Comparison of fast-neutron irradiation effects in YBa2Cu3O7−x (123) and YBa2Cu4O8 (124) ceramics

PflYSICA

Physica C 220 (1994) 181-186 North-Holland

Comparison of fast-neutron irradiation effects in YBa2Cu307_x ( 123 ) and YBazCu408 (124) ceramics A. Wisniewski a, G. Brandst~itter b, C. Czurda b, H.W. Weber b, A. Morawski c and T. Lada c a Institute of Physics, Polish Academy of Sciences, PL-02-668 Warsaw, Poland b Atominstitut der Osterreichischen Universitiiten, A-1020 Wien, Austria ¢ High Pressure Research Center, Polish Academy of Sciences, PL-OI-142 Warsaw, Poland

Received 5 November 1993

A comparison of fast-neutron irradiation effects on the irreversibility line (IL) and the intragrain critical current densities (J¢) in the 90 K and the 60 K phase of YBa2Cu3OT_x ( 123 ) and in YBazCu4Os (124) ceramics is presented. The 124 compound shows a transition into the reversible regime at distinctly lower temperatures and fields than the 90 K phase of 123. However, the IL of 124 lies above the one of the 60 K phase. Upon irradiation we observe a pronounced shift of the position of the IL to higher fields and temperatures for the 60 K phase of 123 and for the 124 compound. The IL remains almost unchanged in the 90 K phase of 123 at the relatively low fluences employed so far. We also observe an increase of J¢ after irradiation. The values of the Jc enhancement as well as their temperature and field dependence vary considerably among these compounds.

1. Introduction Careful e x a m i n a t i o n o f the p r e s s u r e - t e m p e r a t u r e - s t o i c h i o m e t r y phase d i a g r a m o f the YBaCuO system has shown [1 ] that, in a d d i t i o n to the best known compound, YBa2Cu3OT_x ( 123 ) with Tc = 92 K for x < 0.2, two other c o m p o s i t i o n s exist, namely YBa2Cu408 (124) with Tc = 80 K, and Y2Ba4Cu7015+x (247) with 14 K < T ~ < 9 4 K. The crystallographic structure o f 124 is closely related to the one o f 123. The essential difference is that the 124 structure contains double chains o f C u - O , whereas the 123 structure contains only a single chain. This leads to the following differences between these two compounds. The c-axis o f the 124 c o m p o u n d ( c = 2 . 7 2 4 n m ) is much longer than in 123 ( c = 1 . 1 6 7 n m ) and the oxygen content in 124 is thermally stable up to very high temperatures (850 °C). As a consequence, single crystals o f 124 do not have twins in the a - b plane. Studies o f the lower critical field H~l have shown that Hci in 124 is three times lower than in 123 [2] and that the upper critical field H~2 is lower [ 3] as well. Hence, the coherence length in the a - b plane ~ a b ( 0 ) ~ 4 nm is about three times larger for the 124 c o m p o u n d than for 123.

On the other hand, the essential role o f the oxygen content for the superconducting properties o f 123 c o m p o u n d s has been established. The decrease o f Tc from 92 K to 60 K is due to a transfer o f negative charge ( a p p r o x i m a t e l y 0.03 e - / C u ) into the planes. Then, after an additional transfer o f 0.05 e - / C u , superconductivity disappears [4]. The changes o f the oxygen content affect not only To, but also other parameters o f the system. Since, e.g., the upper critical field decreases as oxygen is depleted, the value o f the coherence length increases. In the case o f the 90 K phase m a n y experiments show ~ b ( 0 ) ~ 1.5 nm; for the 60 K phase ~ab(0) ~ 2 n m was reported [5]. N e u t r o n irradiation is a very useful tool for introducing defects into a material in a controlled way. I r r a d i a t i o n leads to significant enhancements o f the critical current density (Jc), modifies the irreversibility line ( I L ) and the activation energy, but also affects the critical temperature. In the case o f reactor neutron irradiation, neutron energies E ranging from m e V to several MeV are available. A large fraction o f the p r i m a r y knock-on atoms have energies o f the order o f tens o f k e V and, hence, produces defect cascades. Transmission electron microscopy [6] has shown that these cascades consist of highly disor-

0921-4534/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved. SSDI 0 9 2 1 - 4 5 3 4 ( 9 3 ) E 1 0 4 6 - 9

