Probing of current-tailoring mechanisms in YBa2Cu3O7−δ films by means of heavy ion irradiation

Physica C 332 Ž2000. 115–121 www.elsevier.nlrlocaterphysc

Probing of current-tailoring mechanisms in YBa 2 Cu 3 O 7yd films by means of heavy ion irradiation E. Mezzetti a,) , A. Chiodoni a , R. Gerbaldo a , G. Ghigo a , L. Gozzelino a , B. Minetti a , C. Camerlingo b, A. Monaco b a

INFM, U.d.R Torino-Politecnico; INFN, Sez. Torino, Politecnico di Torino, c.so Duca degli Abruzzi 24, 10129 Turin, Italy b Istituto di Cibernetica del Consiglio Nazionale delle Ricerche Via Toiano 6, 80072 Arco Felice, Italy

Abstract This paper deals with the origin of the plateau-like feature of the critical current density vs. logarithm of the field in YBCO films. By means of a suitable model, we check the hypothesis that this trend depends on a particular grain boundary ŽGB. network modulated by intrinsic correlated defects, coherently trapping the vortices. Such network consists of a 1D row of high-Jc intergrain Josephson junctions ŽJJ.. Irradiation with gold ions provides extrinsic columnar defects. The fit of the data with the model indicates that the columnar defects change the length and distribution of the JJ. In interpreting our data, it results that at least in a range of temperature up to 0.5 TrTc , Jc depends on the defect-induced modulation Žeither intrinsic or extrinsic. of the JJ network. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Josephson junctions network; Critical current density; Ion irradiation; Irradiation effects

1. Introduction Superconducting YBa 2 Cu 3 O 7y d thin film exhibits both large critical current densities Ž Jc . and plateau-like features in the Jc dependence on the logarithm of the magnetic field. This particular behavior points toward a mechanism involving coherent pinning of vortices inside interdomain channels w1–10x, due to correlated defects. In particular, the

) Corresponding author. Tel.: q0039-115647349; fax: q0039115647399. E-mail address: [email protected] ŽE. Mezzetti..

grain boundary ŽGB. topology and morphology play the main role in determining the dependence of Jc on field and temperature. As long as length scales of a few nanometers are considered Žshorter lengthscales are not., the topology concerns the setup of single crystal-like islands separated by a GB network w6x. The morphology concerns the details of the texture Žintergranular misorientation angles and faceting w5,11x.. It determines modulation of the order parameter across the boundary. Not withstanding the extensive studies of GBs in HTSC, the investigation of the correlation between the GB morphology and its transport properties have been mainly centered on different kinds of bicrystal interfaces w12,13x. The findings, though extremely enlightening,

0921-4534r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 0 0 . 0 0 0 0 8 - 3

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are not statistically meaningful for what concern films, since in a real film, a huge number of different GBs with different natures determine the macroscopic superconducting properties w14,15x. Yet the macroscopic properties carry the signature of what kind of GB determines the transport mechanisms. To focus on this topic, we investigated the critical current densities as a function of field and temperature of two sets of films with different macroscopic properties and ‘‘a priori’’ also different distributions of dominant GBs w11x. As a meaningful probe to investigate length scales of a few nanometers and to modulate the columnarlike intrinsic defects between islands, we used ioninduced columnar defects. The investigation is performed in the framework of a model based on a network of Josephson junctions ŽJJ. along the boundaries between the islands w7x. In such model, the transport currents are determined by an average 1D row of JJ whose length is modulated by columnar defects, either intrinsic or extrinsic. As long as static properties of the vortex lattice are investigated, the defect-modulated network coherently traps vortices along the junction network w5,10x. The vortex distance spectrum corresponds to a spectrum of fields up to a maximum, characteristic of the network w7,8,16x. However, it must be emphasized that in the framework of the model no speculation on the local density of the trapped vortices is made, because the nature of the vortices trapped in the JJ is up to now under debate w17x. In this paper, the previous model w7x was modified as reported in Section 4, in order to get more information about the correlation between macroscopic properties and nanostructure. Actually, we can determine, under suitable hypothesis, the average junction length and distribution. In interpreting our data, we found that the effect of irradiation consists of a further modulation of the junction lengths and of a modified length distribution. Model and fitting data suggest that after irradiation, the network carries more intergranular pits, coherently trapping vortices. This only happens as long as the temperature range involves a length scale of a few nanometers. As the coherence length diverges and the regime goes toward the irreversibility line ŽIL., the structure behaves almost as a continuum, compared to the previous length scale.

