Physica C 191 ( 1992 ) 199-204 North-Holland
Defect size dependence of critical current density enhancement for irradiated YBa2Cu3OT_ J. Bechtold, Y.Y. Xue, Z.J. Huang, E.V. Hungerford, P.H. Hor and C.W. C h u Department of Physics and Texas Center for Superconductivit7 at the University of Houston, Houston 7 X 77204.5932, USA
X.K. M a r u y a m a , H. Backe J, F.R. Buskirk, S.M. C o n n o r s and D.D. Snyder Department of Physics. Naval Postgraduate School, Monterey, CA 93943, USA
Y.C. Jean Department of ChemistrT, University of Missouri, Kansas City MO 64110, USA
J.W. F a r m e r Research Reactor, UniversiO,of Missouri at Columbia, Columbia, MO 65211, USA Received 30 September 199 ! Revised manuscript received 19 November 1991
The effects of 100 MeV electron (to 3.6 X 10 ~a c m - 2 ) and fast neutron irradiation (to 4.0 x I 0 ~a c m - 2 ) on the superconducting properties of melt-textured YBa2Cu3OT_ ~ is studied experimentally. By comparing the critical current enhancement effects normalized to the total atomic displacement damage, it is found that the enhancement depends heavily on the type and energy of radiation and on the beam direction with respect to the crystal. Fast neutron irradiation is more effective than 100 McV electron irradiation (along the c-axis) in enhancing the critical current, and electron irradiation along the c-axis is more effective than electron irradiation along the a, b-plane. A model based on defect size is proposed to interpret the effects.
1. Imrodnetion One of the critical frontiers for practical use of high temperature superconductors is the effective introduction of artificial pinning centers to increase the current carrying capacity. Based on the pinning models and the measured coherence lengths (~) of high temperature superconductors, defects of 30 to I00 A have been commonly accepted as effective pinning centers. However, it has been suggested that much smaller and larger defects may also play an important role in enhancing J~, although efforts made to enhance Jc by introducing such defects have not been entirely successful. Civale et al. [ 1 ] proposed that point defects were responsible for the large J¢ f Permanent address: Institut • r Physik der Universit/it, Postfach 3980, W-65 Mainz 1, Germany.
enhancement of their proton-irradiated YBa2Cu3OT_ ~ (Y 123 ) single crystal. Along this line, various cation substitutions might also be expected to he beneficial. On the other hand, Murakami et al.  suggested that large Y2BaCuO5 precipitates (---lain size) are pinning centers in melt-textured Y 123. Tiny segregated second-phase particles introduced by nonstoichiometric composition would then be a good candidate for Jc enhancement. With the existence of such disparate views on the size of an effective pinning center, it is ~l~,~ . . . . . . ~,~a~L~",,~,,,~,,,-,,~,~d~°~,.~¢ the dependence of the pinning force on the size and shape of the defect is very important. Using fast neutron and electron irradiation we have produced defects with sizes ranging from point displacements to longer track-like defects ( ~ 0. ! ~m). Although irradiation by electrons [3-5] and fast neutrons [6-8] has been previously performed, we
0921-4534/92/$05.00 © 1992 Elsevier Science Publishers B.V. ,All rights reserved.
£ Bechtold et al. /Defect size dependence
present for the first time a systematic comparative study of the;,r effects on J¢. By using only the highest quality mOt-textured Y 123 samples in this study, the problems in comparing J~ enhancements (such as grain size estimation) are avoided. When different irradiation enhancements are compared by the nuclear displacement dose, rather than by fluence, it is found that more displacement energy must be deposited with electrons in order to obtain the same Jc enhancement as with fast neutrons. In other words, the damage structure produced by fast neutron irradiation is more effective in pinning the magnetic flux than the damage structure produced by electron irradiation. From comparison of the beam effectiveness the importance of defect size in flux pinning is induced. Preliminary results show that defects of 10 to 20 A and above are required for substantial increases in Jc. However, much larger track-like defects < 1 lam are counter-productive when perpendicularly oriented to the field, thus placing an upperlimit on the pinning defect size.
