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Vortex dynamics of 250 MeV Ag ions irradiated YBCO films
Nuclear Instruments and Methods in Physics Research B 156 (1999) 44±49
www.elsevier.nl/locate/nimb
Vortex dynamics of 250 MeV Ag ions irradiated YBCO ®lms Sanghamitra Khatua
a,*
, Ravi Kumar b, P.K. Mishra c, V.C. Sahni c, R. Pinto
d
a
JaiHind College, Churchgate, Mumbai 400020, India Nuclear Science Centre, New Delhi 1100 67, India c Bhabha Atomic Research Centre, Mumbai 400085, India Tata Institute of Fundamental Research, Mumbai 400005, India b
d
Abstract YBCO thin ®lms with c-axis orientation were grown in-situ by KrF laser on LaAIO3 substrate. The ®lms were irradiated with 250 MeV Ag ions at doses equivalent to B/ ranging from 1 T to 4 T. Critical currents were measured using SQUID magnetometer with ®eld parallel to columnar defects. There is a marked enhancement of vortice pinning due to generation of additional pinning centres. There seems to be an upper limit to the irradiation doses beyond which there is a Ôsmearing eectÕ on Jc . We also ®nd a shift in Irreversibility Line towards higher ®eld which is attributed to better pinning induced by columnar defects. Defect production is examined in the framework of ``Coulomb explosion'' model and the consequences are analysed in terms of defect-induced behaviour within the purview of models such as ``Bean model'', ÔExponential modelÕ and ``Vortex Glass'' model. We point out that the Ôstress modelÕ provides a qualitative explanation for the reduced Tc and a broad transition at high ¯uence. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 60.Gs; 70.Bx; 62.Db Keywords: Irradiation; Vortice pinning; Columnar defects; Critical currents; Irreversibility line
1. Introduction YBCO is an extensively studied HTSC system [1±6]. Magnetic studies have been reported in bulk and single crystals but scarce work is done in the realm of thin ®lm inspite of its better potential for technological application. The most vexed problem for practical application is the disappearance of superconductivity in moderately high magnetic
* Corresponding author. Tel.: +91 11-6893955/6892601; fax: +91 11-6893666; e-mail: [email protected]
®eld and temperature. Some recent reports show that the columnar defects produced by irradiation of heavy ions are highly eective in pinning the ¯ux lines over wide range of temperature and ®eld [7±10] and that the critical current increases dramatically over the unirradiated material. Moreover, these columnar defects have been found to cause a signi®cant change in position and shape of the Irreversibility Line (IL). Since swift heavy ions provide an excellent tool for creating columnar defects, we decided to explore the vortex dynamics of YBCO ®lms in the presence of ``controllably introduced correlated disorder'' induced by heavy ion irradiation.
0168-583X/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 1 6 1 - 5
S. Khatua et al. / Nucl. Instr. and Meth. in Phys. Res. B 156 (1999) 44±49
45
2. Experimental Stoichiometric YBa2 Cu3 O7 (YBCO) target was made by sintering process. Four thin ®lms (5 were grown inmm ´ 5 mm) of thickness 2000 A situ under identical growth parameters by KrF excimer laser on LaAIO3 (1 0 0) substrate. X-ray diraction showed that the ®lms were c-axis oriented. The critical temperature of the as-grown ®lms was 90 K as measured by the four-probe resistivity method. Three of the ®lms were irradiated with 250 MeV Ag14 ions using the 15 UD accelerator at ¯uence values 5 ´ 1010 , 1 ´ 1011 and 2 ´ 1011 ions/cm2 corresponding to ``matching ®elds'' of 1 T, 2 T and 4 T, respectively. This particular beam energy was chosen using TRIM95 [11] so as to ensure best electronic excitation. In addition, the range of Ag14 ion in this energy range being larger than the sample thickness, any undesirable ion implantation in the sample is avoided. The density of columnar defects produced was high enough to pin all the vortices at least for our measurements performed in an applied DC magnetic ®eld upto 5 T. Field-dependent magnetisation measurements with H//c-axis were done using a SQUID magnetometer. For the hysteresis, we adopted the new ``half-scan'' technique instead of ``full scan'' [12]. The advantage of this method over the conventional one is that it yields results independent of scan length, essentially unaected by any deviation in ®eld homogeneity. Thus more accurate measurement is obtained which may otherwise be masked by any ®eld inhomogeneity inherent in the method. All the measurements were done for a 2 cm scan length upto 5 T DC magnetic ®eld with intervals of 500 Gauss. M±H loops were traced for temperatures ranging from 30 K to 70 K. All the ambient parameters were maintained identical for all the ®lms under study to ensure uniformity and consistency in the measurement. 3. Results A typical plot of M±H for T 30 K is shown for one of the samples (Fig. 1) Critical currents (JCM ) values were obtained from the Bean Model
Fig. 1. Magnetisation hysteresis isothermal at 30 K for 5 ´ 1010 ions/cm2 .
