Critical currents in protonirradiated singlecrystal Ba2YCu3O7−δR. B. van Dover, E. M. Gyorgy, A. E. White, L. F. Schneemeyer, R. J. Felder, and J. V. Waszczak Citation: Applied Physics Letters 56, 2681 (1990); doi: 10.1063/1.103269 View online: http://dx.doi.org/10.1063/1.103269 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/56/26?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Effects of Gd 2 O 3 addition in Y Ba 2 Cu 3 O 7 − δ on the critical current density J. Appl. Phys. 103, 07C714 (2008); 10.1063/1.2839619 High criticalcurrent density of Nd(Ba,Nd)2Cu3O7−δ single crystals Appl. Phys. Lett. 67, 2406 (1995); 10.1063/1.114562 Critical current enhancement in protonirradiated Tl2CaBa2Cu2O8 films J. Appl. Phys. 69, 1119 (1991); 10.1063/1.347384 Magnetization and critical currents of BiSrCaCuO and Ba2YCu3O7 superconductors Appl. Phys. Lett. 53, 2223 (1988); 10.1063/1.100508 Stress and field dependence of critical current in Ba2YCu3O7−δ superconductors Appl. Phys. Lett. 51, 855 (1987); 10.1063/1.98835

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Critical currents in protonMirradiated single~crystal Ba2 YCUa07-fJ R. B. van Dover, E M. Gyorgy, A. E. White, L. F. Schneemeyer, R J. Felder, and J. V. Waszczak AT&T Bell Laboratories, Murray Hill, New Jersey 07974

(Received 18 January 1990; accepted for publication 17 April 1990)

Irradiation with 3.5 MeV protons creates defects in Ba2YCu307_1), which can act as strong pinning sites, comparable to those produced by neutron irradiation. Indeed protons as well as neutrons undergo momentum-transferring collisions that result in atomic displacements; the additional electronic proton-solid interaction does not appear to yield defects which contribute to pinning. Under optimized conditions we have obtained le( T ..... 0, H = 1 T) ~ 2 X 107 A/ cm2 in single crystals. At 77 K the value is still lc - 1. 7 X 105 A/ cm2

, decreasing only threefold in a field of 6 To The ftuence and temperature dependence of Je suggest a model, in which spatially uncorrelated defects pin vortex cores.

Single crystals of Ba2YCu307_1l typically I show criti­cal currents of only O.S-l.OX 104 A/cm2 in the a-b plane at 77 K in a field of 1 T (Hlic). Extensive measurements of magnetization decay2 and thermally activated resistivity3

have shown that this limitation should be ascribed to weak flux pinning4 rather than poor crystalline quality. Indeed, from this perspective one would expect the highest quality crystals, i.e., those with the fewest defects and weakest pinning, to have the lowest critical currents. Preliminary results on crystals irradiated with fast ( > 1 MeV) neutrons have shown that the J, .. of high quality crystals can be dramatically increased, resulting5 in a value Jc (77 K, 0.9 T) = 6x 105 A!cm5

, comparable to that of the best thin films. This demonstrates that there is no disabling limita­tion to Jc at 77 K in bulk BaZYCu307_1i due to, e.g., pu­tative flux lattice melting (although it does not rule out melting closer to Tc). In order to more thoroughly exam­ine the effects of radiation damage on J", we have turned to proton irradiation, which has the advantages that monoen­ergetic collimated MeV proton beams are readily obtained, the flux is easily monitored, and proton irradiation does not induce radioactivity in either the samples or associated paraphernalia. On the other hand, protons have a more limited range in Ba2YCuP7_1i (~60 lIm at E = 3.5 MeV) and transfer much more energy per proton per unit length at the end of their range than at the top surface of the sample. Previous work has shown that proton irradiation can moderately increase Jc in Ba2YCu307 Ii' Willis et al,6 for example, reported a twofold increase in Jc (77 K, H = 0) in a polycrystalline sample irradiated with 800 MeV protons. In such a sample it is difficult to infer the absolute magnitude of Je since the magnetization depends on the length scale over which induced currents circulate, which is not known a priori. Preliminary reports on irra­diation of Ba2YCu307_1i single crystals have been much more promising. 7

