A systematic comparison of transport properties and pinning potentials in YBCO and BiSCCO thin films

Physiea C 235-240 (1994) 2625-2626 North-Holland

PHYSICA

A systematic comparison of transport properties and pinning potentials in YBCO and BiSCCO thin films* R.M.Schalk"', G.Samadi Hosseinali"', H.W.Weber", S.Griindorfer b, D.Biiuerle ~, S.ProyeP, P.Wagner d, U.Frey d and H.Adrian d. a Atominstitut der Osterreichischen Universitliten, SchiittelstrafAe 115, A-1020 Wien, Austria b ELIN Energieanwendung, A-1140 Wien, Austria Institut ftir Angewandte Physik, Universitiit, A-4040 Linz, Austria d Institut fur Festk6rperphysik, D-64289 Darmstadt, Germany c-axis oriented single crystalline YBazCU3OT.~ and Bi2Sr2Ca2Cu30~o thin films were patterned for transport measurements to determine the critical current density J~"t'(H,T,0). Film to film variations of J~ were compared for a large number of YBCO samples to assess the dominant influence on the magnitude of J~ and possible correlations with the film growth conditions. A comparison of J~(0) for YBCO and Bi-2223 demonstrates the fundamental differences in their flux line lattice, especially at high temperatures. 1. I N T R O D U C T I O N While magnetic measurements on bulk single crystals probe a volume average of the local current density, which is entirely determined by the defect structure relevant for pinning, transport measurements are always sensitive to the weakest part of the current path. Therelore, Jc is determined not only by nm-sized defects, but also by larger scale defect structures, which may represent local barriers for the transport current. We try to clarify the complex relation between transport current and ~;~na:l (nmsized) or iarg e (>100nm) defect structures. -7he~v relations also influence the anisotropy of J~. 2. E X P E R I M E N T A L YBCO thin films were produced by pulsed laser deposition on MgO substrates [1], while Bi-2223 was sputtered onto SrTiO~ substrates [2]. In the case of YBCO, three different series (code P3,P4,P6)~ deposited at different substrate temperatures (686700°C) and at different times over a period of several months, were studied. Each sample was etchstructured for transport measurements and split into four parts, each of which containing three current bridges. This enables us to compare twelve separate

current bridges, which should nominally show the same properties. In the case of Bi-2223 only two bridges were measured so far. 3. RESULTS The largest sample variation of J~ was observed in the P3-series of YBCO fihns. It could vary by a factor of 10 among the three bridges of one pattern. Since magnetic hysteresis effects are observed upon cycling the field at fixed angular position and upon rotating the sample at fixed field, this variation is attributed to a weak ii.~k su-uctn,'e of tl.z classical grain boundary type. In the P4 and P6 series (higher deposition temperatare) the J, variation was smaller (factor 2-3). While most of the P4 samples reached J~-values above 10~A/m 2 at 20K, the data on P6 were typically lower by a factor of 2, which may be due to the lower T~ values in P6 (-86-87K) comparediv P4 (87.5-90K). In order to elimin?tc the influence of large scale d-"-~" v,.,,u,,,, C,=~.tS, the "'^~ . . . . . . v ........ ' ' ; " " ~ fo""e ,.. Pv '.". .~. , nc~rm.qliT~cl .............. by its maximum. Good scaling was found in YBCO at those temperatures, where the maximum of Pv was accessible to experiment (60-90K). Fits to the scaled curves are shown for several samples in Fig 1. In the case of weak !ii::ks fP321) the high field tail

*This work is supported in part by the "Fond zur F6rderung der Gcw,.~rblichen Winschaft", Wien (D.B.,',:,.P.), and by the "Bundcsministenum fi.ir Forschung und Technologie", Bonn, under contract #13-N-5748-A (P.W.,U.F.,H.A,). 0921-4534/94/$07.00 © 1994 - F.Iscvier Science I3.V. All rights reserved. SSDI 0921-'~534(94)01878-2

2626

R.M. Schalk et al./Physica C 235-240 (1994) 2625-2626

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Fig. 1: Fitted scaling curves of Pv.

Fig, 2: Pinning potential for YBCO and BiSCCO.

of the scaled curve is strongly suppressed. The other scaling results indicate only small differences in the local defect concentration. It seems to be highest in the P4 series, although no correlation of the magnitude of J~ and the shape of the high field tail was found. Samples with considerable differences in Jc exhibited the same scaled curves in this series. STM investigations of some YBCO films showed the usual growth spirals and a very high density of small defect areas (holes) in the spiral terraces [3]. Therefore, we believe that the local defect density is already optimized and that the variation of Jc is due to large scale dcfect areas such as tilted regions or joints between two growth spirals (step dislocations). This view is suppot'ted by neutron irradiation experiments, which showed no change of Jc after irradiation. The angular dependence of J~ shows 2D-scaling in YBCO up to 20-40K [4], while 2D-behavior is observed in the whole temperature range in Bi-2223. This may be explained by a different flux line structure. An analysis of the E(J)-curves in terms of the Kim-Anderson model [5] yields the temperature .n~'p-nd~'-c" ........ o ~"the ,',;v,n,,,,,~,-; . . . . v ---,~,,,,,-,'°""'~l(~;g.,.,_ox. xxzh;1,:,., ,,,,,.. a strong increase and therefore a more 3D-like structure of the flux lines is observed in YBCO, Uo r e m a i n s small in B i - 2 2 2 3 e v e n at h i g h e r temperatures. At 20K the I t anisotropy of YBCO is similar in all samples and only determined by the

field dependence IIc, while it varies considerably at 77K, with the highest values in the P4 series. The anisotropy at high temperatures seems to be reduced in some cases by a defect structure, which leads to a shoulder behavior of Jc(0) that is strongly field dependent and hysteretic [4], and can be linked-to the elastic properties of a 3D-like flux line. Such effects are absent in Bi-2223. 4. S U M M A R Y

The variation of Jc in different current bridges of the same batch of YBCO films is most likely due to large scale barriers for the transport current and not to a variation in the local defect density. In order to improve Jc, it is necessary to reduce the number of these unwanted growth features and to increase the distance over which the CuO 2 layers remain undisturbed. REFERENCES

[1] D. B~iuerle, Appl.Phys. A 48 (1989) 527 p.579 (1993) [3] J.Sumrr, hammer et al., submitted to Physica C (1994) [4] R.M.Schalk et al., Cryogenics 33 (1993) 369 [5] R.M.Schalk et al., Phys.Rev.B 49 (1994) 3511