[13] JCM 30 DM/d where DM MÿM¯ and d is the thickness of the sample. The applied DC magnetic ®eld being parallel to c-axis is parallel to the ion-tracks created by the Ag ion beam. The critical current closely follows a universal curve with JCM / Bÿn for low ®elds [Fig. 2(a)±(c)]. The correlation of the ¯ux of irradiation and the critical current seems to have a limiting parameter. In contrast to the systematic monotonicity in the JCM vs T plots for the ¯uence values upto 1 ´ 1011 ions/ cm2 , we found that at 2 ´ 1011 ion/cm2 the data points of DM (i.e. JCM ) have an extremely arbitrary and random distribution (Fig. 3). There is no distinctive trend in the JCM vs B plot but rather a kind of ``smudging'' of the distribution pattern. We have also extracted the pinning force FP for dierent ¯uences as well as for the virgin sample. The pinning force FP Jc B is highly critical current dependent. A glance at the plots of FP versus ®eld (Fig. 4) portrays an interesting pattern in its variation. Next we have examined to ®nd the relationship, if any, between the Irreversibility ®eld Hirr and temperature for various ¯uences. For this purpose, we used the constant JCM criterion (JCM 700 A) for the critical current isothermals to extract the Hirr values [14] The JCM ±B plot for the ¯uence 2 ´ 1011 being devoid of any trend or information was not considered for Irreversibility line (IL). The
46
S. Khatua et al. / Nucl. Instr. and Meth. in Phys. Res. B 156 (1999) 44±49
Fig. 3. Variation of critical current with ®eld at 60 K for ¯uence 2 ´ 1011 ion/cm2 (Note the random distribution).
Fig. 4. Variation of pinning force with ®eld at dierent ¯uence values. (Note the position of peak value).
Fig. 2. (a) Variation of critical current with ®eld at dierent temperatures for unirradiated sample. (b) Variation of critical current with ®eld at dierent temperatures for dose 5 ´ 1010 ion/ cm2 . (c) Variation of critical current with ®eld at dierent temperatures for ¯uence 1 ´ 1010 ion/cm2 .
IL plots for dierent samples (Fig. 5) show a rather interesting pattern. The monotonic IL shifts to higher ®eld over the virgin sample for 5 ´ 1010 ¯uence but goes down for the higher ¯uence of 1 ´ 1011 . Moreover, the in¯uence is strong when Hirr is close to B/ and recedes when away from B/ . Finally a simple, nonetheless, signi®cant trend is that of the critical temperature Tc for zero resistivity as measured from the magnetisation
S. Khatua et al. / Nucl. Instr. and Meth. in Phys. Res. B 156 (1999) 44±49
Fig. 5. Variation of irreversible ®eld with temperature for different ¯uence values.