We find that irradiation with 3.5 MeV protons at the optimum fluence (cfJt~5X 1016 cm- 2

) yields results com­parable to those obtained by fast neutron irradiation, with a maximum Jc~2X 105 A/cm2 at the benchmark point T = 77 K and H = 1 T. The values of Jc are reproducible and systematic at moderate and low temperature

(T < -60 K) but less systematic at 77 K, where the results are sensitive to even slight degradation of Te. At T < 60 K, we will show that the critical current at a given tluence correlates with the defect density induced by nuclear col­lisions, as inferred from Monte Carlo calculations,8 rather than the electronic component. The picture is consistent with the data of Summers et al.,9 who have shown that changes in Te and p" i.n Ba2 YCu307 -Ii correlate closely with the nuclear component of energy transfer from ener­getic particles (ranging from electrons to heavy ions) and not at all with the electronic component.

Crystals were prepared for this study as described previously 10; this procedure results in crystals that have been extensively characterized, are of high quality, and are extremely reproducible in their properties. The crystals were annealed in O2 for I week at 500 ·C, followed by :5 months at 400·C ("initial anneal"). With a few excep­tions, to be noted, they were then heated to 650·C for 20 min followed by 450·C for 1 week ("reannealed"). The latter procedure has been found to increase Tc from 88 to 91 K, presumably due to a subtle change in the defect structure of the oxygen sub lattice. J 1)Z Typical crystal di­mensions were O.8XO.8XO.02 mm. Protons with an en­ergy of 2-3.5 MeV were provided by a Van de Graaff accelerator at a flux of 0.6-6 X 1012 cm-2 S··I (10 7_10- 6

A/cm2). As shown below, the data are independent of flux

which suggests that heating effects are negligible. The sam­ples were greased to Al20 j substrates which were clamped to a liquid-nitrogen-cooled stage-the sample temperature was not monitored during irradiation but OUf experience indicates that they were never above 0 0c, All samples were warmed to room temperature before measurement, which is known to anneal low-energy defects. 13 Subsequent stor­age at room temperature for a month had no effect on Jc

(77 K, 1 T). Critical currents were inferred from the magnetization

of the crystals using the Bean critical-state model, assum­ing that persistent currents circulate around the entire sample. The magnetic field was applied parallel to the c axis inducing currents in the a-b plane. M-H loops were measured using a vibrating-sample magnetometer (VSM) at fields up to ± 1.5 T. For comparison, a sample was also

26B1 Appl. Phys. Lett. 56 (26), 25 June 1990 0003-6951/90/562681-03$01.00 (c) i 990 American Institute of Physics 2681

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4000 ------

N 3000~-

'"

PROTON IRRADIATED 8ozYCu30r

10 ~

N

E u E i 2000

5 ~

N "­~o

-, 1000

o o

_l-_l 20 40 60 80

TEMPERATURE [K]

o 100

'" $2

FIG_ 1. Temperature depelldence of the critical current for threc repre­sentative crystals at an applied magnetic field H = 1 T. The lines are linear regressions for J;!2 ( T) .

measured in a superconducting quantum interference de­vice (SQUID) magnetometer and in another VSM at fields up to 7.5 T. The overall absolute error in inferring Je is estimated to be about ± 15%, primarily due to the irreg­ular and imperfectly measured geometry of the samples. Relative errors for a given sample are probably much less (-5%).

Figure 1 illustrates the general trend of the data. In this figure the square root of Je is plotted versus tempera­ture, since this empirically leads to a fairly linear relation. The ordinate and abscissa intercepts will be denoted f;!'(O) and T~, respectively, to make it clear that these are extrapolated values which depend on the function used to fit the data. In general, T~ is markedly lower than the transition temperature. This is a commonly observed fea­ture of critical current measurements, but its significance in the present case is unclear because we do not have a model which explains the temperature dependence of Je. The main trend illustrated in Fig. 1 is that proton irradiation at first increases f;!'(O) without affecting T~, until ~(O) reaches a maximum at a fiuence of 3 X 1016 cm- 2 (see Fig. 2 ), beyond which T* decreases abruptly with little effect

20

~ 15 -E u "­« "' o :=, 10 -

I-

,z S 5 *u ~ --

I /

/'

