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Preparation of biaxially oriented TlCu-1234 thin films
Physica C 320 Ž1999. 39–44
Preparation of biaxially oriented TlCu-1234 thin films Nawazish A. Khan ) , Yoshiyasu Sekita, Fussiko Tateai, Takahiro Kojima, Katuei Ishida, Norio Terada, Hideo Ihara Electrotechnical Laboratory 1-1-4 Umezono, Tsukuba, Ibaraki 305-8568, Japan Received 4 January 1998; received in revised form 1 April 1999; accepted 27 April 1999
Abstract The single phase of TlCu-1234 superconductor thin films is prepared for the first time by the amorphous phase epitaxy ŽAPE. method, which is thallium treatment of sputtered amorphous phase at 9008C for 1 h. The amorphous phase is prepared by sputtering from the stoichiometric target composition CuBa 2 Ca 3 Cu 4O12yy . The films on the SrTiO 3 substrate are aligned biaxially after the thallium treatment. Highly reproducible TlCu-1234 films are prepared by this method. The XRD reflected ˚ This lattice constant value is in between that of a predominant single phase with the c-axis lattice constant of 18.74 A. ˚ . and Tl-1234 Ž19.11 A˚ .. The pole figure measurements of Ž103. reflection of the films showed Cu-1234 Ž17.99 A a-axis-oriented crystals with D f s 0.88. The composition of the films after Energy Dispersive X-ray ŽEDX. measurements is Tl 0.8 Cu 0.2 Ba 2 Ca 3 Cu 4O12yy . From the resistivity measurements, the Tc is 113 K. Preliminary Jc measurements showed a current density of 1.0 = 10 6 Arcm2 Ž77 K, 0 T.. q 1999 Published by Elsevier Science B.V. All rights reserved. PACS: 74.76.y w; 74.76.Bz; 74.72.y h; 74.72.y Jt Keywords: Thin films; TlCu-1234 superconductor; Low anisotropy; APE method; High Jc
1. Introduction Recently, a new bulk superconducting compound CuBa 2 Ca 3 Cu 4 O 12yy with Tc up to 118 K has been prepared w1,2x under high pressure. This is a potentially very important compound with low superconducting anisotropy and long coherence length w2x which make this compound capable of carrying high currents. Since its discovery, there has been a need
)
Corresponding author. Tel.: q81-298-54-3379; fax: q81298-54-5447; E-mail: [email protected]
to prepare this compound at normal pressure to use it for the device and wire applications. Although the normal pressure synthesis of this compound has not yet become possible, the derivatives of this phase in the form of thin films of Cu 1y xTl x Ba 2 Ca 2 Cu 3 O 12yy ŽCuTl-1223. have been synthesized w3x at normal pressure. Thallium has been found to facilitate the formation of CuTl-1223 phase from the amorphous phase thin films deposited on SrTiO 3 substrate. It is observed that these films carry very high current densities, i.e., at 77 K, 2 = 10 7 Arcm2 in zero field and 4 = 10 5 Arcm2 under 10 T. The anisotropy w4,5x in these compounds is found to decrease with increase in number of Cu–O planes which means that
0921-4534r99r$ - see front matter q 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 9 9 . 0 0 2 9 4 - 4
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N.A. Khan et al.r Physica C 320 (1999) 39–44
anisotropy of TlCu-1234 is lower than CuTl-1223 and hence it is capable of carrying higher current densities than CuTl-1223. So there is a dire need to synthesize the TlCu-1234 superconductor thin films for the device and wire applications. In the present study, the results of one such preparation method of predominant single phase of TlCu-1234 is reported.
