Critical current density in quenched oxygen deficient YBa2Cu3O7−δ

PHYSICAG

Physica C 184 ( 199 ! ) 13-20 North-Holland

Critical current density in quenched oxygen deficient YBa2Cu307_a M a n f r e d Diiumling IBM T.J. Watson Research Center, Yorktown Heights, NY 10598, USA Received 3 September 1991 Revised manuscript received 30 September 199 !

The critical current density Jc was determined magnetically in pblycrystalline sintered speci mens of YBa2CusO7_a as a function of magnetic field and temperature for 0.02
1. Introduction

One of the peculiarities in the high temperature superconductor YBazCu307_~ is that very high critical current densities Jc are found magnetically in polycrystalline [ l ] as well as single crystalline specimens [2-4]. Intrinsic flux pinning on the copper oxygen planes themselves has been suggested [ 5 ] as a mechanism for high Jc values for the H Z c orientation. However, this mechanism is inoperative if HIIc. Large Jc values are found also for HIIc, even for single crystals that are free of twin boundaries [6 ], which could act as pinning center for magnetic flux lines. Microscopically, YBa2Cu3OT_,~ appears to be quite clean [ 7 ] within single grains, so that it is not obvious what could act to pin magnetic flux. Recently, variations in oxygenation treatments have been shown to have some effect on the critical current densities in both flux grown single crystals and melt processed specimens [4]. A connection was made between the anomalous excess pinning (or "fish tail") behavior and oxygen deficiencies in the s~ecimens. However, in single crystals it is rather -ifficult to ensure a homogeneous oxygen concentration, especially clove to full oxygenation. This is due to the long diffus:on distances and a small diffusion coefficient at the oxygenation temperature. In this work J~ was measured systematically as a

function of magnetic field, temperature and oxygen deficiency 8. The bulk of the experiment was carried out using open porosity ceramic materials. Here diffusion distances are short, and thus equilibration times are short. Some constant stoichiometry low temperature equilibrmions were carried out as well.

2. Experimental detai~

2. I. Specimen preparai~on Polycrystalline sintered specimens were prepared using a solid state route [ 8 ]. Jet-milled powders were pressed into pellets and then sintered in flowing oxygen at 950°C, after slowly ( 1 K / m i n ) ramping up to the sintering temperature to burn offcarbon. Two batches of ceramic were produced using the same starting powder, but sintering for different times at the same sintering temperature. The first batch had a relative density of about 90%, with a grain size of about 3 ~tm. The pores were not connected in this batch. For the second batch the final relative density was 67%, with a grain size of about 2 lain. The grain size was determined using the linear intercept method. The pores in the ceramics of this second batch are therefore interconnected, and diffusion lengths for oxygen are equal to the grain radius. The

0921-4534/91/$03.50 © 1991 Elsevier Science Publishers B.V. All rights reserved.

M. Diiumling I Jc in YBa2Cuj07_6

14

second phase content is very small, barely detectable by X-ray analysis. The second phase consists of the green phase Y:BaCuOs. Most of the measurements were carried out on the second open porosity batch, since the times to achieve equilibrium for oxygen concentrations were much smaller than in the closed porosity specimens. In order to produce specimens with varying oxygen content the sintered pellet was broken into smaller pieces, which were annealed at various temperatures up to 650°C in purified oxygen. This way the oxygen content can be related to phase diagrams measured by others [ 9 ]. The specimens were held at the annealing temperature for up to 48 h, after which they were quenched to room temperature. Two quenching procedures were used. The specimens were either quenched into liquid nitrogen, or thrown directly onto an aluminium surface. The second method was only used for very thin specimens. It should be noted that neither process is a true fast quenching process compared to rapid quenching processes like melt spinning or splat quenching. The fully oxygenated specim"n was produced by slowly cooling (1 K/rain) the sample in a furnace under flowing oxygen from 940 to 400°C, holding for 8 h at 400°C, and slow cooling to room temperature by turning off the furnace. Analysis of the oxygen content of some samples using a gas evolution method [ 10] confirmed the expected [9 ] oxygen stoichiometry J to _+0.02. Low temperature equilibrations were carried out by sealing the specimen into an evacuated quartz ampoule, which was placed into the furnace. Equilibration temperatures were chosen to be low enough that the oxygen loss due to the outgassing of the specimen into the vacuum was too small to cause a significant change in the stoichiometry of the specimen during the equilibration. After the heat treatment the ampoule was quenched into water.

