Transport and structural properties of thin (Bi, Pb)2Sr2Ca1Cu2O8+δ films prepared by DC sputtering

PHYSICA

Physica C 221 (1994) 405-412 North-Holland

Transport and structural properties of thin (Bi, Pb ) 2Sr2Ca 1Cu208 + films prepared by DC sputtering R. Henn 1, T. Kroener, J. Geerk, G. Linker and O. Meyer lnstitut far Nukleare Festkrrperphysik, Kernforschungszentrum Karlsruhe, POB 3640, 76021 Karlsruhe, Germany

Received 25 November 1993

Superconducting thin films of the two CuO2 layer phase in the Pb-doped Bi-Sr-Ca-Cu-O system have been fabricated on (100) MgO and (100) SrTiO 3 substrates by DC magnetron sputtering and ex-situ annealing. The best films were prepared at annealing temperatures just below the melting temperature. On both substrates the 2212 films were grown with the c-axis perpendicular to the substrate surface. Rocking curves revealed mosaic distributions of the crystal grains between 0.3 ° and 0.8 ° underlining the highly textured growth of the films. Films on SrTiO3 showed in-plane epitaxial growth, while films on (100) MgO showed a random orientation in the a-b plane with four dominating growth directions. Weak links caused by different orientations in the a-b plane for films on MgO may be the reason for reduced transport properties compared to films on SrTiO3. Films on (100) SrTiO3 showed T¢ values up to 88 K, critical current densities in the range of 5 × 105 A/cm 2 at 4.2 K, while films on MgO reached Tc values up to 80 K andjc values up to 6× 104 A/cm 2.

1. Introduction In the superconducting Bi-Sr-Ca-Cu-O (BSCCO) system, first described by M a e d a et al. [ 1 ], three well defined phases exist with a c o m p o s i t i o n o f Bi2Sr2Ca,_ ~Cu,O2,+4+~ with n = 1, 2 a n d 3. The superconducting transition temperature, To, in these phases increases with n, i.e. with the n u m b e r o f adj a c e n t CuO2 planes. M a x i m u m Tc values o f 20 K, 85 K and 110 K, respectively, have been reported for the three phases. These values are strongly depending on a correct oxygen i n c o r p o r a t i o n [ 2 ]. In order to favour the f o r m a t i o n o f the 110 K phase, Pb has been a d d e d by substituting a portion o f Bi in the 2223 c o m p o u n d [ 3 ]. Since the discovery o f the BSCCO c o m p o u n d s quite a n u m b e r o f groups have reported on the preparation o f thin films o f these superconductors, mainly o f the 2212 ( n = 2 ) a n d 2223 ( n = 3 ) phases. Thin films are required for m a n y basic investigations and also for applications especially if their properties are superior to those o f b u l k material. Different prepaPresent address: Max-Planck-Institut •r Festk6rperforschung, Heisenbergstr. 1, 70569 Stuttgart, Germany.

ration methods like laser ablation [4,5 ], molecular b e a m epitaxy [6 ], liquid phase epitaxy [7 ] a n d magnetron sputtering [ 8 - 1 0 ] were used. The results o b t a i n e d for the different p r e p a r a t i o n m e t h o d s are quite similar. The range o f deposition temperatures in which films o f high quality were o b t a i n e d is very small [6,9,10]. Nearly single-phase films o f the superconducting 2212 a n d 2223 c o m p o u n d s can be p r o d u c e d but in c o m p a r i s o n to the YiBa2Cu307 syst e m the growth quality is reduced. Therefore m a n y groups are still involved in the o p t i m i z a t i o n o f the deposition p a r a m e t e r s to i m p r o v e the growth and thus also the transport properties o f thin films in the BSCCO system. In this p a p e r we report on our experience on the p r e p a r a t i o n o f BSCCO superconducting thin films. In our investigations we a p p l i e d in-situ sputtering at elevated substrate temperatures, Ts, a n d also ex-situ annealing at higher temperatures, Tan, o f films deposited at low Ts. The m e t h o d o f ex-situ annealing allowed us to prepare 2212 films with high reproducibility. It gave interesting insight into the growth m e c h a n i s m o f the films and the influence o f different substrates on the structural and transport properties. F u r t h e r we o b t a i n e d strong indications for a

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R. Henn et aL /Pb-doped B i - S r - C a - C u - O films

changing oxygen content in the films by varying the annealing temperature. Therefore we concentrate on the results of the ex-situ annealing experiments.

