Magnetic irreversibility limits of the Abrikosov and Josephson-flux dynamics in doped YBCO-123 superconductors

Physica C 354 (2001) 299±303

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Magnetic irreversibility limits of the Abrikosov and Josephson-¯ux dynamics in doped YBCO-123 superconductors V.N. Vieira *, P. Pureur, J. Schaf Instituto de Fõsica, Univ. Fed. do Rio Grande do Sul, UFRGS-Sala M204, Av. Bento Goncßalves 9500, Campus do Vale, Agronomia, CEP 91501-9700 Porto Alegre, RS, Brazil

Abstract The granular superconducting character and the magnetic irreversibility of YBa1:75 Sr0:25 Cu3 O7 d single crystal and oxygen depleted polycrystalline samples are studied in great detail from low ®eld magnetoresistance and zero-®eldcooled and ®eld-cooled DC magnetization as a function of temperature, for applied ®elds from 3 Oe up to 50 kOe. The results are discussed together with those of a very pure and high quality YBCO-123 single crystal. Coarsely the pro®les of the irreversibility curves are all similar, but the irreversibility data of the granular superconductors develop in the low ®eld region a structure, that systematically falls o€ the general trend and is recognized as the signature of the intergrain Josephson-¯ux dynamics, that is strongly marked by the frustrated grain coupling physics. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Magnetic irreversibility; Flux dynamics; Granular superconductors

The granular superconducting character of the high-Tc oxides has been evidenced in practically all but the best single crystal oxide superconductors [1,2]. This granular superconducting character has a striking e€ect on the magnetic properties because two kinds of magnetic ¯uxons penetrate the sample: Josephson and Abrikosov ¯uxons. However, the activation temperature and the critical ®eld for penetration of Josephson ¯uxons into the intergranular spaces (HclJ ) are much lower than for penetration of of the Abrikosov ¯uxons into the

*

Corresponding author. Fax: +55-51-319-1762. E-mail address: [email protected] (V.N. Vieira).

grains themselves (Hc1g ). Therefore in low ®elds ¯ux dynamics is due to the intergrain Josephson¯ux only [3] and thus is strongly marked by the physics of the frustrated grain couplings, that is very sensitive to the applied ®eld [4]. The magnetic irreversibility in the low ®eld region may thus provide much information on that frustration physics [5]. In high ®elds, however, the intragrain Abrikosov-¯ux dynamics, governed by the more conventional ¯ux-melting and ¯ux-creep physics, may predominate. In nearly perfect single crystals, where the superconducting state is almost homogeneous, the lines of ¯ux melting (TM (H)), of zero resistivity (T0 (H)) and of magnetic irreversibility (Tirr (H)) tend to lie all closely together in the H±T plane [2]. The ¯ux dynamics in such single crystals ®ts into

0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 1 ) 0 0 0 9 6 - X

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the conventional ¯ux-creep physics, within the scenario of ``giant-¯ux creep'' (GFC), leading to a very simple power law for the magnetic irreversibility as a function of applied ®eld: H ˆ H0 …1 t†3=2 , where t ˆ Tirr …H †=Tirr …0† and H0 as well as Tirr (0) are ®tting parameters [6]. However, the conventional ¯ux-creep mechanism is certainly inadequate to describe the ¯ux dynamics of granular oxide superconductors in the low ®eld region. Granularity is in general di€erent from polycrystallinity. As the coherence length of the superconducting order parameter is very short, any lattice defect, due to impurities, oxygen holes or twinning planes contribute a granular character. Even imperfect single crystals exhibit a granular superconducting character. It is known since long time that the behavior of the Tirr data of granular oxide superconductors, in low applied ®elds, totally resembles that of the Tirr (H) lines of spin glasses [7]. This is why M uller and coworkers [1] ®tted the Tirr (H) data of LaBaCuO by a de Almeida±Thouless (AT) like line [8] ‰H ˆ H0 …1 t†3=2 Š, that is mathematically identical to the GFC line, but results from mean ®eld calculations for a frustrated Ising spin systems. Subsequent Tirr (H) data of polycrystalline YBCO-123 [5] and BSCCO-2223 [9] and other oxide superconductors were found to systematically deviate from the AT-like line above a crossover ®eld of about 0.5 kOe. In some cases the data could be well ®tted by Gabay±Toulouse (GT) like 1=2 [10] lines ‰H ˆ H0 …1 t† Š, that di€ers from the AT or GFC power law by only the exponent a ˆ 1=2 and was originally obtained from mean ®eld calculations for frustrated XY or Heisenberg spins. These behaviors of Tirr (H) deepen the analogy between the granular oxide superconductors and the spin glasses, where the AT and GT like Tirr (H) lines, respectively below and above a crossover ®eld are longly known to occur [7]. In spin glasses the dimensionality crossover (Ising-XY or IsingHeisenberg) of the impurity spins occurs when the applied ®eld collapses the random local anisotropy ®elds [11]. The remarkable resemblance between Tirr (H ) of the oxide superconductors and the spin glasses can only mean that the essence of the frustration physics, that a€ects both systems,