2. Experimental Samples of melt-textured Y123 ( ~ 1.I × 1.1 ×0.7 mm s), whose preparation is described in ref. , were dry cut with a diamond wafer blade. Magnetic characterization of these samples was performed using either a VSM or SQUID magnetometer. Only the highest quality samples, with preirradiation values of J¢ (77 K, IT) ~ 10 4 A / c m 2, were used in this study. The material was well oriented, with the c-axis parallel to within 3 degrees of the shortest dimension, as verified by XRD. Electron irradiation at 100 MeV was carried out at the Naval Postgraduate School Linac in flowing oxygen under ambient conditions. An average current of 0.5 laA was applied in a 3.5 mm F W H M beam diameter. The sample temperature during irradiatlOn
o_~l K.,3JollilO. t~l~[ t o
. . b. .b t lLt') U . . .U . l.)~.se(.1 . . . . lI C l V- l-i-l O on
input stopping power of less than 2 W on the sample and aluminum sample holder. The maximum fluence obtained was ( 3 . 6 + 0 . 6 ) × 1 0 ]~ e / c m 2. In all, exposures were obtained at 2.1× l0 ~7, t . 0 X l 0 '8. 2.6X 10 ~8 and 3.6× 10 ~8 e/cm 2. The flux at alt fluences was 2 x 1013 e / ( c m 2 s ) , except at the fluence 2 . 1 > ( 1 0 ~7 e/cm 2 in which the flux was 5 × 1 0 ~2 e/
(cm 2 s). At 1.2× 1017 e/cm 2, four samples were irradiated, each with a different angle of the electron beam to the c-axis. The angles chosen were such that the electron beam made angles of 0, 30, 60 and 90 degrees to the c-axes of the four samples, respectively. At 3.6>( 10 ~s e / c m 2 an identical experiment was carried out using two samples at each angle, excepting 30 degrees where only one sample was irradiated. Dosimetry was established by calibrating the efficiency of a secondary emission monitor with the use o f a thermoluminescence technique (CaF2). The beam diameter was determined by continuously monitoring the transition radiation with remote TV cameras throughout the irradiation. Fast neutron irradiation ( E > 0.1 MeV) was done at the University of Missouri Research Reactor (hole H I ) to a maximum fluence of 4 . 0 X l 0 Is n / c m 2. Cadmium shielding was used in all cases excepting one in which boron shielding was used. The neutron spectrum was determined in an experiment with one of the authors (JF) and L. Greenwood. The dose rate was 1.34×10 ts n (E>0.1 M e V ) / ( c m 2 s ) . The irradiation can was immersed in water during the irradiation. The sample temperature during the irradiation is estimated at 50 ° to 100°C. Due to the similar and relatively low dose rates used in the electron and fast neutron experiments (50 h and 80 h to the maximum dose, respectively ) no significant dose rate effects are expected.
3. Results and discussion
The central point of this work is to compare the effects of different types of irradiation and thus the effects of different defect sizes on the flux pinning force. For that purpose, proper measurement of the total radiation damage is critical. Of the two types of radiation damage, ionizing and nonionizing, it has been shown that ionization energy loss is not important to superconductivity [ i 0 ], excepting the case of high energy heavy ion irradiation [ 11 ]. Summers et al.  show that the decrease in Tc of Y123 is linearly proportional to the nonionizing energy loss for many different panicles and energies. In our case, where we are mainly interested in pinning effects, it is even more reasonable to consider only the nonionizing energy losses. Thus, for comparing the re-
J. Bachtold et al. / Defect size dependence
suits of different irradiation experiments, we will be concerned only with the nonionizing energy deposited into the sample. The nonionizing energy loss (in eV/g) can be calculated from [email protected]
Sd is the nonionizing stopping power (in eVcm2/g), and @t (in cm -2) is the fluence. Values of So for electrons are taken from Summers et al. [ 10]. For 100 MeV electrons So is 1.6× l02 eV cm2/g. For fast neutrons So is calculated from the formula [ 10]:
&= WE where NA is Avogadro's number, W is the molecular weight of Y 123, w~is the number of/-atoms in a unit cell, tr~ is the cross section for fast neutrons with the (th atom, and T~ is the average recoil energy of the ith atom. Using neutron cross-sections [ 12] and limiting T~ to the ionizing to nonionizing crossover energy ( ~ a t o m i c weight × l03 eV) the value of Sd obtained is 1.0X l03 eVcm2/g. The increase in Jc for 100 MeV electron and fast neutron irradiation is plotted in fig. 1 using [email protected]
the dosage parameter. J~ was calculated from M-H loops, measured with HIIc, using the modified Bean formula of Gyorgy et al. [ 12 ]. Due to the high quality material used the grain size was assumed equal to that of the sample dimensions. From the figure it can be seen that the fast neutron irradiation results in a greater increase in J¢. Another feature observable in fig. 1 is the approximately linear increase of J~ for
I !1 fast n e u t r o n ! ~ 100 MeV e I . . . . . . . .