I '1

I I I

I

if I

I

--()-..- ..... "-

.. "-,

"-"-

\ \

\

0_______ I _______ . L I

1x 10'5 1 .10'6 1x 10'7

PROTON FLUENCE 4>1 [cm- 2 ]

FIG. 2. Fluence dependence of J;I'(O), the critical current at I T extrap­olated to zero temperature as illustrated in Fig. 1. The lines are only guides to the eye. The arrow indicates ~(O) for an unirradiated sample. Except as noted, all are "rcannealed" samples. (e) 2 MeV, 6X 1012

cm- 2 s- l . (Al 2 MeV, 6XlOII cm- 2 s- l . (V) 3_5 MeV, 6XlO!2 em 2S--I. (0) samples irradiated at multiple angles at 2 MeV. (., "initial anneal" sample, 3.5 MeV, 6XlO12 cm- 2 s-- I

•

2682 AppL Phys. lett., VoL 56, No. 26, 25 June 1990

on the slope of J~/2 vs T. Curves for ftuences intermediate to those shown are systematically consistent with this de­scription.

A simple interpretation of this trend is that the proton damage creates small regions of depressed superconducting order parameter with a characteristic spatial extent-as long as the density of defects is low enough that these regions do not overlap they act as core pinning site. Each vortex may be pinned at many places along its length, so the more defects the better. But when the depressed regions begin to overlap, then the effective pinning barrier will disappear at a temperature lower than the original rt so T~ decreases. Pc(O) is subject to contrary influences as the depressed regions overlap. The density of pinning centers continues to increase with fiuence, tending to increase .J'! (0). Yet the difference between the order parameter at the center of the pinning site and midway between sites diminishes, tending to decrease f;!'(0). Since we experi­mentally find that Pc(O) decreases at high ftuences, the latter effect must dominate. The reduction of Jc(T) at high fiuences is comparable to that observed in Ne-irradiated Ba2YCu307_8 thin films,14 which was accounted for in terms of the simple concepts described above.

The dependence of ~ (0) on ftuence is shown in Fig. 2 for samples irradiated under a wide range of conditions. The broad peak in ~(O) is well defined and relatively robust with regard to experimental conditions. The solid line of Fig. 2 comprises data on samples irradiated at 2 MeV with a tenfold variation in flux, and samples irradi­ated at 2 and 3.5 MeV at the same flux. Other samples irradi.ated at normal incidence also fit this general shape. The dashed line of Fig. 2 indicates results for samples each of which were irradiated with 2 MeV protons at five angles e from the c axis: e = 0°, 30', 45', 60°, and 70° (equal fluences at each angle for a total ftuence as indicated). At high angles from the normal, the range of 2 MeV protons is less than the effective thickness of the samples (typically 20 pm/cos 8). Therefore these samples will contain a greater degree of end-of-range damage, as well as extensive H implantation. The shape of the dashed curve is compa­rable to that for O· -irradiated samples (solid line) but scaled to lower fluences. This is qualitatively consistent with the expectation that the damage/ion/ ftm is greater at the ion end of range, and can be put on a more quantitative basis through Monte Carlo calculations,8 which show that the total number of atomic displacements per unit volume (assuming 20 eV Idisplacement) is virtually equal at the

k f h 1· 1 4 1020 --3 d· 5 1020 -3 pea sot e two mes: . X em' an LX, em for the solid and dashed curves, respectively. This is fur­ther support that f;!'(O) depends mainly on the nuclear component of the proton energy loss.

The well-defined peak in f;!' (0) at 1.4 X 1020

displacements/em' implies an average spacing between dis­placements of 1.9 nm. Within the framework of our simple model, this means that the spatial extent of a typical defect is no greater than about 1.9 nm. Even if it were smaller, the region over which the order parameter is depressed would be no less than the coherence volume, an ellipsoid with axes ~a-S/;-2 nm and 5c-0.2 nm at T = O. Of course, it

van Dover et al. 2682

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200

o ~

OJ E 150 u "-« '" 9

100 !- .. ,;: "-"-U 50·· .. .. ..., ..