182

A. Wisntewski eta/. /Fast-neutron zrradiamm ~!:]i'c:~

dered or a m o r p h o u s material. In the case o f the 90 K phase o f 123, the diameters of these cascades vary between 1 and 5 nm. The mean d i a m e t e r is 2.5 nm. The surrounding strain field has a p p r o x i m a t e l y the same size. Hence, the majority of defects extends over a spherical volume o f about 5-6 nm in diameter. This size is c o m p a r a b l e to the coherence length in the a b plane o f the Y B C O c o m p o u n d s at 77 K. Cascades are the most effective pinning centers at temperatures for which their sizes match the d i a m e t e r o f the normal core o f the vortex ( ~ 2~) and it can be expected that differences in the value o f ¢~will be relevant for the discussion o f fast-neutron irradiation effects. Lower energy recoils produce clusters of defects with smaller sizes ( 1 nm and less). These may act as pinning centers in a way similar to the cascades. One expects that they are more efficient for currents flowing in the c-direction, since the coherence length in this direction is o f the order o f 0.5 nm. Recoils with the lowest energies produce Frenkel pairs, i.e. pairs of a vacancy and an interstitial atom. In this p a p e r we will report on results o f fast-neutron irradiation effects in 124 ceramics and c o m p a r e them with i r r a d i a t i o n effects in the 123 c o m p o u n d s . Parts o f the data have been published previously [7,81.

signal Z" was used tbr the definition o f T,,.,. because o f convenience (the out-of-phase signal peaks when the AC flux penetrates the sample completely ). Both techniques lead to quite similar results (fig. t). In the case o f the 90 K and the 60 K phases of 123. the IL's were d e t e r m i n e d from susceptibility measurements ( p = 2 1 Hz, AC a m p l i t u d e 0.1 m T ) in fields up to 8 T. The values of the irreversibility temperature were defined as the zero intercept of a linear extrapolation of Z'- The Z" c o m p o n e n t was also rccorded, but showed a strong broadening o f the peak in Z" after irradiation, especially at higher fields. which is related to the damage o f the weak links by radiation. This effect was not so strong in the case of the 124 samples. A comparison o f the position o f the IL's d e t e r m i n e d from the real and from the imaginary part of the AC susceptibility shows that the values of T,~,. inferred from Z' are slightly larger than those inferred from Z". This difference is more pronounced at higher fields and a m o u n t s to about 6% o f T,~ at 8 T. The intragrain .It was d e t e r m i n e d from magnetization curves measured in a vibrating sample m a g n e t o m e t e r ( V S M ) at fixed temperatures in fields up to 6.5 T. The critical t e m p e r a t u r e was det e r m i n e d from AC measurements and defined as the intersection o f tangents constructed on the Z' curve near the onset of superconductivity (in zero external magnetic field ).

2. Experimental 124 ceramic samples p r e p a r e d by the pyrolithic m e t h o d were investigated. X-ray diffraction confirmed that all the samples were single phase, the high quality o f the samples is further d e m o n s t r a t e d by a sharp superconducting transition at T~,=80 K. The samples were subjected to neutron irradiation al room t e m p e r a t u r e up to fluences o f 4.7X 102o, 1.3X 1021 and 5.5X 102~ m -2 ( E > 0 . 1 M e V ) . Samples o f the 90 K and 60 K phases o f 123 were subjected to fluences o f 7 . 7 X 1 0 ~°, 1.7X102t and 2.3X 1021 m - 2 ( E > 0 . 1 M e V ) . In the case o f the 124 samples, the intragrain J~ and the position o f the IL were d e t e r m i n e d from magnetization curves measured in a S Q U I D m a g n e t o m e t e r at fixed temperatures in fields up to 8 T. The IL's were also determined from low-frequency AC susceptibility measurements ( u = 9 Hz, AC a m p l i t u d e 0.1 roT) in fields up to 15 T. The m a x i m u m o f the out-of-phase

i

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40

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A. Wisniewski et al. ~Fast-neutron irradiation effects

3. Results

The changes of T¢ with neutron fluence are found to be only moderate in all samples. For the 124 samples, Tc was 80.0, 79.4, 78.9 and 76.5 K for fluences of 0, 4.7X102°, 1.3×1021 and 5.5×1021 m -2, respectively. For the 90 K phase, Tc was 92.0, 91.7, 91.5, 91.5 K and for the 60 K phase, Tc was 60.2, 60.0, 59.4, 58.8 K for fluences of 0, 7.7×102°, 1.7X 10 z~ and 2.3× 1021 m -z, respectively (fig. 2). The positions of the IL's, determined from the AC susceptibility measurements, before and after irradiation to a fluence of 1.3× 1021 m -2 (124 sample) and of 1.7 X 10 zl m - z (90 K and 60 K phases of 123), are shown in fig. 3. The 124 compound reaches the 95

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2 3 4 fluence (10 21 m -2)

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Fig. 2. Transition temperatures as a function of neutron fluence.

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T/T Fig. 3. IL's of the unirradiated and irradiated 124 and of the 60 K and the 90 K phase of 123.