keep field variations during the scan to a minimum. The field variation at 5 T is about 0.25 mT. In addition some specimens were also measured using a scan length of 3 cm. The field variation for the 3 cm scan is almost one order of magnitude larger than for the 2 cm scan. If the full penetration field h* (see ref. [ l l ] ) of the specimen was much larger than the field variation there was good agreement between the hysteresis from the 2 and 3 cm scans. Significant differences can occur when full penetration fields are small. Results of the hysteresis measurements were only used if the calculated h* from the loop exceeded the field variation at the measuring field by at least a factor of 10. For the specimens in this otudy this corresponds to a arc value of roughly 2 × l05 A / c m 2 at 5 T, or 4 × 104 A/cm 2 at 1 T for a 2 cm scan length. This procedure limits the error in J~ close to these limits to less than 20 %.

3. Results The critical temperature T= for all of the specimens is plotted versus the oxygen deficiency J in fig. 1. These T~ values were extracted from measurements of the reversible magnetization [ 12]. Tran-

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Magnetic measurements were carried out in a commercial Quantum Design SQUID magnetometer. All loops were measured using the hysteresis option. In this mode the superconducting magnet is under the control of the magnet power supply at all times. A scan length of 2 cm was used in order to

201 ......... l ......... i ......... i .........

0.0

0.1

0.2

0.3

0.4

Oxygen Deficiency 6 Fig. 1. Critical ~ ,perature T ~ L vs. oxygen deficiency J. Specimerts in b a t c h . ,~ave open porosity.

M. D~iurnling I Jc in YBa2Cu.~O,-,_,~

sition widths (10 to 90% shielding, measured with an AC inductive method) are less than 2 K for the fully oxygenated specimen, and significantly larger in oxygen deficient specimens. However, it should be noted that transition widths in polycrystalline decoupled specimens usually reflect the penetration depth/grain size ratio rather than inhomogeneities. Figure 2 depicts one half of a hysteresis loop taken on the fully oxygenated specimen. In order to achieve a fully penetrated specimen in small fields all loops were started at a negative field of equal to or larger than 2 T. The loops are not symmetric about the field axis due to the reversible diamagnetic magnetic moment of the specimens. The results of the study of the reversible magnetization in these specimens is described elsewhere [ 12 ]. The value for the critical current density Jc was calculated from the loop using the critical state model [ 11 ] for a cylindrical specimen. In this model (in Sl units) Jc=3AM/d, where AM is the total width of the hysteresis loop, and d is the grain diameter. This expression is exact for long cylindrical specimens of diameter d, but is not entirely correct for grains that are roughly spherical in shape. However, only the prefactor is affected by specimen shape and thus this error will only affect the absolute value of J¢. Comparisons between different oxygenations are not affected since the average grain shape and size are identical for all specimens of the same batch. Figures 3 and 4 show Jc values extracted from the

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eld (T) Fig. 3. J~ vs. field for specimen with 6=0.02 (batch 2 ).

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Fig. 4. J¢ vs. Eeld for specimen with 6=0.07 (batch 2 ).