o .. o

0 h10~5

.... --_.L ... _-'?-:-____ 6--_ ........ :=_~ hW16 1xl0i1

PROTON FLUENCE ¢t [cm-2 ]

FIG. 3. Fluenee dependence of JA77 K), the measured critical current at I T and 77 K. The arrows indicate Je ( 77 K 1 for two unirradiated crystals: (upper arrow: typical data for "reannealed" samples, lower arrow: typical data for "initial anneal" samples). Symbols as defined in Fig. 2.

is the depression in the a-b plane which determines the variation in core energy and hence pinning for Hlle. This decreases rapidly for average defect spacings less than about 2 nm. On the other hand, depression of the order parameter down the c axis is on a much shorter length scale, so, if the defects themselves are small enough, we would expect to see evidence of superconductivity in sam­ples irradiated at high fiuences even wen above T = T~.

This is the case, for example, in a sample irradiated with a fluence of 2X to17 cm- 2

, which had T't = 57 K (Fig. 1) but showed a moderately diamagnetic ac susceptibility (WillS = 0.1 Oe) up to 83 K. A proper model for these properties of irradiated Hal YCu307 s would have to take into account the anisotropic coherence volume and prox­imity effects as well as pinning per se.

Compared to Pc(O), the measured values of Je at 77 K are much less reproducible and less systematic in the pro­ton fiuence. Figure 3 illustrates the scatter of the data. The strongest correlation seen is that the multiple-angle irradi­ation experiments (open circles) consistently yield low val­ues of Je (77 K). While end-of-range damage has little effect on the low-temperature value ~(O), it does decrease T',/, to which J e (77 K) is extremely sensitive. Little is currently known about the detailed nature of defects pro­duced by proton irradiation of Ba2YCu307_b' As more is learned, we hope to identify the causes for such wide vari­ability in Je (77 K), which may be attributed to variations in the original crystals.

The best values of Je (77 K, 1 T) were obtained in the thinnest crystals at 3.5 MeV, where protons are implanted far into the substrate and end-of-range damage in the sam­ple is minimized. As described in an earlier paper,12 the best values also seem to obtain in "initial anneal" rather than "reannealed" samples. At present the most encourag­ing value is J c (77 K, 1 T) = 1.7x 105 A/cm- 2 at the relatively low dose of 1 X 1016 em 2. This sample was also measured in a SQUID magnetometer, yielding Je (77 K, 1 T) = l.4x 105 A/cm 2, in fair agreement with the VSM measurement. The behavior at higher fields was investi­gated using another VSM, and was found to be non mono­tonic above 5 T; we find Je (77 K, 6 T) = 6.3 X 104 A/cm2

.

2683 AppL Phys. Lett, VoL 56, No. 26, 25 June 1990

We expect that the absolute value of Je (77 K) might be substantially improved by optimizing the irradiation pa­rameters, and suggest that the important feature is that Je (77 K) decreases only threefold between 1 and 6 T.

Since the protons are incident from a specific angle, it is possible that defects might be correlated along the irra­diation vector, or that the shape of a defect might be an­isotropic, extended along the irradiation vector. In either case the critical current should be highest when the applied field is parallel to the irradiation vector. A similar effect was seen for grain-boundary pinning in r-.'b bicrystals l5 and for twin-boundary pinning in Ba2YCu3078 single crystals. 16 To evaluate this possibility we irradiated sepa­rate crystals at an angle of 30° and 45" from the normal, and checked the critical current by observing M-Hloops as a function of angle, and by torque magnetometry with a continuously rotating electromagnet, respectively. The lat­ter technique easily resolves the narrow peak in JeC 8) at­tributed to twin-boundary pinning. However, neither tech­nique showed a change in Je (within a resolution of -10%) at the irradiation angle. We conclude that the defects are neither extended nor correlated along the irra­diation vector.

We are pleased to acknowledge W. L. Brown, R. C. Dynes, and K. Short for technical discussions.

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ly' Yeshurun and A. P. Malozemoff, Phys. Rev. Lett. 60, 2202 (1988). 3T. T. M. Paistra, B. Batlogg, R. B. van Doyer, L. F. Schneemeyer, and J. V. Waszczak, App!. Phys. Lett. 54, 763 (1989).

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van Dover et al. 2683

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