measurement. We found a steady decline in Tc with the increasing degree of irradiation (Virgin ®lm Tc ® 90 K, 5 ´ 1010 ® 87 K 1 ´ 1011 ® 85 K, 2 ´ 1011 ® 75 K). Additionally the transition to Tc (DT Tonset ÿ Tc ) is also broadened instead of being sharp. 4. Discussion During the irradiation, a cylindrical shockwave produced by charged ion creates columnar defects all along the ion track as explained by Coulomb explosion model [15]. Such ``Correlated disorder'' generates an increase in critical current. In YBCO single crystals and the thin ®lm any pinning enhancement is strongly angle-dependent and maximises when the magnetic ®eld is parallel to latent ion tracks because then the track radius coincides with that of the vortex core. Keeping this in view, our samples, were subjected to a DC magnetic ®eld parallel to c-axis i.e. the direction of the ion path. Among the ion-induced eects in our measurements, the reduction in transition temperature Tc , we believe, is due to a possible orthogonal to tetragonal crystallographic transition subsequent to irradiation. At higher ¯uences (2 ´ 1011 ions/ cm2 ) amorphisation sets in i.e. there is a severe
47
lattice strain resulting in a crystalline to amorphous phase transition. This apparently manifests as an extraordinarily random and ``Smeared'' effect in our JC values. The broadening of the phase transition with irradiation can be explained by a kind of ÔstressÕ model. The randomly distributed track positions create mutually dissimilar strains spread all over the sample topology. So it is plausible that a wide distribution of Tc will exist. The sample zones with least strain (i.e. higher Tc ) determine Tonset since they belong to the Ôunaffected-by-strainÕ matrix. In contrast, the superconducting path needed for Tc (R O) certainly pass through regions of higher strain (hence low Tc ) and in the worst scenario, no such path may exist, resulting in a very broad transition and loss of any meaningful superconducting state as seen in the case of ¯ux 2 ´ 1011 ions/cm2 . A major trend in the JC ±B plots is a Ô¯atteningÕ of the JC isothermals vis-a-vis the virgin ®lms. A simple interpretation can be that, the ion damages create small regions of depressed superconducting order parameters with their characteristic spatial extent. As long as the density of defects is low enough so that these regions do no overlap they act as core pinning sites. So, in principle, the more the defects the better for JC . But when the depressed zones begin to overlap, the eective pinning barrier can disappear at a temperature lower than Tc . Such weakening of pinning barrier will have a detrimental eect on JC , particularly at high ¯uence and hence it shows a weak plateau like dependence with ®eld. Another feature is that JC decreases rather slowly with increasing ®eld. For low ®elds, JC ®ts reasonably well to a power law curve with JC / Bÿn . This provides a strong evidence that the ratio B/B/ which is just the average number of vortices per ion track is a relevant parameter to assess the volume pinning force. At low ®elds, each vortex is pinned by an ion track and JC exhibits a power law dependence. For higher ®elds, say B P 2B/ there exists an excess of vortices which can only interact with the pinning centres which were present prior to the irradiation. Consequently JC approaches the exponential scaling curve. Next we turn to an understanding of the characteristic feature in the Fp vs B plot. In the virgin
48
S. Khatua et al. / Nucl. Instr. and Meth. in Phys. Res. B 156 (1999) 44±49
sample the curve shows a peak around 3 T indicating a maximum pinning ®eld. In the irradiated samples of 5 ´ 1010 ¯uence and 1 ´ 1011 ¯uence a similar inclination is seen too. However, a comparison of the two ¯ux values show that in the ¯uence range 1 ´ 1011 the curve begins to decline at 3.2T whereas for 5 ´ 1010 ¯uence the curve is still on an upward trail with no such signature. We conjecture that the vortice-pinning due to columnar defects via irradiation reaches an optimum around 5 ´ 1010 ¯uence beyond which it is probably rather inhibited. Coming to the analysis of IL features, we start from the basic concept that vortices are extended elastic objects that interact with each other as well as with crystallographic defects. Unlike conventional superconductors, vortex dynamics of HTSC such as YBCO is strongly in¯uenced by thermal ¯uctuation. The consequence of this is the existence of a melting line in H±T phase diagram. Above this line, the vortex matter forms a liquid phase, and below it, the vortices freeze into several possible phases including variety of ``glasses''. Invoking the ¯ux Creep and Thermally activated ¯ux ¯ow model (TAFF) [16±19], the expression R / exp (ÿU/kT) yields an enhanced activation energy with reduced resistivity. Hence the irradiation induced defects can reduce dissipative ¯ux motion caused by the Lorentz force and an electric ®eld. The strong in¯uence of the matching ®eld B/ in the IL indicates the existence of an empirical scaling law E/ (J, B + B/ ) E0 (J, B). It is suggestive of the fact that in the high ®eld limit dissipation is primarily due to the motion of excess vortices. These excess vortices move independent of those vortices pinned at ion tracks. As the vortices at these sites do not depin, the activation energy of vortices pinned by columnar defects exceeds that of vortices pinned by background defects of preirradiation state. At lower ®elds (B/ < B < 2B/ ) the vortex-vortex interaction becomes more prevalent thereby preventing all the columnar defects from being occupied. Hence the strong signature is seen around Hirr B/ exclusively. The signi®cant upward shift in the position of IL at 5´1010 ¯uence is indicative of a reduced ¯ux creep rate following production of additional pinning centre via columnar defects.