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Field(T) Fig. 2. Hysteresis loop for open porosity specimen with 6=0.02 at 10K.

hysteresis loops for quenched open porosity specimens with oxygen stoichiometries 6=0.02 and 0.07. J~ values for specimens with larger oxygen deficiencies were found to be quite small. Specimens with 6>__0.31 had Jc values that were too small to be measured for T> 10 K in fields larger than 1 T. For all specimens the J~. values decrease monotonically for increasing magnetic field. The "fish tail" or excess pinning peak behavior that occurs in single crystals [2,4] and also polycrystalline specimens [ 3,13 ] is not observed. Values for J,.(H,T) for a closed porosity specimen with 6=0.07 is shown in fig. 5. Thc duration of the

M. Diiumling / Jc in YBa.,Cu.jOr_a

16

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oxygenation treatment was increased so that the average oxygen diffusion distance was larger than the thickness of the macroscopic specimen. Based on published diffusion coefficients it was decided that a 6 of 0.07 was the lowest 6 for which a homogeneous specimen could be obtained using practical annealing times (less than 1 week). This procedure should have resulted in specimens that are nominally identical to the open porosity samples. However, the specimen shows a drastically different field dependence of J~, namely the excess pinning effect. Despite a slightly lower critical temperature the absolute values of Jc are somewhat higher than in the first batch. The stoichiometry dependence of Jc at a measuring temperature of 10 K for the open porosity ceramics is replotted in more detail in figs. 6 and 7. The current density decreases monotonically as the oxygen deficiency increases. Low temperature equilibrations were carried out for the 6=0.07 specimens originally quenched from 500°C. The equilibration treatment does have effects on the Jc values. The changes in J¢ are shown for a measurement temperature of 10 K in fig. 8 (a) and (b). The effect of the equilibration is different for the two batches. The equilibration raises Jc by about a factor of two in the open porosity specimens, but decreases Jc in the closed porosity specimens. This decrease is small but consistent. The reduction

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Field (T) Fig. 6. J¢ (at 10 K) vs, field for specimens (open porosity) of different oxygen stoichiometry.

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Fig. 7. Jc (at 10 K) vs. oxygen deficiency 6 in different applied fields for open porosity specimens.

in excess pinning is therefore not dependent on a change of the oxygen stoichiometry, as was claimed previously [4].

4. Discussion

4. I. General remarks Due to the random grain orientations found in our polycrystalline specimens the measured hysteresis is an average of current densities with HIIc and H ± c .

M. Diiumling/ Jc in YBazCu.jOz_6

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Fig. 8. (a) Jc (at 10 K) vs. field for open porosity specimenswith 6=0.07, post-annealedat different temperaturesfor 2 weeks; (b) Jc vs. fieldfor closed porosity specimenwith 6=0.07, post-annealedat 200°C for 2 days. For HJ.c the critical state is actually anisotropic [ 14 ], because the current must flow parallel and perpendicular to the a,b planes. Since the grains are of roughly spherical shape, for this orientation the critical state will be dominated by currents parallel to the c-axis. In fully oxygenated single crystals [15] these Jc values are roughly 30 times smaller than J¢ values either for Hllc, or HJ_c with Jlla,b. Thus the hysteresis is expected to be reduced by a similar same factor as soon as current along the c-axis has to flow, which is the case any time there is a significant misorientation between the field and the c-axis. Therefore it is expected that in-plane currents completely dominate the magnetic hysteresis behavior. Intrinsic pinning [5] is not expected to be a factor in our measurements. Another concern is how much of the hysteresis in these specimens is being caused by currents across grain boundaries (intergranular currents) compared to currents inside of one grain (intragranular currents). Since the transport critical current density J¢, was not measured in these specimens the various contributions have to be estimated. Some of the highest Jet values have been obtained in magnetically aligned fully oxygenated sintered ceramics [16]. In these specimens Jet < 200 A/cm 2 for B> 0.1 T at 4 K. This leads to a magnetic hysteresis of approximately 500 A/m for a specimen of the size used. This is roughly 5% of the hysteresis measured at 10 K and

1 T. It should be pointed out that this is a worst case estimate for the closed porosity specimen. Firstly, Jet values are expected to be lower in randomly oriented ceramics, and secondly, Jet values decrease rapidly with increasing porosity. In the open porosity ceramic Jet is expected to be at least a factor of l 0 less as compared to maximum values, thus reducing the intergranular contribution to less than 1% of the total hysteresis.