2. Experimental The stoichiometric target of composition CuBa 2 =Ca 3 Cu 4 O x is prepared by mixing the CuO, BaCO 3 and CaCO 3 followed by firing at 9008C for 15 h. The fired material is ground for about 5 h and is pressed into a 3 inch target which is then annealed for 15 h at 9008C in flowing oxygen. In this process, very slow heating and cooling Ž18Crmin. is followed to avoid cracks in the target. The thin film amorphous phase is deposited by rf-sputtering on the SrTiO 3 Ž100. substrate from this target with rf-power of 100 W in a mixture gas of 15 mTorr ArrO 2 Ž10 mTorr Ar and 5 mTorr O 2 .. The thin film of about 1 mm thickness is achieved after sputtering for 6 h. The amorphous phase epitaxy ŽAPE. method, which is thallium treatment of the amorphous phase deposited on a suitable substrate, is used for the preparation of these films. The films of Cu 1y xTl x Ba 2Ca 3 Cu 4 O x ŽTlCu-1234. phase is prepared by treating the amorphous phase with thallium. This process is accomplished in a Au capsule by treating a thin film amorphous phase with pellets containing thallium in the composition Cu 0.5Tl 0.5 Ba 2 Ca 3 Cu 4 O x . The mouth of the gold capsule is then closed and is heated at 9208C for 60 min followed by quenching to room temperature after the heat treatment. The superconducting TlCu-1234 phase is identified by XRD and composition analysis is carried out by Energy Dispersive X-ray ŽEDX. spectroscopy. The surface of the films is analyzed by scanning electron microscopy; the resistivity of the samples is measured by the four probe method. For the transport critical current density measurements, a bridge is made by lithographic patterning. The current density is measured on patterned samples with bridge of 0.3 mm length and 30 mm width, made from a 1 mm thick film and total dimensions of the sample on which the bridge is patterned is 5 mm = 5 mm. The transport
Jc is determined by following the criterion value of 1 mVrcm, the critical current of post-annealed samples is 0.336 A.
3. Experimental results The XRD of the samples of TlCu-1234 is shown in Fig. 1. This XRD pattern reflects predominantly single phase of TlCu-1234. Most of the peaks are marked as Ž00 l . showing c-axis alignment of the films; impurity peaks unindexed are present in a small amount. The length of the c-axis calculated ˚ The c-axis of Tl-1234 from these peaks is 18.74 A. ˚ which show and Cu-1234 w6x are 19.11 and 17.99 A, that the length of the c-axis of our TlCu-1234 is in between these materials. This is also consistent with the observations that the length of the c-axis is found to increase in the Cu-1234 superconductor with the introduction of the thallium in the charge reservoir layer w6x. In our experiments with different Tl treatment temperatures, we have observed an increase in the length of the c-axis in the samples prepared at relatively low preparation temperatures, but in the samples prepared above 9158C, the c-axis length remains the same, which shows that at lower preparation temperatures, there is more intake of Tl ŽFig. 2, with filled squares.. However, c-axis length is found to decrease by decreasing Tl contents in the
Fig. 1. Typical XRD of Tl 0.8 Cu 0.2 Ba 2 Ca 3 Cu 4 O12yy thin film grown on SrTiO 3 substrate, achieved after heating the amorphous phase at 9208C.
N.A. Khan et al.r Physica C 320 (1999) 39–44
Fig. 2. Variation in Tl contents Ž`., Cu contents in Žv . in the Cu 1y xTl x Ba 2 O4y d charge reservoir layer as a function of Tl treatment temperature of amorphous phase and ŽB. change in the length of c-axis as a function of Tl treatment temperature.
charge reservoir layers of our samples, as shown in Fig. 2 Žwith blank circles.. The concentration of the thallium in the films can be manipulated by changing the treatment time or by varying the concentration of the thallium in the pellet used for the thallium treatment. A f scan of Ž103. reflection also shows preferential a-axis alignment of the films, as shown in Fig. 3. The full width at half maximum D f is 0.88 and reflections appear after every 908 as expected for tetragonal phase of TlCu-1234 films. The surface of our thin film samples is analyzed by electron microscopy ŽFig. 4.. Typical grain size is few micrometers and these grains are well-connected, giving high conductivity to the films. The surface roughness is less than 0.2 mm in the 1 mm thick film. The transport measurements also provide another evidence for the quality of the films ŽFig. 5.. Metallic behavior of resistivity is observed from room temperature down to the onset of superconductivity as far as its variation with temperature is concerned. Superconducting transition temperature Tc is found to be 113 K, as manifested in the inset of Fig. 5. These Tc values in our TlCu-1234 thin films are somewhat lower than previously reported w6x in bulk of TlCu-1234 superconductor. The reasons for this discrepancy are off-stoichiometric composition in the thin film, but not yet clear exactly at the present stage. Preliminary Jc measurements showed a current density of 3.3 = 10 5 Arcm2 at 77 K in 0 T ŽFig. 6a..
