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Physica C 326 ­ 327 Z1999. 79 ­ 82

Fabrication and properties of high-Tc ramp junctions with manganite barriers
M.Q. Huang ) , Z.G. Ivanov, P.V. Komissinski, T. Claeson
Department of Microelectronics and Nanoscience, Chalmers Uniуersity of Technology and Goteborg Uniуersity, S-412 96 Goteborg, Ё Ё Sweden Received 25 April 1999; accepted 15 July 1999

Abstract We report on fabrication and characterization of YBa 2 Cu 3 O 7y d ZYBCO. ramp junctions with a barrier of doped manganite, La 0.67 Sr0.33 MnO 3 ZLSMO.. Multilayers of YBCO and LSMO are epitaxially grown on Z100. SrTiO 3 substrates by on-axis pulsed laser ablation. The resistance and current ­ voltage characteristics of the ramp junctions have been measured as a function of barrier thickness, in the range of 10 ­ 30 nm, and of temperature. Junctions with 20 nm thick barriers had RSJ-like current ­ voltage characteristics with a small excess current. The critical current density and normal state resistance of the junctions at 4.2 K were 5 = 10 3 Arcm2 and 300 V , respectively. q 1999 Elsevier Science B.V. All rights reserved.
Keywords: YBCO ramp junction; Manganite; Multilayer

1. Introduction Recently, doped manganites have attracted much attention on account of their colossal magnetoresistance ZCMR. w1,2x. Most studies were on spin-dependent tunneling between two ferromagnetic ZFM. films across an insulator ZI., while only a few reports were focused on the tunneling between oxide superconductors ZS. separated by a manganite barrier w3 ­ 5x. A superconducting current through 200 ­ 500 nm thick La 0. 7 Ca 0.3 MnO x ZLCMO. barriers was reported in

Corresponding author. Fax: q46-31-772-3224; E-mail: huang@fy.chalmers.se

)

c-axis trilayer junctions of YBa 2 Cu 3 O 7y drLa 0.7Ca 0.3 MnO xrYBa 2 Cu 3 O 7y d ZYBCOrLCMOrYBCO. at 9 K w4,5x. However, in other YBCOr LCMOrYBCO junctions, with a barrier thickness of 80 ­ 130 nm, no superconducting current was observed even at 4.2 K w3x. The results are contradictory and reflect the complexity of the epitaxial growth of the superconductive and manganite perovskites. In addition, the supercurrent transport through the LSMOrYBCO interface in these junctions is hindered by the short coherence length in the c-direction of YBCO. Ramp type junctions allow the study of the Josephson effect in the a ­ b plane of the superconductor w6x. We will report on the fabrication and properties of YBCO ramp junctions with thin barriers of LSMO. A prerequisite for epitaxial growth

0921-4534r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Z 9 9 . 0 0 4 1 8 - 9


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M.Q. Huang et al. r Physica C 326 ­ 327 (1999) 79 ­ 82

of such structures is the small lattice mismatch of 2% in the a ­ b planes of YBCO and LSMO and the similar growth conditions at elevated temperatures, 7008C ­ 8008C, in oxygen environment. We deposited YBCOrLSMOrYBCO multilayers by on-axis pulsed laser ablation and fabricated YBCO ramp-type Josephson junctions by using La 0. 67 Sr0.33 MnO 3 as a barrier material. The surface smoothness of the ramps in the YBCO bottom electrode was studied by AFM and correlated with junction transport properties. The resistance and current ­ voltage characteristics of the junctions were measured at different temperatures.

electrode were different from that of the bottom electrode. A deposition temperature of 7408C and oxygen pressure of 0.2 mbar were applied to avoid an interdiffusion at the interface. The thickness of barriers was chosen to be in the range of 10 ­ 30 nm. A 200 nm thick Au film was deposited and contact pads were defined by ion milling. The top electrode was patterned by Ar ion milling under an angle of 458. The texture of the multilayer YBCOrLSMOr YBCO was analyzed by X-ray diffraction u ­2 u scan and v scans. The temperature dependence of the resistance and current ­ voltage characteristics of junctions were measured by using a four-point method.

