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APPLIED PHYSICS LETTERS 89, 213111 2006

Grain boundary weak link in a-b plane in MgB2 film
E. Stepantsov
Institute of Crystallography, RAS, Moscow 119333, Russia

M. Tarasova
Institute of Radio Engineering and Electronics, RAS, Moscow 125009, Russia

M. Naito and A. Tsukada
Tokyo University of Agriculture and Technology, Tokyo 184-8588, Japan

D. Winkler
Chalmers University of Technology, GЖteborg 41296, Sweden

Received 27 July 2006; accepted 16 October 2006; published online 22 November 2006 The authors have fabricated the MgB2 bridges 850 ­ 6000 nm in width on bicrystal 111 MgO substrates with in-plane grain boundaries of the two types: 13 ° / 13° 110 and 13 ° / 13° 112 . Annealing in oxygen of the films on 110 bicrystal substrates leads to a systematic decrease of critical current, a widening of the transition temperature width, and an improvement of the shape of IV curve that finally looks more like a Josephson junction. They have measured a response of such samples to the microwave radiation at 110 GHz with the voltage amplitude up to 0.5 mV. © 2006 American Institute of Physics. DOI: 10.1063/1.2396917 We suppose that a 111 magnesium oxide substrate is suitable for a 001 MgB2 film growth due to their close lattice constants in the planes, that is, a = 0.298 nm in 110 for the former and a = 0.308 nm in 110 for the latter. In order to obtain a weak link we grow a MgB2 film on a bicrystal substrate. This provides an inheritance of the artificial grain boundary from the substrate into the film. We would expect a formation of an oxide barrier between two superconducting parts due to more active oxygen diffusion in the region of boundary. The B2O3 boundaries of 1 ­ 3 nm thickness between MgB2 grains were observed by scanning transmission electron microscopy in Ref. 1. It could also be a metallic MgB4 Ref. 2 or amorphous MgB2 boundary of 5 ­ 20 nm thickness separating MgB2 crystalline grains as was reported in Ref. 3. These types of boundaries can give rise to superconductor-normal metal-superconductor SNS or superconductor-insulator-superconductor SIS types of Josephson junctions. Bicrystal 111 MgO substrates were produced by a solid phase intergrowing method.4 For that the two pieces of MgO single crystals were put together in a symmetric crystallographic position, contacting by flat surfaces oriented parallel to 110 -13 ° and 110 +13 ° correspondently for the one bicrystal type and parallel to 112 -13 ° and 112 +13 ° for the other. The pieces were rotated in the surface of contact in such a way that their 111 planes were parallel to each other. In such position the systems were welded. The produced bicrystals were cut parallel to common 111 plane and polished for fabricating bicrystal substrates. Thus, we fabricated the two types of bicrystal substrates oriented as 111 with artificial grain boundary perpendicular to the surface and with symmetric rotation of crystal lattice by 2 13°. In the first bicrystal the boundary makes angles 13° to the axes 112 , and in the second one to the axes 110 . For comparison the 111 single crystal substrates were also fabricated.
a

MgB2 films were grown by coevaporation in a customdesigned ultrahigh-vacuum chamber the base pressure 5 10-10 Torr from pure Mg and B sources using multiple electron guns.5 The growth temperature was 280 ° C and the

Electronic mail: mikhail.tarasov@mc2.chalmers.se

FIG. 1. X-ray diffraction phi scan of 001 MgB2 film from planes of 101 type in top panel a and 111 MgO substrate from planes of 311 type in bottom panel b .

0003-6951/2006/89 21 /213111/2/$23.00 89, 213111-1 © 2006 American Institute of Physics Downloaded 22 Nov 2006 to 129.16.137.132. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp


213111-2

Stepantsov et al.

Appl. Phys. Lett. 89, 213111 2006

FIG. 3. Voltage response at 110 GHz in dependence on bias current. FIG. 2. IV curves for annealed at 450 ° C bicrystal bridge 0.9 m wide measured at temperatures from 25.2 to 29.8 K.

growth rate was 3.8 е / s. The evaporation rate of Mg to that of B was set three times as high as the nominal rate so as to compensate the loss of Mg due to reevaporation. Heterostructures consisting of 111 MgO substrate and 001 MgB2 film were studied with a four-circle x-ray diffractometer X-Pert Philips in a phi-scan mode. As an x-ray reflection plane for 111 MgO substrate we chose 311 plane types. For a film analysis the 112 reflection planes were used. One can see that the reflection angles from the film 112 planes Fig. 1 a are shifted by 60° compared to the reflections from 311 planes of substrate Fig. 1 b . This means that our 001 MgB2 films are epitaxial, single domain, and their crystal lattice is linked to a 111 MgO substrate lattice. The MgB2 films were patterned in a form of microbridges 0.9 m in width and 6 m in length and integrated with log-periodic planar antennas. The IV curves measured immediately after the fabrication show critical currents of several milliamperes and a normal resistance of 20 ­ 60 . The annealing in the oxygen of films on bicrystal substrates leads to a systematic decrease of critical current, a widening of the transition temperature width from 2 to 5 K, and an improvement of the curve shape that finally looks more like a Josephson junction see Fig. 2 . In a single-crystal case IV curves did not show significant changes. For the second bicrystal sample 112 the annealing at 600 ° C completely suppressed critical current and increased resistance over 1 k for widest junction and to infinity to the rest ones. We can also mention a specific ripple structure in IV curves at the temperatures close to the critical one. It is similar to regular step structures due to successive establishment of

phase slip centers that were observed in long superconducting thin film microbridges.6 We have also measured a voltage response for such bicrystal MgB2 samples to the microwave radiation at 110 GHz see Fig. 3 . Response maxima, in general, follow to the dependencies of dynamic resistance around phase slip features. For lower temperatures down to 4 K the IV curves are hysteretic and further improvement can be achieved by optimizing misorientation angle, annealing time, and annealing temperature. For comparison with other MgB2 weak links we can refer to Ref. 7 where Josephson junctions were fabricated using focused ion beam milling. For a junction with small value of the critical current Ic =3 A at 4.2 K , they observed Shapiro steps when the junction is irradiated with microwave power at 9 GHz. Response in this case does not exceed few microvolts. This work was supported by Swedish agencies VR and SI, and Russian agencies RFBR 05-02-19650 and FASI 02.434.11.1010.
1

R. F. Klie, J. C. Idrobo, N. D. Browning, A. Serquis, Y. T. Zhu, X. Z. Liao, and F. M. Mueller, Appl. Phys. Lett. 79, 1837 2001 . 2 J. Q. Li, L. Li, Y. Q. Zhou, Z. A. Ren, G. C. Che, and Z. X. Zhao, Chin. Phys. Lett. 18, 680 2001 . 3 H. Narayan, S. B. Samanta, A. Gupta, A. V. Narlikar, R. Kishore, K. N. Sood, D. Kanjilal, T. Muranaka, and J. Akimitsu, Physica C 377, 1 2002 . 4 E. A. Stepantsov, A. S. USSR No. 1116100 cl. C30B33/00 1982 Bull. Izobreteni 36, 77 1984 . 5 K. Ueda and M. Naito, J. Appl. Phys. 93, 2113 2003 . 6 W. Skocpol, M. R. Beasley, and M. Tinkham, J. Low Temp. Phys. 16, 145 1974 . 7 A. Malisa, M. Valkeapaa, L.-G. Johansson, and Z. Ivanov, Physica C 405, 84 2004 .

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