2. Experimental The films were deposited by DC sputtering applying an inverted cylindrical magnetron device [ 11 ] in a 1 : 1 mixture of oxygen and argon at a total pressure of 0.5 mbar. We used a single compound target with a nominal composition of Bil.6Pbo.4Sr2CazCu3Ox. The substrates, polished MgO (100) and SrTiO3 (100) single crystals, were placed unclamped on a stainless steel oven heated resistively with a coaxial heating wire. The substrate temperature, Ts, was measured with a Ni-CrNi thermocouple mounted in the center of the oven. Usually the deposition occurred at temperatures of about 440 ° C on substrates having a dimension of 5 × 10 × 1 mm 3. The deposition rate amounted to 16.5 nm/min. The deposition time was 20 min resulting in film thicknesses of about 330 nm. After deposition the films were annealed ex-situ in air at different temperatures Tan (780-920°C) usually for 45 min. The temperature in the furnace was controlled by a thermocouple situated near the sample. The films were heated up at a rate of 15 K / m i n and cooled down to room temperature at a rate of about 3 K/min. We determined the composition and thickness of the films by Rutherford backscattering spectrometry (RBS) using 2 MeV He + ions. Because of the insufficient mass separation of RBS for high masses we cannot distinguish between the elements Bi and Pb. Additional measurements by energy dispersive analysis of X-rays (EDAX) allowed us to prove the Pb content in the films. We found Pb in the films with a content of about 1 at.%. The structure and growth quality were measured by X-ray diffraction on a twoaxis goniometer in Bragg-Brentano focusing geometry with Cu Kctl radiation supplied by a rotating anode generator operated at 8 kW. In addition co and scans were performed to analyse the grain orientation perpendicular and parallel to the substrate surface. We searched for polycrystalline fractions and foreign phases in the films by X-ray diffraction in the Seemann-Bohlin focusing geometry. The surface

of the films was investigated by scanning electron microscopy (SEM). The temperature dependence of the resistance, R (T), was measured using a standard four-probe DC method. In addition to resistive Tc measurements inductive Tc measurements were performed. For resistive Jc measurements the films were covered with a special mask and patterned with bridges of 2 mm length and 0.2 mm width by 300 keV He + ion irradiation at RT. The fluence of the He + ions was chosen such that the irradiated part of the film was completely transferred into an insulating amorphous phase. For the determination of the critical current density we applied a 5 g V / c m criterion.

3. Results and discussion The as-deposited films were insulating, brown and shiny and had an average composition of (Bi, Pb)2Sr2Ca~.sCu2.60,. In the X-ray diffraction patterns no reflections could be found, indicating an amorphous state of the films. After the ex-situ annealing procedure films treated below 920°C were black and shiny, those annealed at temperatures above 920°C became grey and rough which we interpret as a sign of melting. Therefore, in the following the temperature of 920 °C on our scale is referred to as the melting point, Tm. In films annealed at temperatures below Tm we found no significant change in the composition as compared to that before annealing. For annealing temperatures, Tan, ranging from 100 K below Tm up to Tm we obtained superconducting films of the 2212 phase on (100) MgO and (100) SrTiO3 substrates. The films were all grown with the c-axis perpendicular to the substrate surface. An example of a resistance versus temperature curve of a 2212 film on SrTiO3 is shown in fig. 1. In the inset the AC signal versus temperature is presented. Both signals, the DC and the AC signal, show a sharp transition into the superconducting state. Typical for all films discussed here is the linear behavior of the resistance versus temperature above 120 K. The resistivity at 120 K, p( 120 K), typically was about 400 p.D cm. A small resistance drop at 110 K was also typical for all films discussed here and is obviously caused by the presence of the 2223 phase.