2. Experimental details The YBa 2 Cu 3 O 7y d films were fabricated in situ by dc sputtering from inverted cylindrical magnetron on SrTiO 3 Ž100. oriented substrates. Since we aimed at studying YBCO films with different grain properties, we slightly modified the standard growth process. In fact, these properties can be controlled, preserving the crystalline orientation, by varying the cooling rate. During deposition an ArrO 2 Ž4:3. gas mixture with a total pressure of 93 Pa was used, with a dc power of 110 W, resulting in a growth rate of about 2 nmrmin. The substrates were heated during deposition at a temperature TS s 8208C. As mentioned above, we used different cooling processes. When fully oxygenated films were required, the vacuum chamber was filled with oxygen at a pressure of 40 kPa and the heater power was slowly decreased, cooling the sample at a rate of 108Crmin until TS s 6008C was reached. After 15 min, the O 2 pressure was raised to 80 kPa and the sample temperature was decreased at a slower rate Ž58Crmin. to 4508C and kept at this temperature for 60 min; then, the heater was turned off. Otherwise, when slightly deoxygenated films were required, the vacuum chamber was filled with 80 kPa of oxygen and the temperature slowly decreased Ž108Crmin. until TS s 4508C was reached. After 30 min the heater was turned off. Two sets of samples were obtained: fully oxygenated samples ŽF. and oxygen deficient samples ŽD.. Each set comes from a single deposition, by cutting twin square samples Ž2.5 = 2.5 mm2 . from the same film. The oxygen content was inferred from Raman measurements. The samples ŽD. exhibit disorder in the oxygen sublattice, with broken CuO chains and likely, the setup of out of phase oxygen microdomains w18x. These features do not affect the crystalline properties of the films, which exhibit a strong crystal orientation in the u y 2u X-ray diffraction, with c-axis normal to the surface plane. An island structure is clearly visible by AFM analysis w7x. The surface morphology consists in islands of about 100 nm diameter. 0.25 GeV 197Au ion irradiations were performed at room temperature and in vacuum at the 15 MV Tandem accelerator of the INFN-Laboratori Nazion-

E. Mezzetti et al.r Physica C 332 (2000) 115–121

ali del Sud ŽCatania, Italy.. The samples were irradiated perpendicularly to the surface with fluences of 1.5 = 10 10 and 7.5 = 10 10 cmy2 , corresponding to dose equivalent fields1 Bf s 0.3 and 1.5 T, respectively. Magnetic susceptibility measurements were carried out by means of a commercial ac susceptometer in a range of temperatures from 28 K to Tc . Both ac and dc fields were oriented parallel to the sample c-axis. The critical current densities at zero field range from 1.4 = 10 10 to 3.7 = 10 10 Arm2 at T s 36 K. On average, the Jc values are slightly higher for the specimens ŽF.. However, it must be stressed out that the absolute values of the critical current density are slightly sample-dependent.

3. Zero field characterization The electrical resistance for both specimens ŽD. and ŽF. was measured as a function of temperature. The curve ŽD. exhibits a pronounced tail in the low temperature region, likely due to the oxygen miscomposition, and a lower Tc w7x. In Fig. 1, the real part of the first harmonic ac susceptibility as a function of temperature is shown. The reported curves concern three samples ŽD.: D0 Žvirgin., D1 Ž Bf s 0.3 T., and D2 Ž Bf s 1.5 T.. Curves for two samples ŽF. are also reported: F0 Žvirgin. and F2 Ž Bf s 1.5 T.. For the lower dose, the corresponding average distance between the tracks is d s 83 nm Ž d s Ž f 0rBf .1r2 where f 0 is the quantum flux.; for the higher dose, the average distance is d s 37 nm. The susceptibility characterization of all the mentioned films does not show the double transition typical of weakly coupled granular materials w19x. Hence, we assume that all samples exhibit a prevailing amount of GBs not behaving as usual weak links, normally responsible for the so-called magnetic granularity. On the contrary, most of the involved GBs are high-Jc junctions, playing the role of ‘‘hidden’’ weak links. This assumption accounts