5. Conclusion Summing up, we have attempted to provide a detailed magnetisation study of the eect of irradiation on the vortex dynamics of YBCO thin ®lms. YBCO ®lms, prepared by pulsed laser deposition were irradiated with 250 MeV Ag ions. Using the Critical state model, the critical current at dierent temperatures and ¯uences were inferred from magnetisation data. The result shows the bene®cial eect of columnar defects on the vortices-pinning although it has an upper limiting value. So, in principle, it establishes the technological potential of columnar defects as a mechanism for pinning enhancement. The variation of pinning force with ¯uence is consistent with our assumption of a optimum ¯uence value of 5 ´ 1010 ion/cm2 . We also sought to gain an insight into the vortex dynamics through the analysis of IL. A point of interest is the creep rate above and below the Ômatching ®eld B/ (where the number of columnar defects and the number of vortices are equal) with a distinct character to the IL in this region. The ``stress model'' suggested corroborates with the eect seen at high ¯uences. In conclusion, new results on the eect of Ag irradiation on YBCO ®lms, adequate models to ®t all the features in our results, as well as reasonable explanations for them, have been provided. References [1] R. Wheeler, M.A. Kirk, A.D. Marwick, L. Civale, Appl. Phys. Lett. 63 (1993) 1573. [2] L. Civale, Supercond. Sci. Tech. 10 (1997) A11. [3] J. Thomas, P. Vergese, P. Scatzle, A. Wetzig, U. Kramer, Physica C 251 (1995) 315. [4] J.T. Kim, C.H. Kim, K.E. Gray, S.S. Choi, H.R. Lim, Physica C 301 (1998) 99. [5] G. Blatter, M.V. Feiglman, A.I. Larkin, V.M. Vinokure, Rev. Mod. Phys. 66 (1994) 1125. [6] E.H. Brandt, Rep. Prog. Phys. 58 (1995) 1465. [7] L. Civale, A.D. Marvik, M.A. Kirk, Y. Sun, J.R. Clem, F. Holtzberg, Phy. Rev. Lett. 67 (1991) 648. [8] M. Konezykowski, F. Alberque, A. Shanlov, P. Lejay, Phy. Rev. B 44 (1991) 7167. [9] R.C. Budhani, W.L. Holstein, M. Suenaga, Phy. Rev. Lett. 72 (1994) 566. [10] T. Schuster, M. Leghissa, M.R. Koblischa, G. Ischenko, Physica C 203 (1992) 203.
S. Khatua et al. / Nucl. Instr. and Meth. in Phys. Res. B 156 (1999) 44±49 [11] F.J. Ziegler, J. Biersack, Stopping and Ranging of Ions in Matter, Pergamon, New York, 1985. [12] G. Ravikumar, T.V. Chandrasekhar Rao, V.C. Sahni et. al., Physica C, 276, 1997, 9. [13] C.P. Bean, Phy. Rev. Lett. 8 (1962) 250. [14] T. Ishida, K. Ukuda, O. Michikami, S. Okagasu, Physica C 247 (1995) 206. [15] D. Lesuer, A. Dunlop, Radiat. E. Def. in Solids 126 (1993) 163.