the fast neutron irradiation followed by a saturation. This feature has also been observed in other studies such as 3 MeV proton [ 1 ], 3 MeV electron [ 5 ], and 2 MeV proton [ 13] irradiation. While there is insufficient data to make this conclusion for the 100 MeV electron irradiation, it is reasonable to conclude that a similar behavior exists. In the case of the individual pinning model a linear dependence of J¢ on dose is expected. However, the prediction of linear behavior is not restricted to individual pinning. In the 2D [ 14] and the ID [ 15] collective pinning theories, an approximately linear dose dependence of Jc is also predicted. Our goal in comparing the effectiveness of different irradiations is simplified by the presence of the nearly linear increase in Jc. To compare the beam effectiveness on an equal footing we calculate q=AjcB/ [email protected]
, where B is the magnetic field, in the linear region. This amounts to normalizing the increase in pinning force to the total atomic displacement damage. To give the electron irradiation the benefit-ofthe-doubt, q is calculated from the lowest dose point which yields the highest value ofq. Even so, q for the electron irradiation is about four times smaller than for the fast neutron irradiation. Values of q at 77 K and 1 T are shown in table 1 for our electron and fast neutron irradiation. Also included in table 1 are values of AJcB/[email protected]
calculated from refs. [ 1,4,6,7,13 ] for various irradiations of single crystal and quasisingle crystalline Y123. In choosing other work for the purposes of comparison, care has been taken to select only work on the highest quality materials so that the comparison will be valid. The effectiveness parameter 2dcB/[email protected]
similar, Table I Values o f the normalized J~ e n h a n c e m e n t and the average primary recoil energy for various beams and energies are shown for this work a n d other studies on high quality Y 123
q (Nm-3/(eVg-~)) ,~, v , T)
3 MeVe3MeVp 3.5 McV p I00 M e V e fast neutron fast neutron fast n e u t r o n
   thiswork this work  
8.4X 10 -~s 2.5× 10 -t6 7 - 4 × 1 0 -~6 2.0× 10 -~6 8. I x 1O- ~6 7.7xI0 -~ 4.6X 10 -16
6 .8×10~ 2.0~ 102 3.0X 102 2.1XlO 2 2.0 × ! 04 2.0× 104 2.0X 104
3 4 5 6 0 1 2 NONIONIZING ENERGY LOSS (1020 eV/g) Fig. I. Increase in Jc for fast n e u t r o n and 100 Me-, electron irradiation at 77 K and 1 T c o m p a r e d using the atomic d i s p l a c e m e n t d a m a g e parameter [email protected]
J. Bechtold et al. / Defect size dependence
bui not identical, to the parameter Q introduced by Kramer in the study of defect size effects in Nb metal [ i 6 l, the difference being that q is the pinning force il~crease per unit displacement energy deposited, while Q is the pinning force enhancement per defect. Due to the normalization, differences in q among different irradiation techniques are caused solely by the distribution (not the amount ) o f the induced damage. Iv fast neutron damage, for example, nearly all atomic displacements occur in cascades of energy l keV < E < 200 keV [ 17 ]. These cascades generate clusters of atomic displacements which may be interpreted as large defects. On the other hand, electrons and low energy protons which interact mainly through the long range weak Coulomb potential, predominantly produce isolated point defects. The greater effectiveness of fast neutron irradiation suggests that larger size defects are more effective in pinning the magnetic flux. Any subsequent migration of the damage structure introduced would tend to equalize, rather than extend, the differences in effectiveness of the various types of irradiation. An independent test of defect-size effects was made by irradiating with l O0 MeV electrons at four different angles to the c-axis, Measurements of J~ were performed magnetically with Hllc in all cases. The results of this experiment are shown in fig. 2. As can be seen, the optimal angle for the irradiation is along the c-axis, the results becoming progressively worse