belongs to a universality class and does not depend on the particular interacting objects. Regardless the large survey of Tirr (H) data, the origin of the magnetic irreversibility of the oxide superconductors is still matter of strong controversy. Some groups see in it the signature of a genuine phase transition of an inhomogeneous, disordered and frustrated system, [1,2,5,9] others view it as a conventional ¯ux-creep phenomenon [6]. Apparently the controversy persists because most Tirr (H) data are too sketchy and too restricted in ®eld and the sample quality also plays a fundamental role. With the purpose of solving the old controversy, we have performed very detailed and precise determination of the magnetic irreversibility of two YBa1:75 Sr0:25 Cu3 O7 d samples, a single crystal with an intermediate granular character and an oxygen depleted and strongly granular polycrystalline sample known from previous works to have well di€erent superconducting granularities [12,13]. The irreversibility data of these samples are compared with those of a pure and high quality YBa2 Cu3 O7 d single crystal with vanishing granularity. Although the granularity of our samples is caused by Sr impurities, oxygen vacancies, twinning planes, etc., it seems that granularity is not at all speci®c. The samples were prepared and analyzed by the usual well known methods. The electric resistivity measurements were performed with a very precise conventional AC zero technique, having a sensitivity of 10 5 X and temperature resolution of 10 3 K and the DC magnetization measurements were performed with a SQUID-MPMS-XL magnetometer from Quantum Design. The resistive transition q(T) of a granular superconductor occurs in two steps, the temperature derivative dq/dT normally showing a sharp peak and a hump or shoulder at its lower temperature side. While the peak marks the superconducting transition within the grains, the hump is due to the phase coupling of the Gisnburg±Landau (GL) order parameter between the superconducting grains. The weak links, that connect the superconducting grains, are very fragile and thus an applied ®eld weakens the coupling between the grains [4] while letting almost intact the super-

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Fig. 1. The magnetoresistive transition and temperature derivatives of the indicated samples and ®elds.

conducting transition within the grains. These effects strongly extend the lower temperature foot of the resistive transition and increase the hump in the temperature derivative, while the peak of the intragrain transition remains sharp. Fig. 1 displays the resistive transitions of the pure single crystal and of our doped polycrystalline sample before oxygen depletion. While the resistive transition of the former occurs in only one step its temperature derivative showing only a narrow peak, the resistive transition of the polycrystalline sample has two intragranular steps plus a foot, due to grain coupling and its dq/dT displays two sharp peaks (88.7 and 89.8 K) plus a hump. The e€ect of ®eld shows that the grain coupling is much more sensitive than the bulk transition. Not rarely high-Tc oxide superconductors exhibit a double Tc , that is apparently related to the presence of large irregular crystallite grains imbedded in a large number of much smaller ellipsoidal grains that form during sintering at somewhat too high temperatures. After oxygen depletion to

7 d  6:85, the sample still showed double but more spaced Tc , one at 80.5 K and the other at 83.5 K and its granularity was considerably higher. In reality the occurrence of the double Tc represents here no trouble at all, as Tirr occurs at considerably lower temperatures. DC magnetization measurements were performed while slowly warming the samples (0.1 K/ min) after zero-®eld cooling (MZFC (T)) and subsequent ®eld cooling MFC (T). The irreversibility limit is the temperature point where the di€erence data, MD …T † ˆ MFC …T † MZFC …T † leave the zero base line, de®ned by the high temperature data. We determined Tirr as a function of applied ®eld from 3 Oe to 50 kOe. Plotting the irreversibility limits for the whole ®eld range in a H±T diagram, de®nes the irreversibility line Tirr (H) of the sample. The exact behavior of Tirr (H) provides much information on the physical mechanism causing the magnetic irreversibility. The H±T diagram of Fig. 2 shows the magnetic irreversibility data of the polycrystalline sample

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Fig. 2. Tirr (H) before and after oxygen depletion the continuous lines are ®ttings.

