauvais, Y.Y. Sun, Y.Y. Xue and C.W. Chu, Physica C 213 (1993) 261. [ 6 ] J.L Wagner, B.A. Hunter, P.G. Radaelli, J.D. Jorgensen and D.G. Hinks, to be published. [ 7] J.L. Wagner, P.G. Radaelli, D.G. Hinks, J.D. Jorgensen, J.F. Mitchell, B. Dabrowski, G.S. Knapp and M.A. Beno, Physica C210 (1993) 447. [8] D.H. Kim, K.E. Gray, R.T. Kampwirth, J.C. Smith, D.S. Richardson, T.J. Marks, J.H. Kang, J. Talvacchio and M. Eddy, Physica C 177 ( 1991 ) 431. [9] U. Welp, G.W. Crabtree, J.L. Wagner, D.G. Hinks, P.G. Radaelli, J.D. Jorgensen, J.F. Mitchell and B. Dabrowski, Appl. Phys. Lett. 63 (1993) 693. [ 10] J. Schwartz, S. Nakamae, G.W. Raban Jr., J.K. Heuer, S. Wu, J.L. Wagner and D.G. Hinks, Phys. Rev. B, to be published. [11] M. Paranthaman, J.R. Thompson, Y.R. Sun and J. Brynestad, Physica C 213 (1993) 271. [12]A. Umezawa, W. Zhang, A. Gurevich, Y. Feng, E.E. Hellstrom and D.C. Larbalestier, Nature (London) 364 (1993) 129. [ 13 ] J.S. Satchell et al., Nature (London) 334 ( 1988 ) 33 l; J.D. Hettinger et al., Phys. Rev. Lett. 62 (1989) 2044. [ 14 ] J.A. Lewis, C.E. Platt, M. Wegmann, M. Teepe, J.L. Wagner and D.G. Hinks, Phys. Rev. B 48 (1993) 7739.