183

reversible regime at distinctly lower temperatures and fields than the 90 K phase of 123, but the IL of 124 lies above the line of the 60 K phase. After irradiation, we observe the following changes of the IL's. For 124, the IL's shift with fluence systematically to higher fields and temperatures. For the 60 K phase, we also observe a shift of the IL to higher fields and temperatures, but almost no change with fluence. On the other hand, the IL in the 90 K phase remains almost unchanged at these relatively low fluences. With regard to the temperature dependence of the IL, the data have been analyzed in terms of the thermally activated flux-creep model, according to which the depinning fields are proportional to ( 1 - T / T c ) ~ with ot=l.5 [9]. For the 124 sample the exponent a = 1.76 _+0.07 decreases systematically with fluence to 1.74_+0.06, 1.6_+0.05 and 1.49+0.04. This decrease might be related to a gradual crossover from a quasi-2D to a 3D superconducting state. For the 60 K phase, the exponent a changes from 1.8 + 0.15 for the unirradiated sample to 1.45 _+0.1 for the irradiated samples. In the case of the 90 K phase, the exponent a is found to be 1.4 + 0.1 before as well as after irradiation. The influence of irradiation on the intragrain critical currents J¢ was inferred from M - H hysteresis loops. For 124, the measurements were done at 5, 10, 20, 30, 40, 50, 60, and 70 K, for 123 at 4.2, 10, 20, 30, 40 K and at 4.2, 20, 40, 60, 77 K for the 60 K and the 90 K phase, respectively. In all cases, an accurate determination of the absolute values of Jc is difficult, because detailed information on the sizes and shapes of the grains is not available. However, with regard to the irradiation effects on Jc, the widths of the hysteresis loops before and after irradiation can be compared, which are proportional to the enhancement factor of J¢, i.e. Jcirr /J.cunirr. Enhancements Of Jc were found after irradiation for all compounds. However, the values of the Jc enhancement, their temperature and field dependence are quite different. For the 124 samples the enhancement factor of Jc depends on fluence, temperature, and field and varies between a minimum value of 2 and a maximum value of 7.5 (fig. 4). At low temperatures, e.g. at 5 and 10 K, the highest enhancement (factor of 5) is observed at 3 T. With increasing temperature the highest enhancement shifts to lower fields, e.g. to 1 T at 40 K (factor of 7.5 ) and to 0.5 T at 60 K (factor

A. Wisniewski el a/. / t:ast-neutron ~rradiatum
184

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_=k :1 [ "-

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.

( 1)

Only at the highest temperatures (close to the IL) a more rapid decrease of J~ occurs. A comparison of the J~(T, B = c o n s t ) lines shows a slight increase of the slopes with increasing fluence. This indicates that the radiation-induced defects are more effective pin-

.

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of 4). The m a x i m u m occurs in all cases at temperatures below 40 K, i.e. at relatively low temperatures. The field dependence of the e n h a n c e m e n t factors at T = 5 , 40 and 60 K is shown in fig. 5 for different fluences. The values of J,, were d e t e r m i n e d using a simple isotropic Bean model (fig. 6). We find that .1~ increases systematically with fluence only at the lowest temperature ( 5 K), For temperatures above I 0 K, ,l~ increases at first, but decreases at the highest fluence (5.5X 102~ m 2). We observe the following radiation-induced changes in the J~(B, T = c o n s t ) dependence. J~ of the unirradiated sample drops sharply at low fields ( < 1 T ) and further decreases only slightly'. This p r o n o u n c e d drop at low fields is absent after irradiation. Furthermore, the decrease of J,.(B, T = c o n s t ) is similar over a wide range of fields and can be described by an exponential law,

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ning centers at higher temperatures than the ones exisling in the unirradiated sample. The following results were obtained for lhc 123 compounds. For the 90 K phase the increase of,I, is very pronounced (up to a factor of 36 at 77 K and I T) and particularly large at higher temperatures and lower fields. The e n h a n c e m e n t factor increases systematically with temperature and a m o u n t s to 3, 10 and 36 at 4.2. 40 and 77 K, respectively', at I T, For higher magnetic fields, the e n h a n c e m e n t factors are smaller, i.e. 3, 6 and 16 at 4.2, 40 and 77 K. respectively, at 3 T. For the 60 K phase the increase of J¢ is rather moderate (factors of 1.5 to 5 ) and onl.~ weakly temperature and field dependent. Finally, the influence of irradiation on the value of the penetration depth 2 was investigated for the 124 c o m p o u n d using the method proposed in ref. [10]. Accordingly, the penetration depth was deter-

A. Wisniewski et al. ~Fast-neutron irradiation effects

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185

value obtained for single crystals, 2ab(O)=180 nm [3 ]. 2 increases with fluence and reaches 200 nm at the highest fluence.