2. Experimental details Four-layer structures consisting of a base YBCO superconducting electrode, 150 nm thick, and a complex insulator, PBCO Z50 nm.rSTO Z50 nm.rPBCO Z50 nm., were deposited on Z100.SrTiO 3 substrates by on-axis pulsed laser deposition at 8008C in 0.25 mbar oxygen pressure. The laser energy density was 1.2 Jrcm2 at a pulse frequency of 10 Hz. The films were cooled rapidly to 5008C at the pressure specified above, treated at 5008C in oxygen atmospheric pressure during 30 min and slowly cooled to 3808C for 30 min at the rate of 18Crmin. In order to optimize the deposition conditions of LSMO, YBCOrLSMOrYBCO multilayers were separately deposited on STO substrates at oxygen pressures of 0.2 mbar and temperature 7408C. The ramps were etched by an Ar ion beam under an angle of 458 normal to the substrate plane while the substrate was rotated. An ion energy of 300 eV and ion beam current density of 0.2 mArcm2 were used for the ramp etch. The ramp layout was defined by a mask of hard baked S 1813 photoresist. After resist removal by an O 2 plasma, the ramps were cleaned, in sequence, by an Ar ion beam with ion energy of 100 eV and beam current density of 0.1 mArcm2 for 30 min, and by a low energy ArrO 2 plasma. The substrates were then heated again to the deposition temperature in 0.4 mbar oxygen and held for 30 min to recrystallize the ramp surface. The La 0. 67 Sr0.33 MnO 3 barrier was epitaxially deposited and covered in situ with the top YBCO electrode. The deposition conditions of the LSMO and the top 3. Results and discussion The critical temperature of the YBCO films was 88 ­ 90 K with a transition width less than 0.5 K. The ramps were characterized by AFM. Ramps with roughness of less than 8 nm and angles of 268 were obtained under the above specified conditions. In order to minimize the interdiffusion between LSMO and YBCO at elevated temperature the deposition processes for LSMO and YBCO films were optimized. With an oxygen pressure of 0.2 mbar, the temperature of the substrate during the deposition of the barrier and the counter electrode was about 608C lower than that during the deposition of the bottom electrode. To test the film quality under these conditions, a test sample of a YBCOrLSMOrYBCO multilayer was grown on an STO substrate. YBCO and LSMO films were epitaxially grown with a pure c-axis alignment as indicated by the X-ray diffraction u ­2 u scan pattern and by rocking curves shown in Fig. 1. Ramp-type junctions with different thicknesses of the LSMO barriers were studied. Junctions with 30 nm LSMO barriers had no superconducting current at 4.2 K. The temperature dependence of the resistance is shown in Fig. 2 by curve Za.. A drop in the R ­ T curve at 90 K indicates the transition of the YBCO electrodes into the superconducting state. At this temperature, a single LSMO film should be in a metallic state and a resistance of a few ohms should be measured. However, in our case we observe an


M.Q. Huang et al. r Physica C 326 ­ 327 (1999) 79 ­ 82

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Fig. 1. X-ray diffraction pattern of a YBCOrLSMOrYBCO multilayer deposited on an STO substrate. The inset shows the rocking curves of LSMO Z002. and YBCO Z005. reflections.

nm had a large superconducting current with fluxflow type I ­ V characteristics. This probably indicates the existence of shorts in the barrier. When the barrier thickness was 20 nm, the junction had a temperature dependence of the resistance as shown by curve Zb. in Fig. 2. Fig. 3 shows Za. the I ­ V characteristics and Zb. their dVrd I ­ V curves for an 8-m m wide junction with a 20-nm thick barrier at different temperatures. Additional conductivity is observed at high bias voltage in I ­ V curves at low temperature, while the conductivity is constant with bias voltage at 46 K. Subgap structures were seen in the dVrd I ­ V curves, and their locations are independent of temperature. A critical current density of the junction is 5.0 = 10 3 Arcm2 at 4.2 K. The normal resistance of 300 V was determined at a voltage of about 100 mV. This means that the char-

increase in the resistance that can be related to the interface between YBCOrLSMO or to oxygen depletion of the LSMOrYBCO films. The inset shows the I ­ V curve of the junction at 4.2 K. The normal resistance of the junction was 330 V at 4.2 K. Junctions with LSMO barrier thicknesses of about 10

Fig. 2. The temperature dependence of the resistance of YBCO ramp-type junctions with Za. 30-, and Zb. 20-nm thick LSMO barriers. The inset shows the I ­ V curve of the junction in Za. at 4.2 K.

Fig. 3. Za. The I ­ V curves and Zb. dV rd I ­ V curves of a YBCO ramp-type junction with a 20 nm thick LSMO barrier at different temperatures.


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M.Q. Huang et al. r Physica C 326 ­ 327 (1999) 79 ­ 82

4. Conclusion In summary, we have deposited YBCOrLSMOr YBCO multilayers by on-axis pulsed laser ablation and fabricated YBCO ramp-type Josephson junctions with LSMO barriers on STO substrates. The temperature dependence of the resistance and I ­ V characteristics of the junctions were measured. The thickness of the LSMO barrier has obvious effects on the properties of the junctions. A superconducting current and too high normal resistance were observed in some junctions, but the origin of the high normal resistance is unknown at the moment.
Fig. 4. The temperature dependence of critical current and normal resistance of an 8-m m wide ramp junction with a 20-nm thick barrier. ZThe lines are used only to guide the eyes..

Acknowledgements The author would like to thank P. Larsson, J. Blomgren, and A.Ya. Tzalentchuk for useful discussions. The work is supported by the Swedish foundation for strategic development ZSSF. under contract ``Oxide heterostructures and novel electronic devices'' and by the Materials consortium on superconductivity.

acteristic voltage of the junction is about 40 mV. The temperature dependences of the critical current and the normal resistance of the junction are depicted in Fig. 4. Comparing the parameters of junctions with different barrier thicknesses, an interface resistance of 240 V was estimated for these junctions. It means that the junction properties are dominated by the SrFM interfaces. Even if the interface resistance were subtracted from the total resistance of the junctions, the calculated value of the barrier resistivity at 4.2 K was 0.36 V cm and this is still about two orders of magnitude higher than the usual value in bulk materials. A high resistance was reported in c-axis trilayer junctions with LCMO barriers by Bari et al. w3x. So far, we cannot figure out whether the higher resistance is due to an interface diffusion or is an intrinsic effect of the LSMO barrier. An oxygen depletion at the interface and inside the barrier may be a possible origin for the high resistivity. A further investigation is in progress.

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