41
Fig. 3. f scan of the Ž103. reflection of Tl 0.8 Cu 0.2 Ba 2 Ca 3Cu 4 O12yy thin film.
The current density is improved to 1.0 = 10 6 Arcm2 by annealing the samples in oxygen atmosphere at 4508C for 20 h ŽFig. 6b.; however, the Tc is not changed by oxygen annealing. In the Hall effect measurements of Cu-1234 w7x, the full oxidation of CuBa 2 O4y d charge reservoir layers is suggested to result in excellent superconducting properties. In our thin films, the improvement in the current density after annealing may possibly be due to decrease of O 2 and reduction of Tl from Tl 3q to Tl1q in the Cu 1y xTl x Ba 2 O4y d charge reservoir layers of TlCu1234, which results in to the optimum carrier density and hence higher Jc w8x. These Jc measurements are
Fig. 4. Electron micrograph of Tl 0.8 Cu 0.2 Ba 2 Ca 3 Cu 4 O12yy thin film.
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N.A. Khan et al.r Physica C 320 (1999) 39–44
Fig. 5. Temperature dependence of electrical resistivity of Tl 0.8 Cu 0.2 Ba 2 Ca 3 Cu 4 O12yy thin film grown on SrTiO 3 substrate, inset shows the blowup of the transition region.
determined by following the criterion value of 1 mVrcm. Higher Jc values are expected for TlCu1234 compared to CuTl-1223 as the former has lower anisotropy Žg ; 3.2. than later Žg ; 4.0. w9x. These films seem to be the potential candidate for the device and wire applications.
4. Discussion and conclusions
Fig. 6. Jc measurements Žat 77 K, 0 T. of Tl 0.8 Cu 0.2 Ba 2 =Ca 3 Cu 4 O12yy thin films Ža. before annealing, and Žb. after annealing at 4508C for 20 h.
In the previous studies w6x, it is mentioned that coherence length along the c-axis j c depends on the Fermi velocity Õ Fc along the c-axis, the carrier concentration in the CuO 2 planes and the thickness of superconducting layers. If we compare Cu-1234 with Tl-1234, it is observed that the thickness of superconducting layers is the same in both compounds. Then coherence length along the c-axis depends on Õ Fc , which in turn depends on the conductivity along
N.A. Khan et al.r Physica C 320 (1999) 39–44
c-axis in the charge reservoir layers. The conductivity along the c-axis of Cu-1234 is higher than Tl-1234 because of higher conductivity of CuO 2y y than of Tl 2 O 3 . Therefore, a longer coherence length along c-axis j c is inspected in Cu-1234 than Tl-1234; the former compound shows lower anisotropy than the latter, since g s j a brj c . In the TlCu-1234 system, the superconducting anisotropy is intermediate of above two compounds as charge-reservoir-layer Cu 1y xTl x Ba 2 O4y d contains both Cu and Tl atoms. The anisotropy in our Tl 0.8 Cu 0.2 Ba 2 Ca 3 Cu 4 O 12yy compound could be reduced by further reducing the Tl concentration. One of possible ways for reducing the Tl in our TlCu-1234 material is by heating it as high as possible. The reduction of relative amount of Tl results in an increment of Cu in the charge reservoir layer Cu 1y xTl x Ba 2 O4y d which can increase the carrier concentration and decrease the superconducting anisotropy. The high carrier concentration results in better superconducting properties Ž Jc and Hirr .. In the earlier studies on Tl-1223 w10x and Hg-1223 w11x, the transport critical current densities at 77 K under zero magnetic field are G 10 6 Arcm2 . These values are comparable with the current density 1 = 10 6 Arcm2 observed in TlCu-1234 at 77 K and zero magnetic field. The Jc could be increased over 1 = 10 6 Arcm2 up to 2 = 10 7 Arcm2 either by decreasing the Tl contents or by reducing the treatment temperature. The current density of the thin films depends significantly on the surface roughness and intergranular coupling; by improving surface roughness and intergranular coupling, Jc could be improved. In the Hg-system, the high toxicity of mercury makes it difficult to handle and the high volatility coupled with high sensitivity to the air of mercury hinder the growth of its phases like Hg-1223. Many different methods w13–15x have been used for the improvement of the critical temperature and current density in Hg-1223; a Tc, zero s 130 K and transport Jc s 4.4 = 10 5 Arcm2 Žat 77 K, 0 T. is achieved w12x. Krusin-Elbaum et al. w16x observed very high Jc ) 10 7 Arcm2 Ž; 77 K. in the Hg-1212 samples; however, mercury is highly toxic and difficult to handle during preparation stages. Thallium system is preferred over Hg-system because it is less toxic, less volatile, has relatively less sensitivity to the air, and acts as structure stabilizer.