1

The dose equivalent field is the magnetic field which would be ideally required to fill each track with a quantum flux, i.e., Bf s n f 0 , where f 0 is the quantum flux and n is the track density.

117

Fig. 1. Real part of the first harmonic ac susceptibility as a function of temperature, in zero applied dc field.

for a strong proximity coupling across the defects w17x. Most of them are considered as insulating zones of nanometric size with modified carrier density and reduced order parameter. Moreover, the GB network constitutes a network of planar defects. Simple topological considerations lead to the hypothesis that the meeting point of three or more islands constitutes a columnar-like defect site across the film thickness. Other linearly correlated defects could originate as linear dislocations in low-angle GBs w20x or as dislocation chains w6,21x. Although all these defects are less controllable than ion-induced defects, they are effective in determining the pinning capability of these films. Irradiation of the fully oxygenated specimens did not induce a critical temperature decrease ŽTc s 89 K.. This fact is probably due to the good-quality crystal structure of the islands. On the contrary, the underdoped specimens show an ion-induced Tc degradation, probably due to the pre-irradiation oxygen disorder, both in the island and in the boundaries. Critical temperatures were evaluated as the temperatures where the ReŽ x . signal significantly emerges from the noise Ž1% of the signal at T s 4.2 K.. The critical temperature decreases from Tc s 88 K Žvirgin film D0. to Tc s 83.8 K Žirradiated film D1. and to Tc s 82.3 K Žirradiated film D2.. For the specimens ŽD., the Tc degradation increases with the fluence. This fact and the lack of the Tc damage in the ŽF. specimen provide the evidence that Tc damage sets up mainly in samples characterized by an intrinsic oxygen disorder.

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4. Model As mentioned above, we focus on the analysis of the plateau-like features in the dependence of Jc on the logarithm of the field. We check the hypothesis that most of our GBs are high-Jc junctions, playing the role of ‘‘hidden’’ weak links. Inside the boundary interfaces the nanosized defects enclose magnetic flux. The simplest way to account for such complex systems is to model the film as a random array of parallel uniform Short JJ ŽSJJ., with statistically distributed junction lengths. Such a network can be thought as a one-dimensional array meandering in a two-dimensional space. Moreover, the considered array must be regarded as the average high-Jc interface w22x. Each junction will pass current according to its own Fraunhofer pattern w23x, and interference among these patterns will smear out the structure. As pointed out by Fistul and Giuliani w24x, the usual Fraunhofer pattern must be modified to take into account the penetration of vortices into the grains. In fact, the order of magnitude of the magnetic thickness L0 , for B ) Bc1 , should match the distance between Abrikosov vortices in the junction electrodes. Neglecting the equilibrium magnetization w5x, the magnetic thickness, usually written as L 0 s d q 2 l Ž d is the geometrical thickness and l is the London penetration depth., becomes L 0 ( z f 0rB . Here z is a number of order unity, which depends on the bulk pinning and on the topology of the GB. In order to take into account the statistical distribution of the contact lengths, we consider a suitable probability density function pŽ L.. The critical current is then obtained as the average, weighted by pŽ L., of the critical current of the single junction, each of them characterized by its own value of length L and magnetic thickness of the barrier L0 . The macroscopic critical current density is thus expressed by

(

`

Jc Ž B . s

H0 d Lp Ž L . J

SJJ

H0

d Lp Ž L .