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.3.102 m-2 4. Discussion

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g0H (T) Fig. 6. Comparison of J¢(#oH) for different fiuences at 5 and 40 K (124).

mined from the M(H) isotherms at high fields, where M is reversible and linear in the logarithm of the field H. For ceramic samples, an average value of 2, 5, can be calculated from

22=Oog(y )d(ln H) / ( 64n2dM) ,

(2)

where the function g(y) is given by

g(y) = ~

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'

(3)

and y is the anisotropy factor, y = 10 for 124 [ 3 ]. To determine the value of the penetration depth at 0 K, a two-fluid-like equation was used: ).(T) =).(0) ( 1 - (T/T~) 4) -o.s.

(4)

The resulting penetration depth in the unirradiated state is 5 ( 0 ) = 165 nm, which is close to the

One of the most prominent features of this study is the considerable variation in the temperature dependence of the Jc enhancement factor in the different compounds. To explain this behavior, we argue along the following line. For the 90 K phase of the 123 compound, the flux-line diameter at 77 K, 2~ab(77 K) ~ 5.6 nm, is very close to the average cascade size. The coherence length of the 124 compound is almost three times larger than the one of the 90 K phase. Hence, the average cascade size matches the flux-line diameter at lower temperatures. A more accurate estimate of this temperature is not possible at present, since no studies of defect sizes caused by fast neutrons are available for 124 compounds. To explain the differences between the 90 K and the 60 K phase of 123, we assume that the cascade size increases with decreasing oxygen content. Although there is no direct experimental evidence (TEM investigations) for this behavior in neutron-irradiated 123 samples, heavy-ion irradiation of 123 has shown [ 11 ], that the average diameter of the columns of amorphous material along the ion trajectories depended strongly on oxygen content. The average diameter of the columnar region was 7.8 nm for YBa2Cu306.3, 5.3 nm for Y B a 2 C u 3 0 7 _ x ( x < 0 . 1 ) and 3.3 nm for Y B a 2 C u 3 0 7 (ozone oxygenated). It can be expected that a similar trend will occur in the case of neutron irradiation. Moreover, some additional hints in this direction come from computer-simulation studies (molecular dynamics) of the collision cascade development in Y B a 2 C u 3 0 7 , YBa2Cu306.5 (60 K phase) and YBa2Cu306 [ 12 ]. These simulations were performed for a temperature of zero K and (because of computer limitations) with an energy of 150 eV for the primary knock-on atoms and are, therefore, not directly comparable with a "real" neutron-irradiation experiment. The results for the average cascade diameter are 2.7, 3.8 and 6 nm for Y B a 2 C u 3 0 7 , YBa2Cu306.5 and Y B a 2 C u 3 0 6 , respectively. Based on these results the observed differences in the temper-

186

A. Wisniewski et al. ~Fast-neutron irradiatton (~{~bc[~

ature dependence of the J~ enhancement factors could be explained as follows. A continuous increase of the J~ enhancement with temperature in the 90 K phase indicates that the number of cascades, which become effective pinning centers, increases. The maximum of the distribution of the effective cascade sizes (cascade diameter plus surrounding strain field of approximately the same size) matches the diameter of the vortex at higher temperatures, i.e. at 77 K. Therefore, the enhancement factor is largest at this temperature. For the 60 K phase we observe a moderate, in fact almost temperature-independent (4.2 to 40 K) enhancement of J~. This indicates that the number of cascades which become effective is almost the same in this temperature range. We assume that the maximum of the cascade sizes distribution for the 60 K phase is shifted to distinctly larger values (in comparison with the distribution in the 90 K phase). In the temperature range from 4.2 to 40 K, the vortex diameter is significantly smaller than the mean cascade size. Hence, only a small fraction of the cascades is really effective and the enhancement of J~ is moderate. On the other hand, the different behavior of the IL's in the 90 K and 60 K phases could also be well explained by the larger sizes of the defects and the corresponding larger values of the activation energies of the defects producted by neutrons in the 60 K phase, the pronounced shift of the IL in the 60 K phase indicates that the pinning centers introduced by the irradiation are able to keep the vortex lattice pinned at higher temperatures and fields than those existing in the material before irradiation. The assumption that the cascade size depends on the oxygen content, is also supported by the well known fact that an amorphization of the grains starts from the boundaries (oxygen-deficient regions) in 123 ceramics. Finally, we wish to point out that the maximum achievable J~ enhancement occurs for the 124 compound at a fluence somewhere between 1.3 and

5.5X 1021 m -e, i.e. at rather low fluence levels in comparison to results on 123 compounds.

Acknowledgements This work has been supported in part by Fonds zur F6rderung der Wissenschafllichen Forschung, Wien, under contract No. 8849 and by the Polish Goverment Agency KBN under contract No. 2 0484 91 01. Technical support by H. Niedermaier and valuable discussions with R.M. Schalk and M. Reissner are gratefully acknowledged.

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