2 x l O Iv e c m ~ ~
4x101~ e c m
as the beam approaches alignment with the a, b-plane. In apparent contradiction with this result is a similar experiment performed using low energy protons [ 18 ] where no angular dependence was observed. The short 60 ~tm penetration depth of 3.5 MeV protons may be responsible for inhibiting the effect, since this effect has been observed in 160 and 210 MeV proton irradiation [ 19 ]. Nevertheless, the number and consistency of the data points in fig. 2 suggest the validity of the result. As mentioned previously, the main defects created by 100 MeV electrons are point defects, which are small enough that they should behave isotropically to the field. Only the collisions with energy transfer > 105 eV are expected to produce long track-like defects which could behave anisotropically to the field. Due to the Coulombic cross-section, only one such high energy recoil occurs for every million point-defect-producing recoils. Since angular dependence of the J¢ enhancement is observed, these rare defects must play a disproportional role either as pinning centers when the field is parallel to the defects, or as weak links when the field is perpendicular to them. This anisotropy is strong evidence that point defects are not as effective as the larger defects created by the electron irradiation. In order to obtain a more quantitative picture of the importance of the type of damage introduced in pinning the magnetic flux, we consider the average primary knock-on recoil energy E o produced by different types of radiation. Cascades are initiated by the primary knock-on atom with total energy Ep. The size of cluster defect generated in these cascades is an increasing function of Ep. Due to the Coulomb interaction, electrons and low energy protons obey the relation: Ep ~ E d l n ( E p m a , / E d ) ,
where Ed ( ~ 20 eV ) is the minimum energy to create one displacement [ 18 ]. For the electron and proton irradiation considered here, E o averaged over collisions with Y, Ba, Cu, and O is on the order of 10 z eV. Fast neutrons, interacting through a nearly hardsphere potential  obey the relation:
0 0 ~n~le
of e ~ R,~,rr~ t o
Fig. 2. T h e d e p e n d e n c e o f J ~ e n h a n c e m e n l o n t h e angle o f t 0 0 MeV e l e c t r o n s to t h e c - a x i s o f Y 1 2 3 a l tv, o d o s e s : 2 × l 0 ~7 e / c m = a n d 4 x I (J~~"e / c m ~'.
Ep = Epma,/2
The value of Ep for fast neutrons is about 104 eV averaged over all collisions in Y 123. A plot of the normatized flux pinning force enhancement versus Ep
J. Bechtold et al. / Defect size dependence
is shown in fig. 3. Although there is a significant spread in the values, it is apparent that the enhancement is higher for higher Ep. Depending on the reliability of the 3 MeV point two conclusions can be drawn. First, we can conclude that the defects formed by fast neutron irradiation are more than 10 times more effective in enhancing the critical current per unit displacement energy than are the point defects. Second, it is apparent from fig. 3 that a threshold in effectiveness exists at an average recoil energy of approximately 200 eV. If the threshold exists, it is more likely at somewhat higher energy due to the tail in the distribution of E # above E p . The data above is taken as evidence of far greater flux pinning enhancement for larger defects. Nevertheless, the low density of cluster defects in 3 MeV proton irradiation [ 1 ] and the absence of visible defects > 15 A in TEM images from 1 MeV electron irradiated Y123 has been offered [21 ] as evidence for point defect pinning. It is therefore important to estimate the possible pinning enhancement produced by the cluster defects alone. For example, by calculating the total cross section for collisions of 100 MeV electrons with Y123 it is found that for a fluence of 3.6× 10 ~8cm -2 there are 3 × 10 t7 collisions/ cm 3 with recoils of 103 eV or greater. From TRIM91  simulations it is found that recoils of this energy produce damaged regions of 15 ik and larger.