4.2. Field and temperature dependence of J, In all high-T¢ superconductors thermally activated flux creep has been shown to influence the critical currents, especially at high temperatures. For this reason classical scaling relations [ 17 ] for the global pinning force Fp=JcB ( B = poll) are not expected to hold over an extended temperature regime since flux creep contributions are superimposed, especially at high temperature. Only the results for the open porosity specimens near ~ - 0 were examined more closely for scaling. We were unable to find a field scaling relationship of the Fietz-Webb type [18] (F,~.b'(1-b)'", with b=B/Bc:) in these specimens. However, all specimens show an exponential decrease of Fp with increasing temperature in low magnetic fields ( 1 T or below) from l0 to about 50 K. The decrease is faster than exponential for T> 50 K. Significant departures from this exponential be-

18

M. D~iumfing / J¢ in ~'Ba~CuzOz_6

havior are found for higher magnetic fields (for example 5 T). The slope d(inFp)/dT (at B=const.) does not correlate with the oxygen stoichiometry.

4.3. Dependence of J,. on stoichiometry For specimens with small ~ values (up to 0.18) the pinning force Fp (B) at low temperature ( 10, 20 and 30 K) can be collapsed to a single curve (for each temperature) by a simple numerical factor. If the Fp values for the specimen with 6=0.02 represent 1, then the specimens with 6=0.04 have a Fp value of 1/1.44, the specimen with 6=0.07 a Fp value of 1/ 2.4, and the sample with d=0.12 a Fp value of 1/4.2. This is shown graphically in fig. 9. If the reversible properties of the specimens did not change with ~ then the argument could be made that the only parameter changing is the number density of pinning centers in the specimen. This density would have to decrease with increasing d in order to match the data. However, in these specimens the reversible parameters change quite significantly. These changes have not been taken into account in a previous study [ 19 ]. Measurements of the reversible magnetization on the same specimens [ 12 ] show that the thermodynamic critical field/~V¢ ( T= O) changes 1011

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eid 0") Fig. 9. Scaled global pinning force Fp vs. magnetic field at 10, 20 and 30 for specimens of different oxygen stoichiometry. The Fp values for all specimens shown were scaled so that a single curve results at T= 10 K. The scaling factor is 1 for the specimen with ~=0.02. The inset shows the inverse scaling factor vs. thermodynamic critical field He (see text ).

from 1.5 T for the 6=0.02 specimen to 1.2 T (~=0.04), 1.07 T (6=0.07) and 0.54 T (6=0.12). The~.efore the critical field decreases roughly by a factor of 3, slightly less than the J¢ values. This correlation between H¢ and the factors needed to normalize Fp is plotted in the inset to fig. 9, together with a line of slope 1. We end up with the result that there is a close to linear scaling relationship between Fo and the critical field H~. Typically, surface currents are of order H¢/2 in zero magnetic field, and less in magnetic fields [20]. However, surface currents of this type (to be distinguished from hysteretic surface flux pinning) are such that they always cause the specimen to appear diamagnetic. The direction of this current does not depend on the history of the magnetic field change, but only on the applied field and temperature. Therefore it does not cause a hysteretic behavior of the type measured. Surface pinning due to a Bean-Livingston type surface barrier [ 17,21 ] leads to hysteretic behavior. However, the pinning force (per unit length of fluxon) is expected to scale with H¢2"5, much stronger than observed experimentally. Thus we assume that the non-zero J¢ values are a result of bulk flux pinning. However, then this scaling is an unusual result, since pinning forces (in lowT¢ superconductors) usually scale with Hi, but n > 2 [ 17], depending on the pinning mechanism. The value n is at least 2 because the condensation energy is proportional to H¢2 [22 ]. However, it should be noted that Fp is the global pinning force, which is related to the elementary pinning forcefp acting on each fluxon by possibly complicated summation rules [ 23,24 ]. Therefore in order to obtain fp, knowledge of the number and spatial distribution of the pinning centers is essential. However, usually direct summation holds for strong pinning systems [ 17,24,25 ]. If the assumption is made that the elementary pinning force fp is proportional to H¢2, then it follows that the pinning site densi:y must increase with increasing oxygen deficiency in order to obtain the measured Fp versus 6 curve, at least up to 6=0.12. Flux pinning by twin boundaries in YBa2Cu307_~ has been the subject of discussions [ 6,26,27 ]. In our specimens it is expected that the density of twin boundaries varies as the annealing temperature (and oxygen stoichiometry) varies. One report [28 ] shows a decrease in the twin spacing as the annealing tern-