Here, the parameter Jc Ž0. is supposed to be the same for all the junctions. Previous AFM characterization suggests that the contact lengths have a well characterized mean value and both very low and very large L values are unlikely. This consideration leads to a bell-shaped probability density function. A simplicity criterion, pŽ L. s m2 Ley m L , was used to obtain a reliable analytical result w7x. Nevertheless, a one-parameter probability density function is not suitable to provide transparent information about the hidden network of JJ. In this paper, we consider a two-parameter distribution, with average SJJ length ² L: and standard deviation sL .

mn p Ž L. s

G Žn .

Lny1 ey m L

Ž 1.

where

Ž B.

`

s Jc Ž 0 .

Fig. 2. Normalized critical current densities as function of magnetic field for the two oxygen deficient samples. The lines are fitting curves obtained by Eq. Ž2.. The fitting parameters are reported in Table 1. The inset shows the contact length distribution ŽEq. 1. as deduced from the fitting procedure Ž z s1.35 was assumed..

(

sin pz L Brf 0

ž

(

pz L Brf 0

/

.

ms

² L:

sL2

and

ns

² L:

ž / sL

2

.

E. Mezzetti et al.r Physica C 332 (2000) 115–121

119

5. Experimental results and discussion

Fig. 3. Normalized critical current densities as function of magnetic field for the two fully oxygenated samples. The lines are fitting curves obtained by Eq. Ž2.. The fitting parameters are reported in Table 2. The inset shows the contact length distribution ŽEq. 1. as deduced from the fitting procedure Ž z s1.35 was assumed..

The critical current density then becomes:

mn Jc Ž B . s Jc Ž 0 .

G Žn .

`

n y1 y m L

=

H0 L

(

sin pz L Brf 0

ž

/

(

pz L Brf 0

dL

1

s Jc Ž 0 .

G Ž n . q'B

`

=

H0 x

where q s p

e

ny2 yx

e

sin Ž q'B x . d x ,

Ž 2.

z ² L:

(

n f0 . The two parameters, n and q, as well as Jc Ž0., can be obtained with a last-square fitting of the experimental data.

In this section, we focus on the ratio between the critical current density at a given field to its value at zero dc applied field. The considered samples, coming from two sets of twin specimens, are ŽD0., ŽD1. and ŽF0., ŽF2.. The ILs of the samples ŽD2. and ŽF2. will also be compared. First harmonic susceptibility data were analyzed in the framework of the Clem and Sanchez w25x model for the case of a thin sample in a transverse field. In Figs. 2 and 3 we report the experimental data at TrTc s 0.41 for the two sets of samples fitted by using Eq. Ž2., with n , q and Jc Ž0. as fitting parameters. Both experimental and fitting curves are normalized to the parameter Jc Ž0.. The experimental values of the critical current densities at TrTc s 0.41 and zero applied field are 1.80 = 10 10 Arm2 Žsample D0., 1.35 = 10 10 Arm2 Žsample D1., 3.66 = 10 10 Arm2 Žsample F0. and 2.15 = 10 10 Arm2 Žsample F2.. The fitting procedure is remarkably successful in the low temperature regime, while deviations from the model arise if the reduced temperature is increased over 0.54 TrTc . The results support the conclusion that in this range of temperature, the main morphology responsible for the Jc vs. B trend in YBCO good quality films is a network of JJ modulated by correlated defects. The high-Jc paths should therefore percolate across the hidden GBs w8,10x. The fit allows us to obtain two independent physical quantities: ² L:z and ² L:rsL , reported in Tables 1 and 2 for the two sets of samples at different reduced temperatures. The same tables also show the behavior of z , normalized to the value at T s 28 K, in the hypothesis that the mean value ² L: do not depend on temperature. The maximum temperature variation of z in this range is of the order of 60%

Table 1 Values of the parameters obtained by fitting with Eq. Ž2. the Jc Ž B . curves of the oxygen deficient samples. The temperature dependence of z , normalized to its value at TrTc s 0.32, is also reported. Examples of fitting curves are shown in Fig. 2 TrTc 0.32 0.41 0.54 0.65

Sample ŽD0.

Bf s 0

Sample ŽD1.

² L:z Žnm.

² L:rs

zrz ŽT s 28 K.

² L:z Žnm.