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In the recent studies on the preparation of Tl-1223 thin films w12x, it is observed that after the film deposition on the substrate, a long annealing time Ž30–70 h. at elevated temperatures Ž8308C. is required for better quality films. In these studies, the longer annealing time is found to improve the surface morphology and produces dense structure. In our TlCu-thin films, we get even better quality thin films with improved phase purity for a short treatment time Ž1 h. employing APE method. In another report w17x, Tl-1223 phase is grown by MOCVD as an intergrowth of Tl-2223 phases which are metastable phases and partially decompose to Tl-1223 after long-time annealing Ž4 h., but these long annealing time are not suitable for the mass production of the wire and device applications. In our APE method, a stable TlCu-1234 phase is produced in which the carrier concentration in the superconducting layers could be controlled by Cu 1y xTl x Ba 2 O4y d charge reservoir layer containing both Cu and Tl atoms. The advantage of the CuTl-system is that it is possible to reduce Tl concentration in the compound, it gives a lower superconducting anisotropy g along c-axis and higher hole concentration. In conclusion, we have synthesized predominant single phase of TlCu-1234 films for the first time. These films are well-oriented and along a and c-axes with D f s 0.88. The critical temperature of these films is 113 K and preliminary Jc values are 1.0 = 10 6 Arcm2 Ž77 K, 0 T..
References w1x H. Ihara, K. Tokiwa, H. Ozawa, M. Hirabayashi, A. Negishi, M. Matuhata, Y.S. Song, Jpn. J. Appl. Phys. 33 Ž1994. 503. w2x H. Ihara, Advances in Superconductivity VII Ž1995. 255. w3x H. Ihara, Y. Sekita, F. Tateai, N.A. Khan, K. Ishida, E. Harashima, T. Kojima, H. Yamamoto, K. Tanaka, Y. Tanaka, N. Terada, H. Obara, IEEE Transactions on Applied Superconductivity, 1999, in press. w4x H. Ihara, A. Iyo, K. Tokiwa, T. Tokamoto, N. Terada, M. Tokumoto, M. Umeda, Czechoslovak Journal of Physics 46 Ž1996. 3185. w5x H. Ihara, A. Iyo, K. Tokiwa, N. Terada, M. Tokumoto, M. Umeda, Advances in Superconductivity VIII Ž1996. 247. w6x H. Ihara, K. Tokiwa, K. Tanaka, T. Tsukamoto, T. Watanabe, H. Yamamoto, A. Iyo, M. Tokumoto, M. Umeda, Physica C 282–287 Ž1997. 957.
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w7x M. Ogino, T. Watanabe, H. Tokiwa, A. Iyo, H. Ihara, Physica C 258 Ž1995. 384. w8x K. Tanaka, A. Iyo, Y. Tanaka, N. Terada, H. Ihara, in preparation. w9x H. Ihara, A. Iyo, K. Tanaka, K. Tokiwa, K. Ishida, N. Terada, M. Tokumoto, Y. Sekita, T. Tsukamoto, T. Watanabe, M. Umeda, Physica C 282–287 Ž1997. 1973. w10x J.Y. Yuang, J.H. Horng, S.P. Chen, C.M. Fu, K.H. Wu, T.M. Uen, Y.S. Gou, Appl. Phys. Lett. 66 Ž1995. 885. w11x S.H. Yun, J.Z. Wu, Appl. Phys. Lett. 68 Ž1996. 862. w12x T. Nabatame, Y. Saito, K. Aihara, T. Kamo, S. Matsuda, Supercond. Sci. Technol. 9 Ž1996. 17.