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[16] K.A. Muller, M. Takashige, J.G. Bednorz, Phy. Rev. Lett. 58 (1987) 1143. [17] D.S. Fisher, P.A. Mathew, P.A. Fisher, A. Huse, Phy. Rev. B 43 (1991) 180. [18] Y. Yeshurun, N. Bontemps, L. Burlachkov, A. Kapitulnik, Phy. Rev. B 49 (1994) 1548. [19] G. Blatter, Physica C 282 (1997) 19.
www.elsevier.nl/locate/nimb
Vortex dynamics of 250 MeV Ag ions irradiated YBCO ®lms Sanghamitra Khatua
a,*
, Ravi Kumar b, P.K. Mishra c, V.C. Sahni c, R. Pinto
d
a
JaiHind College, Churchgate, Mumbai 400020, India Nuclear Science Centre, New Delhi 1100 67, India c Bhabha Atomic Research Centre, Mumbai 400085, India Tata Institute of Fundamental Research, Mumbai 400005, India b
d
Abstract YBCO thin ®lms with c-axis orientation were grown in-situ by KrF laser on LaAIO3 substrate. The ®lms were irradiated with 250 MeV Ag ions at doses equivalent to B/ ranging from 1 T to 4 T. Critical currents were measured using SQUID magnetometer with ®eld parallel to columnar defects. There is a marked enhancement of vortice pinning due to generation of additional pinning centres. There seems to be an upper limit to the irradiation doses beyond which there is a Ôsmearing eectÕ on Jc . We also ®nd a shift in Irreversibility Line towards higher ®eld which is attributed to better pinning induced by columnar defects. Defect production is examined in the framework of ``Coulomb explosion'' model and the consequences are analysed in terms of defect-induced behaviour within the purview of models such as ``Bean model'', ÔExponential modelÕ and ``Vortex Glass'' model. We point out that the Ôstress modelÕ provides a qualitative explanation for the reduced Tc and a broad transition at high ¯uence. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 60.Gs; 70.Bx; 62.Db Keywords: Irradiation; Vortice pinning; Columnar defects; Critical currents; Irreversibility line
1. Introduction YBCO is an extensively studied HTSC system [1±6]. Magnetic studies have been reported in bulk and single crystals but scarce work is done in the realm of thin ®lm inspite of its better potential for technological application. The most vexed problem for practical application is the disappearance of superconductivity in moderately high magnetic
* Corresponding author. Tel.: +91 11-6893955/6892601; fax: +91 11-6893666; e-mail: [email protected]
®eld and temperature. Some recent reports show that the columnar defects produced by irradiation of heavy ions are highly eective in pinning the ¯ux lines over wide range of temperature and ®eld [7±10] and that the critical current increases dramatically over the unirradiated material. Moreover, these columnar defects have been found to cause a signi®cant change in position and shape of the Irreversibility Line (IL). Since swift heavy ions provide an excellent tool for creating columnar defects, we decided to explore the vortex dynamics of YBCO ®lms in the presence of ``controllably introduced correlated disorder'' induced by heavy ion irradiation.
0168-583X/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 1 6 1 - 5
S. Khatua et al. / Nucl. Instr. and Meth. in Phys. Res. B 156 (1999) 44±49
45
2. Experimental Stoichiometric YBa2 Cu3 O7 (YBCO) target was made by sintering process. Four thin ®lms (5 were grown inmm ´ 5 mm) of thickness 2000 A situ under identical growth parameters by KrF excimer laser on LaAIO3 (1 0 0) substrate. X-ray diraction showed that the ®lms were c-axis oriented. The critical temperature of the as-grown ®lms was 90 K as measured by the four-probe resistivity method. Three of the ®lms were irradiated with 250 MeV Ag14 ions using the 15 UD accelerator at ¯uence values 5 ´ 1010 , 1 ´ 1011 and 2 ´ 1011 ions/cm2 corresponding to ``matching ®elds'' of 1 T, 2 T and 4 T, respectively. This particular beam energy was chosen using TRIM95 [11] so as to ensure best electronic excitation. In addition, the range of Ag14 ion in this energy range being larger than the sample thickness, any undesirable ion implantation in the sample is avoided. The density of columnar defects produced was high enough to pin all the vortices at least for our measurements performed in an applied DC magnetic ®eld upto 5 T. Field-dependent magnetisation measurements with H//c-axis were done using a SQUID magnetometer. For the hysteresis, we adopted the new ``half-scan'' technique instead of ``full scan'' [12]. The advantage of this method over the conventional one is that it yields results independent of scan length, essentially unaected by any deviation in ®eld homogeneity. Thus more accurate measurement is obtained which may otherwise be masked by any ®eld inhomogeneity inherent in the method. All the measurements were done for a 2 cm scan length upto 5 T DC magnetic ®eld with intervals of 500 Gauss. M±H loops were traced for temperatures ranging from 30 K to 70 K. All the ambient parameters were maintained identical for all the ®lms under study to ensure uniformity and consistency in the measurement. 3. Results A typical plot of M±H for T 30 K is shown for one of the samples (Fig. 1) Critical currents (JCM ) values were obtained from the Bean Model
Fig. 1. Magnetisation hysteresis isothermal at 30 K for 5 ´ 1010 ions/cm2 .
