Direct measurement of the magnetic field penetration depth in YBa2Cu3O7 superconducting films by polarized neutron reflectometry

ELSEVIER

Physica B 241 243 (1998) 1095-1097

Direct measurement of the magnetic field penetration depth in YBazCu30 v superconducting films by polarized neutron reflectometry V. L a u t e r - P a s y u k a'b'*, H.J. L a u t e r c, V . L . A k s e n o v b, E.I. K o r n i l o v h, A.V. P e t r e n k o b, P. L e i d e r e r a Universitiit Konstanz, Fakultiit [~r Physik. Post#wh 5560, D-78434 Konstanz, Germany hJoint hlstituteJbr Nuclear Research, 141980 Dubna, Moscow Region, Russian Federation lnstitut Laue Langet?in, B.P. 156, F-38#42, Grenoble Cedex 9, France

Abstract

A study of the magnetic field penetration depth in a high-temperature (HT~) superconducting film was performed with polarized neutron reflectometry in two scattering geometries. The fit to the reflectivity curves yielded the neutronscattering length density profile and thus a precise image of the composition of the film. The film had a good quality seen by a not noticeable off-specular scattering. The fit to the spin asymmetry gave a magnetic-penetration depth of 1350 _+ 150 ~, along the c-axis at a temperature T = 2 K. The model included an intrinsic exponential decay of the penetration depth. For the first time the spin asymmetry was determined with high resolution over an extended Q-range for a HT~-film. (~, 1998 Elsevier Science B.V. All rights reserved. Kew,ords: High-T~ superconducting film; Magnetic penetration depth; Polarized neutron reflection

In this work, we present the results of the measurement of the London penetration depth 2 [1] of HT,:-superconducting films. Polarized neutron reflectometry (PNR) offers the unique possibility to measure the absolute value of the magnetic penetration depth and the shape of the magnetic flux profile. The discussion in Ref. [2] led to the conclusion that all existing PNR-measurements [3-5] * Corresponding author. Present address: lnstitut Laue Langevin, BP 156, F-38042 Grenoble Cedex 9, France. Tel.: + 33 4 76 20 75 56: fax: + 33 4 76 20 71 20; e-mail:[email protected].

were plagued by the presence of a sizable surface roughness. Also, a more recent study [6] could not overcome this problem. The magnetic flux penetrates from both sides of a superconducting film with decaying magnitude into the film in an external field oriented parallel to the film surface and below the critical field H¢~. The neutron-scattering length density (NSLD) profile for a thin film containing the intrinsic exponential decay can be found e.g., in Ref. [6]. Two reflectivity curves R + and R will be obtained due to the two spin states of the neutron in a magnetic field.

0921-4526/98/$19.00 ,c, 1998 Elsevier Science B.V. All rights reserved PII S 0 9 2 1 - 4 5 2 6 ( 9 7 ) 0 0 8 5 I-X

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Phvsica B 241 243 (1998) 1095 1097

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The experiment was performed at two scattering geometries (discussed in Ref. [7]): reflection from the air film side or from the substrate-film. The advantage of the scattering through the substrate is that the amplitude of the oscillation is higher compared to the reflection from the air film side. This effect is even more pronounced in the spin asymmetry SA = (R + - R )/(R + + R ).However, a disadvantage is that an intensity factor of 5 is lost when the neutrons traverse the SrTiO3substrate. The HTc-films of YBa2Cu307 were epitaxially grown on SrTiO3 with a CeO2 seed layer. The c-axis was perpendicular to the film surface. The quality of the samples was tested by various methods (AFM, X-ray and electron diffraction). The neutron experiments for HTc-films have been performed on the spectrometers S P N / D u b n a and D17/ILL [8]. In Fig. la, two measured reflectivity curves are depicted with the fit to the data. Curve {1) was obtained with the reflection from the substrate side and curve (2) with the reflection from the

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Fig. I. Reflectivity curves of the YBaCuO-film on SrTiO-substrate: 1 indicates the reflection from the substrate film interface and 2 from the air film interface, respectively, at room temperature. The reflection geometries are shown in the sketch on the right side. The spin-asymmetry curves for the two geometries at low temperature are in (bt and (c). In (d) the deduced NSLD profile is shown.

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K Lauter-Pasyuk et al. / Phvsica B 241-243 (1998) 1095 1097

air side half a year later. In the resulting N S L D diagram (Fig. ld) an additional layer is visible at the air-film interface. This surface layer has grown during this half-year from 25 to 125 ,~ consisting presumably of BaCO3. We did not detect any noticeable off-specular scattering from the HTc-film (see Fig. 2) proving the good film quality (see Ref. [6] for a HTc-film with off-specular scattering). The off-specular scattering should be visible at critical Q~ which is marked by an arrow in Fig. 2. Reflectivity curves at low temperatures were taken with an external field of 500 Oe at 4.8 K with reflection from the air side and 300 Oe at 2 K with reflection from the substrate side. The SA is shown in Fig. lb and Fig. lc for the two scattering geometries. 1400 ± 100 and 1350 ± 150,~ were obtained for the penetration depth for the reflection from the air side and substrate side. The complete result is depicted in Fig. id. In this experiment, various parameters of the HT~-film (sample quality, sample size, film thickness, reflection geometries and the film interfaces of the HT~-film) were investigated and optimized. We obtained the details of the film structure in particular at the interfaces. An appreciable Q-range was available

1097

for the determination of the magnetic penetration depth. In this study an exponential character of the penetrating magnetic flux was assumed. The present work was supported by the BMBF under contract Le03Kon and Le04Kon/Dubna.

References [1] F. London, H. London, Proc. Roy. Soc. (London) A 149 (1935) 71. [2] G.P. Felcher, Physica B 192 (1993) 137. [3] R. Felici, J. Penfold, R.C. Ward, E. Olsi, C. Matacotta, Nature 329 (1987) 523. [4] A. Mansour, R.O. Hilleke, G,P. Felcher, R.B. Laibowitz, P. Chaudhari, S.S.P. Parkin, PhysicaB 156&15711989)867. [5] S.V. Gaponov, E.B. Dokukin, D.A. Korneev, E.B. Kljuenkov, V. Lebner, V.V. Pasyuk,A.V. Petrenko, H. Rzany,L.P. Chernenko, Pis'ma Zh. Eksp. Teor. Fiz. 49 (19891277. [6] H. Zhang, J.W. Lynn, C.F. Majkrzak, S.K. Satija, J.H. Kang, X.D. Wu, Phys. Rev. B 52 (1995) 10395. [7] V. Lauter-Pasyuk,H.J. Lauter, V.L.Aksenov,E.1. Kornilov, A.V. Petrenko, P. Leiderer, Physica B, to be published. [8] V.V. Pasyuk, D.A. Korneev, A.V. Petrenko, E.B. Dokukin, Proc. 2nd Int. Conf. Surface X-rayand Neutron Scattering, Bad HonneL Germany, 25 28 June 1991 and Guide to Neutron Research Facilities at the ILL, ILL, Grenoble, 1994, p. 74.