R. Henn et al. / Pb-doped B i - S r - C a - C u - O films

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between 0.3 ° and 0.8 ° underlining the highly textured growth of the films. But in addition, X-ray diagrams in Seemann-Bohlin geometry showed many small peaks indicating polycrystalline fractions and foreign phases. Our investigations of the surfaces of the films by SEM gave a possible explanation for the reflection found in the Seemann-Bohlin geometry. A typical SEM picture of a surface is shown in fig. 3. The dendrite-like morphology of the surface in principle is polycrystalline and probably gives rise to some small peaks in the Seemann-Bohlin spectra. This morphology of the surface may also be a reason why the ex-situ annealed films revealed no channeling behavior. In the following we present in more detail the results of growth and properties obtained by the variation of the deposition temperature Ts and mainly of the annealing temperature Tan. Films sputtered at Ts below 550°C showed no reflection in the diffraction patterns including those measured with high sensitivity in Seemann-Bohlin geometry indicating an amorphous structure, while films deposited at temperatures above 550°C were at least partially crystalline. A variation of the deposition temperature below 550°C, i.e. below crystallization, had no influence on the properties of the films in the subsequent annealing procedure. Films deposited at Ts above 550°C, however, revealed after annealing a decreased quality compared to those deposited at lower T~ resulting in a lower zero-resistance temper-

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Fig. 2. X-ray diffraction pattern in 0--20 geometryand m scan through the (0012) planes (inset) for a 2212 film on MgO. The fraction of this phase, however, must be very small because we found no sign of the 110 K phase in the AC signal and in the X-ray diffraction diagrams. A typical X-ray diffraction pattern in Bragg-Brentano geometry ( 0 - 2 0 scan) and an oJ scan through the 0012 planes (inset) of a 2212 film on MgO is shown in fig. 2. The X-ray diffraction measurements in Bragg-Brentano geometry revealed only 00l peaks of the 2212 phase with a lattice parameter c of about 30.85 ]~. This indicates textured growth with the caxis oriented normal to the substrate surface, o~ scans revealed mosaic distributions of the crystal grains

Fig. 3. SEM picture of a surface of a 2212 film on MgO (marker 1 ~m).

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ature. T~o, a lower resistivity ratio, r = R ( 2 7 3 K ) / R( 120 K ) , and a b r o a d e n i n g o f the mosaic spread. Therefore, the films usually were deposited at a temperature of 440 ° C. As already m e n t i o n e d we found a 100 K wide range o f annealing temperatures, in which superconducting 2212 films were obtained. In fig. 4 the onset o f superconductivity, T °" and T~o is plotted versus the annealing t e m p e r a t u r e for films on SrTiO3 and MgO. F o r films on both substrates we observed an increase o f 1) with higher annealing temperatures. Especially for films on SrTiO3 the highest values for Tco (up to 88 K ) were reached just below the melting temperature. Films on SrTiO3 reached slightly higher values as c o m p a r e d to films on MgO. Similar to the behavior o f Tc we measured for films on both substrates an increase of the resistivity ratio with increasing 7",,, with m a x i m u m values o f 2.3 for films on MgO and for films on SrTiO3. As a measure o f the growth quality o f the films we have taken the mosaic spread d e t e r m i n e d from rocking curves in X-ray diffraction experiments. In fig. 5 the full width at half m a x i m u m ( F W H M ) o f c~ scans with the detector set for the detection of the (0012) line is plotted versus 7",,, for films on both substrates. We observed a small decrease o f the F W H M with increasing Tan with best values o f 0.3 ° for films on MgO and 0.45 ° for films on SrTiO~ reached at the m a x i m u m a p p l i e d temperature. In addition to an i m p r o v e m e n t o f the growth quality we also observed an increase o f the c-axis lattice con-