M. Diiurnling / Jc in YBa2Cu30;,_6

perature is increased, and the oxygen deficiency is increased. However, Shaw et al. [29 ] report strongly different twin structures in specimens of identical oxygen content, produced by a different heat treatment. However, hysteresis measurements on a crystal in the twinned and untwinned state have shown that for the H]lc orientation the effect of the twin boundaries on the critical current density is rather small [ 6 ], certainly less than the variation in Jc "~at is shown in fig. 7. Therefore it must be concluded that a change in twin boundary density cannot explain the decrease of J¢ with increasing t$. Obviously, the number of oxygen vacancies is directly proportional to the oxygen deficiency t~, if the vacancies are distributed randomly. However, the average in-plane distance d between these vacancies is approximately a/x/~ (a is the in-plane lattice parameter). For t~= 0.05 one obtains d ~ 1.6 nm, which is much smaller than the flux line spacing ao ~ 20 nm in an applied field of B = 5 T. Therefore Fp should be rather insensitive to the number density of single oxygen vacancies, because compared to the flux line spacing they are spaced too closely together. The fact that low temperature annealing keeping constant stoichiometry has significant effects on Jc also cannot be explained by a simple oxygen vacancy pinning model, since the number of vacancies does not change during the anneal. However, the pinning sites do not have to be single oxygen vacancies [ 30]. The existence of two phase fieltis in the phase diagram for oxygen deficient YBa2Cu307_a has long been postulated [ 31,32 ]. According to calculated phase diagrams YBa2Cu307 coexists with a second phase, either the ortho II [ 31 ] or the ortho III phase [ 33 ]. Even small localized regions of ortho III would pin flux rather well, since its thermodynamic critical field H~ is small with respect to the more oxygen rich matrix [ 12], and since the coherence length is very small. Experimental electron scattering results show that an ortho III-like structure exists in all oxygen deficient YBa2Cu3OT_,~ [34,35 ]. It was also shown [34] that the microstructure u,~w,,,~,,,, "~. . . . "~"'~ on ..... ,,,ho,h,,r. .... the specimens were quenched or slowly cooled to room temperature. In a two-phase system increasing the oxygen deficiency (in an equilibrated specimen) simply increases the volume percentage of this second phase, leading to either an increased number of pinning centers, or larger pinning centers. Homo-

19

geneous (from a high temperature single phase ortho I regime quenched) oxygen deficient specimens with 0
5. Conclusions

The critical current density Jc was measured magnetically in oxygen deficient ceramics of YBa2Cu307_~ with 0.02<~<0.31. The specimens were produced by quenching. In open porosity samples Jc was highest in the near stoichiometric sampies, and decreased monotonically with increasing temperature, magnetic field and ~. Taking into account the strongly changing revers:,ble properties (in particular the thermodynamic critical field He) it is concluded that the density of pinning centers increases with increasing oxygen deficiency t~, even though the absolute magnitude of Jc decreases. Low temperature annealing of these quenched specimens (keeping the stoichiometry constant) increases crit-

20

M. Diiumling / Jc in YBa2Cu.~07_,~

ical current densities by up to a factor of two. In nominally identical closed porosity specimens excess pinning was observed. As previously observed [4], annealing at low temperature reduces the amount of excess pinning.

Acknowledgements I thank T.M. Shaw for discussions, T. McGuire for the use of the magnetometer, and M.M. Plechaty for performing the oxygen analysis. The starting powders of YBa2Cu3OT_6 were made by E. Olsson and P. Duncombe. The work was partially funded by DARPA under contract No. N00014-89-0112.

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