10 1~ i
IO 10 1
l O: -'
l O "~
Fig. 3. Plot showing the relationship o f the flux p i n n i n g enhancement per unit d a m a g e AJcB/Saqbt at 77 K and 1 T, to the average primary knock-on energy Ep. Bold data points are from this work. Other data is drawn from refs. [ 1.4,6,7 and 13] a n d are identified in table 1,
The maximum pinning force per unit volume from this many defects can be estimated by [23 ]:
= (He 2 / 8 n ) n ~ 2 n ,
where Hc is the thermodynamic critical field, ~ is the coherence length, and n is the number of defects per unit volume. Taking He= 1 T, n = 3 × 1017 cm -3, and ~= 15 A, we find F v = 3 × 10 II N m -3. The measured Jc (77 K, 1 T ) = 4 . 0 × 104 A / c m 2 of the irradiated sample corresponds to a pinning force per unit volume F ~ = J ~ B = 4 × l0 s N m - 3 . Thus, the number of pins with size 15 A, and above is more than sufficient to produce the measured Jc values. It should be noted that a typical damaged region of 15 A, as created above may not be visible by TEM [24 ], under various conditions of sample thickness and defocus values. In fact, no defects were observed by TEM in four samples of Y 123 irradiated with 100 MeV electrons to a fluence of 3.6× 1018 c m - 2 . The existence of a defect-size window for effective flux pinning is consistent with other experimental observations. The lack of significant enhancement of J¢ in chemical substitution experiments supports the conclusion that point-like defects are not effective [25 ]. Furthermore, the angular data shows that when the defect is too large in the direction of vortex motion it can act as a weak link. This implies that large scale chemical impurities such as Y21 l precipitates may not be viable flux pins. Finally, fast neutron irradiation is expected to be far superior to charged particle irradiation for increasing transport Jr since the percentage of mobile point defects, which can migrate to grain boundaries, is much lower. This seems to be the case based on fast neutron [ 26 ], proton [27 ], and electron [28 ], irradiation of thin films, as well as our bulk electron and fast neutron irradiation. In summary, it has been observed that enhancement of Jr strongly depends on the type and energy of radiation and on the beam direction. The dependence of the normalized pinning . . . . . . . L. . . . . . . . (q) on the average primary knock-on energy suggests a threshold defect-size for effective flux pinning exists. The angular dependence of the critical current enhancement by the electron irradiation reinforces the interpretation of a strong defect size dependence in critical current enhancements. These results indicate that the elementary pinning force created by
J. Bechtold et al. / Defect size dependence
detectl in the high temperature superconductors is size -.iependent as has been shown for conventional 'mperconductors. We are currently planning a 3 MeV electron irradiation of our high quality melt-texture Y123 material to confirm its low effectiveness q.
Acknowledgements We would like to acknowledge R.L. Meng for the preparation of the melt-textured Y123, D. Ramirez for assistance in the electron irradiation, and H.W. Zandbergen, J. Kulik, K.B. Ma and W.K. Chu for beneficial discussions. This work is supported in part by the NSF Low Temperature Physics Program Grant No. DMR 86-126539, DARPA Grant No. MDA 97288-G-002, NASA Grant No. NAGW-977, Texas Center for Superconductivity at the University of Houston, and the T.L.L. Temple foundation. One of the authors, JWF, would like to acknowledge MURR support through grant number DE-FG0290ER45427. The work at NPS is sponsored by tlae NSWC and the Naval Postgraduate School.
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