before and after oxygen depletion. The continuous line through the high ®eld data is a ®tting with a GFC line and the continuous lines through the low ®eld data are AT like lines. Dotted lines are guides to the eye. The result shows that our oxygen depletion was not restricted to grain boundaries, but a€ected the bulk as well. The change in the slope of the low ®eld curves clearly shows that the superconducting granularity has increased. Fig. 3, displays the Tirr data of our granular samples together with those of the pure YBCO-123

single crystal, for ®elds applied along the ^c axis in the case of the single crystals. From the data of the doped single crystal we see that substitution of Ba by Sr lowers the superconducting transition temperature Tc and the Tirr (H) line, without any more drastic changes in its overall trend. It also introduces a granular character, that becomes evident from the decreased slope of Tirr (H), as well as from the structure, that arises in the low ®eld data. The overall trend of the Tirr data of the oxygen depleted polycrystalline sample in Fig. 3 also does not di€er drastically from those of the other samples, excepting for a much smaller slope and the reinforced low ®eld structure. The deviation of the low ®eld data from the general trend, de®ned by the data in the major high ®eld region, is the e€ect of the granular character, caused by the Sr doping and oxygen depletion and deserves our special attention. The magnetic irreversibility as a function of the applied ®eld is usually expressed by simple power laws. The continuous lines through the main data in Fig. 3 are ®ttings with GFC line. This power law, that ®ts well the data of the pure YBaCuO single crystal in the whole ®eld range for a ˆ 1:50  0:05, H0 ˆ 820:5 kOe and Tirr …0† ˆ 92:7 K, can ®t well only the high ®eld data of the granular samples. The Tirr (H) data of the doped single crystal, above 5 kOe, are well ®tted by this power law for a ˆ 1:50  0:09, H0 ˆ 590:27 kOe and Tirr …0† ˆ 88:5 K, while those of the oxygen depleted polycrystalline sample can be well ®tted only above 10 kOe for a ˆ 1:51  0:1, H0 ˆ 173:8 kOe and Tirr …0† ˆ 81:5 K.

Fig. 3. The Tirr (H) data of all samples. The continuous lines are ®ttings with the power law H ˆ H0 …1

t†3=2 .

V.N. Vieira et al. / Physica C 354 (2001) 299±303

While the high ®eld data of the pure single crystal can be well ®tted by a GFC line in the whole ®eld region, this is impossible in the case of the granular samples, where the low ®eld data fall systematically o€ those ®ttings. This is clear from Figs. 3 and 4. Besides the high ®eld regime, each granular sample exhibits two quite di€erent low ®eld regimes. In the very lowest ®elds the data curves of the doped single crystal and of the polycrystalline sample are well represented by AT like lines [12], for respectively a ˆ 1:57  0:12, H0 ˆ 831:8 kOe and Tirr …0† ˆ 87:9 K and a ˆ 1:51  0:05, H0 ˆ 57:4 kOe and Tirr …0† ˆ 81:4 K. Above a crossover, near to 1 kOe the behavior of Tirr (H) suddenly changes over to a GT like line [14], respectively for a ˆ 0:52  0:07, H0 ˆ 36:6 kOe and Tirr …0† ˆ 87:1 K up to 4.5 kOe and for a ˆ 0:50  0:05, H0 ˆ 30:3 kOe and Tirr …0† ˆ 75:7 K up to 9 kOe.

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We attribute the high ®eld behavior of our Tirr (H) data to the intragrain Abrikosov-¯ux dynamics, while the AT and GT like behaviors in the low ®eld region are the signature of the Josephson¯ux dynamics of a granular and frustrated superconductor. These AT and GT-like magnetic irreversibility limits, con®rm several previous but ®eld restricted observations [1,8,13] and are a quasi-perfect analogy to the magnetic irreversibility of the spin-glass systems. They corroborate thoroughly the idea that the frustration physics has a universal character, that does not depend on the particular physical system involved. Our results clearly prove the existence of an intergrain Josephson-¯ux dynamics, that predominates in the low ®eld region of granular superconductors and bears the physics of frustrated grain couplings and also delimitates place for the intragranular Abrikosov-¯ux dynamics. This in part elucidates the old controversy about the origin of the magnetic irreversibility of oxide superconductors, but lets open a number of important questions, that need farther investigations before a completely acceptable outline can be given.

Acknowledgements The authors acknowledge the Brazilian agencies CNPq and the PRONEX program.

References

Fig. 4. The extended view of the low ®eld details, pointing out the AT and GT-like behaviors of the Tirr (H) in (b) and (c) and their absence in (a).

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