[13] JCM 30 DM/d where DM MÿM¯ and d is the thickness of the sample. The applied DC magnetic ®eld being parallel to c-axis is parallel to the ion-tracks created by the Ag ion beam. The critical current closely follows a universal curve with JCM / Bÿn for low ®elds [Fig. 2(a)±(c)]. The correlation of the ¯ux of irradiation and the critical current seems to have a limiting parameter. In contrast to the systematic monotonicity in the JCM vs T plots for the ¯uence values upto 1 ´ 1011 ions/ cm2 , we found that at 2 ´ 1011 ion/cm2 the data points of DM (i.e. JCM ) have an extremely arbitrary and random distribution (Fig. 3). There is no distinctive trend in the JCM vs B plot but rather a kind of ``smudging'' of the distribution pattern. We have also extracted the pinning force FP for dierent ¯uences as well as for the virgin sample. The pinning force FP Jc B is highly critical current dependent. A glance at the plots of FP versus ®eld (Fig. 4) portrays an interesting pattern in its variation. Next we have examined to ®nd the relationship, if any, between the Irreversibility ®eld Hirr and temperature for various ¯uences. For this purpose, we used the constant JCM criterion (JCM 700 A) for the critical current isothermals to extract the Hirr values [14] The JCM ±B plot for the ¯uence 2 ´ 1011 being devoid of any trend or information was not considered for Irreversibility line (IL). The
46
S. Khatua et al. / Nucl. Instr. and Meth. in Phys. Res. B 156 (1999) 44±49
Fig. 3. Variation of critical current with ®eld at 60 K for ¯uence 2 ´ 1011 ion/cm2 (Note the random distribution).
Fig. 4. Variation of pinning force with ®eld at dierent ¯uence values. (Note the position of peak value).
Fig. 2. (a) Variation of critical current with ®eld at dierent temperatures for unirradiated sample. (b) Variation of critical current with ®eld at dierent temperatures for dose 5 ´ 1010 ion/ cm2 . (c) Variation of critical current with ®eld at dierent temperatures for ¯uence 1 ´ 1010 ion/cm2 .
IL plots for dierent samples (Fig. 5) show a rather interesting pattern. The monotonic IL shifts to higher ®eld over the virgin sample for 5 ´ 1010 ¯uence but goes down for the higher ¯uence of 1 ´ 1011 . Moreover, the in¯uence is strong when Hirr is close to B/ and recedes when away from B/ . Finally a simple, nonetheless, signi®cant trend is that of the critical temperature Tc for zero resistivity as measured from the magnetisation
S. Khatua et al. / Nucl. Instr. and Meth. in Phys. Res. B 156 (1999) 44±49
Fig. 5. Variation of irreversible ®eld with temperature for different ¯uence values.