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Fig. 5. Full width at half maximum (FWHM) of ¢o scans with the detector set for the detection of the ( 0012 ) line vs. annealing temperature for films on MgO (left) and ( b ) on SrTiO~ ~right L stant with increasing 7],. In bulk samples of the 2212 phase it frequently has been observed [ 12-15 ], that the c-axis lattice constant and T~. d e p e n d on the oxygen content o f the samples which leads to a mutual 7~ versus c correlation revealing an increase of T,. with increasing c. We have c o m p a r e d our thin film T, versus c data with those o f Groen and Leeuw [ 14 ] and o f Triscone et al. [ 15 ], who varied the oxygen content in bulk samples by annealing in different oxygen atmospheres and subsequent quenching. The plots are displayed in fig. 6 showing in both cases a reasonable agreement between thin-film and bulk data. It should be a d d e d that Triscone et al. measured lower oxygen contents at higher annealing temperatures. We therefore argue that also in the thin films the variation o f the c-axis lattice constant is due to different oxygen contents and the increase of c and 7~. with increasing 7an are caused by a reduction of the oxygen concentration. The described continuous i m p r o v e m e n t o f growth and properties o f the films with increasing 7a,, underlines our previous statement that the best films were prepared at annealing temperatures just below the melting point. The best films on SrTiO3 had the following properties: Too=88 K ( T c = 8 2 K by AC measurement), p(120 K)=350 g~cm, j,(4.2 K ) = 7 X 105 A / c m 2 and F W H M (0012 ) = 0.45 °. The best films on MgO showed Too = 80 K ( T~ = 76 K by AC m e a s u r e m e n t ) , p ( 1 2 0 K ) = 4 2 0 laf~ cm, .L(4.2

409

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Fig. 6. Toovs. lattice parameter c for our 2212 films (0) compared to the data of Groen ( O, left) [ 14] and of Triscone ( O, right) [15]. K) = 6 X 104 A / c m 2 and FWHM(0012) =0.3 °. The systematic difference in the properties of the annealed amorphous films on different substrates and the well oriented growth of the films demonstrated by the small mosaic spreads are indications for a crystallization and growth starting at the film-substrate interface in a quasi-epitaxial manner. The crystalline fractions present already in films deposited at Ts above 550 a probably act as nuclei for crystallization during the annealing process. The reordering of atoms starts in these films not only at the substrate surface but also at the crystalline fractions having a random orientation throughout the film. Consequently, the structure of the films after annealing is polycrystalline causing the decreased quality as described before. The results of our experiments show that high annealing temperatures leading to an increased mobility of the atoms are favorable for the reordering process and quasi-epitaxial growth. The better growth quality and especially the appropriate oxygen incorporation are thought to be the reason for the continuous improvement of the transport properties with TanIt is known that not only the orientation of the grains perpendicular to the surface has a large influence on the superconducting properties of the films but also their in-plane orientation. This orientation can be examined by TEM [ 16 ], channeling experiments [ 17 ], Raman spectroscopy [ 18 ] or by X-ray

diffraction measurements performing ~ scans. In scans the intensity of a reflection caused by lattice planes inclined to the surface of a film is measured as a function of the rotation angle ~ around an axis perpendicular to the film. The intensity distribution then gives in addition to symmetrically equivalent peaks the orientation of the grains parallel to the surface and their frequency. It is also possible to determine the orientation of the grains relative to the substrate axes by comparing ~ scans performed on substrate and film. In fig. 7 @scans measured on the (0 1 3) reflection of the SrTiO3 substrate and the (0 2 20) reflection of a 2212 film on this substrate are shown. For the (100) SrTiO3 substrate we found four reflections as expected for a four-fold rotation axis normal to the substrate. For the 2212 film we also found four reflections positioned each 90 °. The c-axis in the 2212 orthorhombic structure is two-fold.