² L:rs

zrz ŽT s 28 K.

Bf s 0.3 T

24 24 24 33

1.4 1.4 1.4 1.4

1.0 1.0 1.0 1.5

16 18 22 26

1.4 1.5 1.8 2.4

1.0 1.1 1.3 1.6

E. Mezzetti et al.r Physica C 332 (2000) 115–121

120

Table 2 Values of the parameters obtained by fitting with Eq. Ž2. the Jc Ž B . curves of the fully oxygenated samples. The temperature dependence of z , normalized to its value at TrTc s 0.32, is also reported. Examples of fitting curves are shown in Fig. 3 TrTc

Sample ŽF0. ² L:z Žnm.

0.32 0.41 0.54 0.65

23 23 25 29

² L:rs 1.2 1.2 1.2 1.3

Bf s 0

Sample ŽF2.

zrz ŽT s 28 K.

² L:z Žnm.

1.0 1.0 1.1 1.2

and we cannot determine possible sample-dependent z variations. Nevertheless, in order to allow qualitative considerations, we report in the insets of Figs. 2 and 3 the contact-lengths distributions resulting from the fits. In this case, we assume for z , the value reported in Ref. w5x Ž z s 1.35.. The first comment on these results concerns the order of magnitude of ² L:, properly modulated by irradiation. As already emphasized, we assume that both unirradiated and irradiated samples exhibit ² L: values which do not represent the whole GB Ža non-uniform interface. but rather the modulations of the boundary itself. A coarse quantitative agreement is found with the values reported in Ref. w5x, in the case of GBs in bicrystals. The outstanding result is that the effect of Au–ion irradiation consists of an increase in the maximum accommodation field BU w16x. From a phenomeno-

10 9.7 12 14

Bf s 1.5 T ² L:rs 1.4 = 10 1.6 1.2 0.9

zrz ŽT s 28 K. 5

1.0 1.0 1.2 1.4

logical point of view, we evaluate BU as the field at which the critical current reaches 90% of the zero field value. BU increases with the dose: after irradiation, for a given value of the ratio JcrJc Ž0., more vortices can be coherently accommodated inside the GB channels. In interpreting our data, we found that in the critical state Jc depends on the defect-induced modulation Žeither intrinsic or extrinsic. of the JJ network. The length distribution of the junctions is sharpened by the irradiation, though in the case of the irradiated samples, a temperature dependence of sL cannot be excluded. As a final remark, it is worthy to emphasize that the quality of the fit becomes less satisfying as the temperature increases above 0.53 TrTc . Above this value, a regime which is gradually less dependent on the GB network and likely more dependent on the bulk distribution of the pinning centers sets up. This interpretation of the data can be supported by the irradiation-induced shift and kink of the irreversibility lines ŽFig. 4.. In fact, the presence of a kink at B f Bfr2 in the ILs of D2 and F2 irradiated samples points toward the setup of a Bose glass regime w7,16x. This result seems to indicate that, near the IL, the Cooper pairs do not ‘‘see’’ the islands: the film almost behaves as a continuum, with respect to the mentioned-scale granularity w26x.

Acknowledgements

Fig. 4. Irreversibility lines of ŽD. and ŽF. samples irradiated at Bf s1.5 T. The lines are power-law fits and the exponents a are reported for the different regimes. The kink in the curves approximately corresponds to Bf r2.

This work is partially supported by INFM under PRA project ‘‘High Temperature Superconductor Devices’’ and by a MURST COFIN98 program ŽItaly.. We wish to thank G. Cuttone and A. Rovelli for the meaningful suggestions concerning the irradi-

E. Mezzetti et al.r Physica C 332 (2000) 115–121

ation set up and the dose evaluation, and the INFNLNS staff for their technical support during the irradiation runs.