measurement. We found a steady decline in Tc with the increasing degree of irradiation (Virgin ®lm Tc ® 90 K, 5 ´ 1010 ® 87 K 1 ´ 1011 ® 85 K, 2 ´ 1011 ® 75 K). Additionally the transition to Tc (DT Tonset ÿ Tc ) is also broadened instead of being sharp. 4. Discussion During the irradiation, a cylindrical shockwave produced by charged ion creates columnar defects all along the ion track as explained by Coulomb explosion model [15]. Such ``Correlated disorder'' generates an increase in critical current. In YBCO single crystals and the thin ®lm any pinning enhancement is strongly angle-dependent and maximises when the magnetic ®eld is parallel to latent ion tracks because then the track radius coincides with that of the vortex core. Keeping this in view, our samples, were subjected to a DC magnetic ®eld parallel to c-axis i.e. the direction of the ion path. Among the ion-induced eects in our measurements, the reduction in transition temperature Tc , we believe, is due to a possible orthogonal to tetragonal crystallographic transition subsequent to irradiation. At higher ¯uences (2 ´ 1011 ions/ cm2 ) amorphisation sets in i.e. there is a severe
47
lattice strain resulting in a crystalline to amorphous phase transition. This apparently manifests as an extraordinarily random and ``Smeared'' effect in our JC values. The broadening of the phase transition with irradiation can be explained by a kind of ÔstressÕ model. The randomly distributed track positions create mutually dissimilar strains spread all over the sample topology. So it is plausible that a wide distribution of Tc will exist. The sample zones with least strain (i.e. higher Tc ) determine Tonset since they belong to the Ôunaffected-by-strainÕ matrix. In contrast, the superconducting path needed for Tc (R O) certainly pass through regions of higher strain (hence low Tc ) and in the worst scenario, no such path may exist, resulting in a very broad transition and loss of any meaningful superconducting state as seen in the case of ¯ux 2 ´ 1011 ions/cm2 . A major trend in the JC ±B plots is a Ô¯atteningÕ of the JC isothermals vis-a-vis the virgin ®lms. A simple interpretation can be that, the ion damages create small regions of depressed superconducting order parameters with their characteristic spatial extent. As long as the density of defects is low enough so that these regions do no overlap they act as core pinning sites. So, in principle, the more the defects the better for JC . But when the depressed zones begin to overlap, the eective pinning barrier can disappear at a temperature lower than Tc . Such weakening of pinning barrier will have a detrimental eect on JC , particularly at high ¯uence and hence it shows a weak plateau like dependence with ®eld. Another feature is that JC decreases rather slowly with increasing ®eld. For low ®elds, JC ®ts reasonably well to a power law curve with JC / Bÿn . This provides a strong evidence that the ratio B/B/ which is just the average number of vortices per ion track is a relevant parameter to assess the volume pinning force. At low ®elds, each vortex is pinned by an ion track and JC exhibits a power law dependence. For higher ®elds, say B P 2B/ there exists an excess of vortices which can only interact with the pinning centres which were present prior to the irradiation. Consequently JC approaches the exponential scaling curve. Next we turn to an understanding of the characteristic feature in the Fp vs B plot. In the virgin
48
S. Khatua et al. / Nucl. Instr. and Meth. in Phys. Res. B 156 (1999) 44±49
sample the curve shows a peak around 3 T indicating a maximum pinning ®eld. In the irradiated samples of 5 ´ 1010 ¯uence and 1 ´ 1011 ¯uence a similar inclination is seen too. However, a comparison of the two ¯ux values show that in the ¯uence range 1 ´ 1011 the curve begins to decline at 3.2T whereas for 5 ´ 1010 ¯uence the curve is still on an upward trail with no such signature. We conjecture that the vortice-pinning due to columnar defects via irradiation reaches an optimum around 5 ´ 1010 ¯uence beyond which it is probably rather inhibited. Coming to the analysis of IL features, we start from the basic concept that vortices are extended elastic objects that interact with each other as well as with crystallographic defects. Unlike conventional superconductors, vortex dynamics of HTSC such as YBCO is strongly in¯uenced by thermal ¯uctuation. The consequence of this is the existence of a melting line in H±T phase diagram. Above this line, the vortex matter forms a liquid phase, and below it, the vortices freeze into several possible phases including variety of ``glasses''. Invoking the ¯ux Creep and Thermally activated ¯ux ¯ow model (TAFF) [16±19], the expression R / exp (ÿU/kT) yields an enhanced activation energy with reduced resistivity. Hence the irradiation induced defects can reduce dissipative ¯ux motion caused by the Lorentz force and an electric ®eld. The strong in¯uence of the matching ®eld B/ in the IL indicates the existence of an empirical scaling law E/ (J, B + B/ ) E0 (J, B). It is suggestive of the fact that in the high ®eld limit dissipation is primarily due to the motion of excess vortices. These excess vortices move independent of those vortices pinned at ion tracks. As the vortices at these sites do not depin, the activation energy of vortices pinned by columnar defects exceeds that of vortices pinned by background defects of preirradiation state. At lower ®elds (B/ < B < 2B/ ) the vortex-vortex interaction becomes more prevalent thereby preventing all the columnar defects from being occupied. Hence the strong signature is seen around Hirr B/ exclusively. The signi®cant upward shift in the position of IL at 5´1010 ¯uence is indicative of a reduced ¯ux creep rate following production of additional pinning centre via columnar defects.