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Therefore we expect only two reflections separated by 180 ° for a perfect untwinned c-axis oriented film. However, the difference o f the a- and b-axis lattice constants is so small that we can not separate them in our X-ray e q u i p m e n t and so cannot exclude 9 0 grain b o u n d a r i e s in our films. Except for these possible 90 ° grain b o u n d a r i e s we found in-plane epitaxial growth o f the films on SrTiO3. The ~ scans o f film and substrate revealed an angle o f 45 ¢ between the a- and b-axis o f the film and that of the substrate, respectively. This orientation o f the film with respect to the substrate seems reasonable since the aaxis o f SrTiO3 (3,905 A ) has a low mismatch (2%) to the half diagonal of the basal plane o f the 2212 phase (3.829 A,) and is also observed for the growth on LaAIO3 substrates [ 19 ] with a similar mismatch. The ~ scans for films on MgO were different from those on SrTiO3. figure 8 shows scans for a 2212 film measured at the ( 0 2 2 0 ) reflection and the (100) MgO substrate. Again, four peaks are observed for the substrate d e t e r m i n e d at the ( 113 ) reflection, but the film - in contrast to that on SrTiO3 - shows 16 m a x i m a e m b e d d e d in a b r o a d background. It is possible to c o m b i n e four reflections each separated b~ 90 ° to four groups. In fig. 8 the reflections belonging to one group each, i.e. to one in-plane orientation, are m a r k e d by the same number. Because o f the broad b a c k g r o u n d we have a more or less r a n d o m orientation o f the grains in the a - b plane with four d o m i n a n t growth directions of films on MgO. By c o m p a r i n g the 0 scans o f film and substrate we could d e t e r m i n e the four d o m i n a n t orientations in the a b plane with respect to the substrate which are diagramatically presented in fig. 9: (1) [100] BSCCO II [100] MgO: For this orientation the m i s m a t c h between the 2212 film and the substrate a m o u n t s to about 10%. It was also observed by refs. [ 20 ] and [ 22 ]. ( 2 ) [100] BSCCO Ii [100] MgO: For this case the m i s m a t c h is very, high a n d a m o u n t s to 22%. This inplane orientation was also observed by other groups for the growth o f the 2212 phase starting with a 2201 layer on (100) MgO [20,21]. (3) [100] BSCCO H [510] MgO: This orientation is characterized by a low m i s m a t c h of 1% between the spacing o f neighboring (510) MgO planes and the four-fold lattice p a r a m e t e r a o f the 2212 phase. This orientation was also observed by ref. [23].

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Fig. 8. ~ scans of a MgO substrate (above) and a 2212 film (bclow). The reflections belonging to one group each, i.e. to one inplane orientation, are marked b~ the same number, ( 4 ) [100] BSCCO [I [150] MgO: This orientation is symmetrically equivalent to case (3). A r a n d o m orientation in the a - b plane for films on MgO was also observed by Golden et al. [19]. We did not observe any influence of the annealing temperature on the d o m i n a t i n g in-plane orientations as it was reported for 2212 films deposited on MgO at elevated T~ by Kataoka et al. [ 21 ]. Weak links caused by the observed different orientations in the a - h plane may explain the degenerated transport properties o f films on MgO especially the one order lower j~ as c o m p a r e d to films on SrTiO3 which showed inplane epitaxial growth.

R. Henn et al. / Pb-doped Bi-Sr-Ca-Cu-O filrns

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plane epitaxy. These observations are in accordance with the low lattice mismatch for 2212 films on SrTiO3 and the much higher mismatch for 2212 films on MgO. Because of the different in-plane orientations for films on MgO weak links occurred and caused reduced transport properties compared to films on SrTiO3. Films on (100) SrTiO3 showed Too values up to 88 K, critical current densities in the range of 5 × l05 A / c m 2 at 4.2 K, while films on MgO reached Too values up to 80 K and j¢ values up to 6 × 104 A / c m 2.

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4. Conclusion

We produced superconducting 2212 films on (100) MgO and (100) SrTiO3 substrates by DC magnetron sputtering and ex-situ annealing. Too values higher than 80 K were obtained reproducibly. The range of annealing temperatures leading to superconducting 2212 films reached from 100 K below the melting temperature (Tin) up to Tin. In this range of Ta. we detected better superconducting properties with increasing Ta, for films on both substrates. This is possibly caused by the higher mobility of the atoms at higher temperatures during the annealing procedure resulting in a more perfect growth and an advantageous oxygen incorporation. Our T~ versus lattice parameter c data are in accordance with a lower oxygen content of films annealed at higher temperatures corresponding to higher T~ values. All films were grown with the c-axis perpendicular to their surface, to scans revealed mosaic distributions of crystal grains down to 0.3 ° for films on MgO and down to 0.45 ° for films o n S r T i O 3 underlining the highly textured growth of these films. Films on MgO showed a more or less random orientation in the a-b plane with four dominating in-plane orientations, in contrast to films on SrTiO3 revealing in-