References w1x M.A. Itzler, M. Tinkham, Phys. Rev. B 51 Ž1995. 435. w2x V.V. Moshchalkov, V. Bruyndoncx, E. Rosseel, L. Van Look, M. Baert, M.J. Van Bael, T. Puig, C. Strunk, Y. Bruynseraede, Lectures on superconductivity in networks and mesoscopic systems, in: C. Giovannella, C.J. Lambert ŽEds.., AIP Conference Proceedings Vol. 427 Woodbury, New York, 1998. w3x E. Mezzetti, R. Cherubini, S. Colombo, R. Gerbaldo, G. Ghigo, L. Gozzelino, B. Minetti, D. Zafiropoulos, J. Supercond. 8 Ž1995. 321. w4x E. Mezzetti, S. Colombo, R. Gerbaldo, G. Ghigo, L. Gozzelino, B. Minetti, R. Cherubini, Phys. Rev. B 54 Ž1996. 3633. w5x X.Y. Cai, A. Gurevich, I.-F. Tsu, D.L. Kaiser, S.E. Babcock, D.C. Larbalestier, Phys. Rev. B 57 Ž1998. 10951. w6x B. Dam, J.M. Huijbregtse, F.C. Klaassen, R.C.F. van der Geest, G. Doornbos, J.H. Rector, A.M. Testa, S. Freisem, J.C Martinez, B. Stauble-Pumpin, R. Griessen, Nature 399 Ž1999. 439. w7x E. Mezzetti, R. Gerbaldo, G. Ghigo, L. Gozzelino, B. Minetti, C. Camerlingo, A. Monaco, G. Cuttone, A. Rovelli, Phys. Rev. B 60 Ž1999. 7623. w8x E. Mezzetti, E. Crescio, R. Gerbaldo, G. Ghigo, L. Gozzelino, B. Minetti, IEEE Trans. Appl. Supercond. 9 Ž1999. 2227.

121

w9x A. Gurevich, IWCC extended abstract, July 7–10 1999, Madison, WI, USA. w10x J.E. Evetts, IWCC extended abstract, July 7–10 1999, Madison, WI, USA. w11x D. Dimos, P. Chaudari, J. Mannhart, Phys. Rev. B 41 Ž1990. 4083. w12x F. Tafuri, F. Miletto Granozio, F. Carillo, A. Di Chiara, K. Verbist, G. Van Tendeloo, Phys. Rev. B 59 Ž1999. 11523. w13x Y. Zhu, Y.N. Qiang Li, M. Tsay, G.D. Suenaga, N. Gu, N. Koshizuka, Phys. Rev. B 57 Ž1998. 8601, and references therein. w14x J. Halbritter, ISEC ’99 extended abstract, June 21–25 1999, Berkley, USA. w15x H. Hilgenkamp, J. Mannhart, Appl. Phys. Lett. 73 Ž1998. 265. w16x L. Krusin-Elbaum, L. Civale, G. Blatter, A.D. Marwick, F. Holtzberg, C. Feild, Phys. Rev. Lett. 72 Ž1994. 1914. w17x A. Gurevich, L.D. Cooley, Phys. Rev. B 50 Ž1994. 13563. w18x M.N. Iliev, P.X. Zhang, H.U. Habermeier, M. Cardona, J. Alloys Compd. 251 Ž1997. 99. w19x V. Calzona, M.R. Cimberle, C. Ferdeghini, M. Putti, A. Siri, Physica C 157 Ž1989. 425. w20x Y. Gao, K.L. Merkle, G. Bai, H.L.M. Chang, D.J. Lam, Physica C 174 Ž1991. 1. w21x R.M. Schalk, K. Kundzins, H.W. Weber, E. Stangl, S. Proyer, D. Bauerle, Physica C 257 Ž1996. 341. w22x J. Rhyner, G. Blatter, Phys. Rev. B 40 Ž1989. 829. w23x A. Barone, G. Paterno, ` Physics and Applications of the Josephson Effect, Wiley, New York, 1982. w24x M.V. Fistul, G.F. Giuliani, Phys. Rev. B 51 Ž1995. 1090. w25x J.R. Clem, A. Sanchez, Phys. Rev. B 50 Ž1994. 9355. w26x J. Jung, H. Darhmaoul, H. Yan, Supercond. Sci. Technol. 11 Ž1998. 973.