5. Conclusion Summing up, we have attempted to provide a detailed magnetisation study of the eect of irradiation on the vortex dynamics of YBCO thin ®lms. YBCO ®lms, prepared by pulsed laser deposition were irradiated with 250 MeV Ag ions. Using the Critical state model, the critical current at dierent temperatures and ¯uences were inferred from magnetisation data. The result shows the bene®cial eect of columnar defects on the vortices-pinning although it has an upper limiting value. So, in principle, it establishes the technological potential of columnar defects as a mechanism for pinning enhancement. The variation of pinning force with ¯uence is consistent with our assumption of a optimum ¯uence value of 5 ´ 1010 ion/cm2 . We also sought to gain an insight into the vortex dynamics through the analysis of IL. A point of interest is the creep rate above and below the Ômatching ®eld B/ (where the number of columnar defects and the number of vortices are equal) with a distinct character to the IL in this region. The ``stress model'' suggested corroborates with the eect seen at high ¯uences. In conclusion, new results on the eect of Ag irradiation on YBCO ®lms, adequate models to ®t all the features in our results, as well as reasonable explanations for them, have been provided. References [1] R. Wheeler, M.A. Kirk, A.D. Marwick, L. Civale, Appl. Phys. Lett. 63 (1993) 1573. [2] L. Civale, Supercond. Sci. Tech. 10 (1997) A11. [3] J. Thomas, P. Vergese, P. Scatzle, A. Wetzig, U. Kramer, Physica C 251 (1995) 315. [4] J.T. Kim, C.H. Kim, K.E. Gray, S.S. Choi, H.R. Lim, Physica C 301 (1998) 99. [5] G. Blatter, M.V. Feiglman, A.I. Larkin, V.M. Vinokure, Rev. Mod. Phys. 66 (1994) 1125. [6] E.H. Brandt, Rep. Prog. Phys. 58 (1995) 1465. [7] L. Civale, A.D. Marvik, M.A. Kirk, Y. Sun, J.R. Clem, F. Holtzberg, Phy. Rev. Lett. 67 (1991) 648. [8] M. Konezykowski, F. Alberque, A. Shanlov, P. Lejay, Phy. Rev. B 44 (1991) 7167. [9] R.C. Budhani, W.L. Holstein, M. Suenaga, Phy. Rev. Lett. 72 (1994) 566. [10] T. Schuster, M. Leghissa, M.R. Koblischa, G. Ischenko, Physica C 203 (1992) 203.
S. Khatua et al. / Nucl. Instr. and Meth. in Phys. Res. B 156 (1999) 44±49 [11] F.J. Ziegler, J. Biersack, Stopping and Ranging of Ions in Matter, Pergamon, New York, 1985. [12] G. Ravikumar, T.V. Chandrasekhar Rao, V.C. Sahni et. al., Physica C, 276, 1997, 9. [13] C.P. Bean, Phy. Rev. Lett. 8 (1962) 250. [14] T. Ishida, K. Ukuda, O. Michikami, S. Okagasu, Physica C 247 (1995) 206. [15] D. Lesuer, A. Dunlop, Radiat. E. Def. in Solids 126 (1993) 163.
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[16] K.A. Muller, M. Takashige, J.G. Bednorz, Phy. Rev. Lett. 58 (1987) 1143. [17] D.S. Fisher, P.A. Mathew, P.A. Fisher, A. Huse, Phy. Rev. B 43 (1991) 180. [18] Y. Yeshurun, N. Bontemps, L. Burlachkov, A. Kapitulnik, Phy. Rev. B 49 (1994) 1548. [19] G. Blatter, Physica C 282 (1997) 19.