[ 1 ] H. Maeda, Y. Tanaka, M. Fukitomi and T. Asano, Jpn. J. Appl. Phys. 27 (1988) 209. [2] C. Allgeier and J.S. Schilling, Physica C 168 (1990) 499. [ 3 ] S.A. Sunshine, T. Siegrist, L.F. Schneemeyer, D.W. Murphy, R.J. Cava, B. Batlogg, R.B. van Dover, R.M. Fleming, S.H. Clarum, J.V. Waszczak, J.M. Marshall, P. Marsch, L.W. Rupp Jr. and W.F. Peck, Phys. Rev. B 36 (1988) 893. [4] M. Kanai, T. Kawai and S. Kawai, Jpn. J. Appl. Phys. 27 (1988) L1293. [ 5 ] P. Schmitt, L. Schultz and G. Saemann-lschenko, Physica C 168 (1990) 475. [6] I. Bozovic, J.N. Eckstein, D.G. Schlom and J.S. Harris, Science and Technology of Thin Film Superconductors 2, eds. R.D. McConnel and R. Noufi (Plenum, New York, 1990) p. 267. [7]G. Balestrino, V. Foglietti, M. Marinelli, E. Milani, A. Paoletti and P. Paroli, Appl. Phys. Lett. 57 (1990) 2359. [8] J. Geerk, X.X. Xi, H.C. Li, W.Y. Guan, P. Kus, M. Hrbel, G. Linker, O. Meyer, F. Ratzel, C. Schultheiss, R. Smithey, B. Strehlau and F. Weschenfelder, Int. J. Mod. Phys. B 3 (1989) 923. [9] P. Wagner, H. Adrian and C. Tome-Rosa, Physica C 195 (1992) 258. [ 10] A. Marino, T. Yasuda, E. Holguin and L. Rinderer, Physica C210 (1993) 16. [ l 1 ] J. Geerk, G. Linker and O. Meyer, Mater. Sci. Rep. 4 (1989) 206. [12 ] R.G. Buckley, J.L. Tall•n, I.W.M. Brown, M.R. Presland, N.E. Flower, P.W. Gilberd, M. Bowden and N.B. Milestone, Physica C 156 (1988) 629. [ 13] D.E. Morris, C.T. Hultgren, A.M. Markelz, J.Y.T. Wei, N.G. Asmar and J.H. Nickel, Phys. Rev. B 39 (1989) 6612. [14]W.A. Groen and D.M. de Leeuw, Physica C 159 (1989) 417. [ 15 ] G. Triscone, J.Y. Genoud, T. Graf, A. Junod and J. Muller, Physica C 176 ( 1991 ) 247. [ 16] X.F. Zhang, B. Kabius, K. Urban, P. Schmitt, L. Schultz and G. Saemann-Ischenko, Physica C 183 ( 1991 ) 379.

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R. Henn et al. / Pb-doped B i - S r - C a - C u - O fihn~

[ 17 ] Q. Li, O. Meyer, X.X. Xi, J. Geerk and G. Linker, Appl. Phys. Lett. 55 (1989) 1792. [18] C. Thomsen, R. Wegerer, H.-U. Habermeier and M. Cardona, Solid State Commun. 83 (1992) 199. [ 19 ] S.J. Golden, F.F. Lange, D.R. Clarke, L.D. Chang and C.T. Necker, Appl. Phys. Lett. 61 ( 1992 ) 351. [20]H. Furukawa and M. Nakao, 3rd FED Workshop on HiTcSC-ED, Kuamoto, Kyushu, Japan, 1991, p. 72.

[21 ] J. Fujila, 1". Yoshitake, T. Saloh, H. Tsuge, N. Matsukura, S. Miura, I. Takeuchi, J.S. Tsai and H. Igarashi, 3rd Workshop on HiT,.SC-ED, Kuamoto, Kyushu, Japan, 1991. p. 80. [22] M. Lelental. S. Chen, S. Tong Lee, G. Braunstein and 'I. Blanlon, Physica C 167 (1990) 614. [231 M. Kataoka, O. Wada, J. Tanimura, T. Ogama, K. Kuroda, T. Takami and K. Kojima, Physica C 201 ( 1992 ) 13 I.