w13x Y. Moriwaki, T. Sugano, C. Gasser, A. Fukuoka, K. Nakanishi, S. Adachi, K. Tanabe, Appl. Phys. Lett. 69 Ž1996. 3423. w14x S.H. Yun, J.Z. Wu, B.W. Kang, A.N. Ray, A. Gapud, R. Farr, G.F. Sun, S.H. Yoo, Y. Xin, W.S. He, Appl. Phys. Lett. 67 Ž1995. 2866. w15x S.H. Yun, J.Z. Wu, S.C. Tidrow, D.W. Eckart, Appl. Phys. Lett. 68 Ž1996. 2565. w16x L. Krusin-Elbaum, C.C. Tsuei, A. Gupta, Nature 373 Ž1994. 679. w17x G. Malandrino, G.G. Condorelli, I.L. Fragala, F. Miletto Granozio, U. Scotti di Uccio, M. Valentino, Supercond. Sci. Technol. 9 Ž1996. 570.
Preparation of biaxially oriented TlCu-1234 thin films Nawazish A. Khan ) , Yoshiyasu Sekita, Fussiko Tateai, Takahiro Kojima, Katuei Ishida, Norio Terada, Hideo Ihara Electrotechnical Laboratory 1-1-4 Umezono, Tsukuba, Ibaraki 305-8568, Japan Received 4 January 1998; received in revised form 1 April 1999; accepted 27 April 1999
Abstract The single phase of TlCu-1234 superconductor thin films is prepared for the first time by the amorphous phase epitaxy ŽAPE. method, which is thallium treatment of sputtered amorphous phase at 9008C for 1 h. The amorphous phase is prepared by sputtering from the stoichiometric target composition CuBa 2 Ca 3 Cu 4O12yy . The films on the SrTiO 3 substrate are aligned biaxially after the thallium treatment. Highly reproducible TlCu-1234 films are prepared by this method. The XRD reflected ˚ This lattice constant value is in between that of a predominant single phase with the c-axis lattice constant of 18.74 A. ˚ . and Tl-1234 Ž19.11 A˚ .. The pole figure measurements of Ž103. reflection of the films showed Cu-1234 Ž17.99 A a-axis-oriented crystals with D f s 0.88. The composition of the films after Energy Dispersive X-ray ŽEDX. measurements is Tl 0.8 Cu 0.2 Ba 2 Ca 3 Cu 4O12yy . From the resistivity measurements, the Tc is 113 K. Preliminary Jc measurements showed a current density of 1.0 = 10 6 Arcm2 Ž77 K, 0 T.. q 1999 Published by Elsevier Science B.V. All rights reserved. PACS: 74.76.y w; 74.76.Bz; 74.72.y h; 74.72.y Jt Keywords: Thin films; TlCu-1234 superconductor; Low anisotropy; APE method; High Jc
1. Introduction Recently, a new bulk superconducting compound CuBa 2 Ca 3 Cu 4 O 12yy with Tc up to 118 K has been prepared w1,2x under high pressure. This is a potentially very important compound with low superconducting anisotropy and long coherence length w2x which make this compound capable of carrying high currents. Since its discovery, there has been a need
)
Corresponding author. Tel.: q81-298-54-3379; fax: q81298-54-5447; E-mail: [email protected]
to prepare this compound at normal pressure to use it for the device and wire applications. Although the normal pressure synthesis of this compound has not yet become possible, the derivatives of this phase in the form of thin films of Cu 1y xTl x Ba 2 Ca 2 Cu 3 O 12yy ŽCuTl-1223. have been synthesized w3x at normal pressure. Thallium has been found to facilitate the formation of CuTl-1223 phase from the amorphous phase thin films deposited on SrTiO 3 substrate. It is observed that these films carry very high current densities, i.e., at 77 K, 2 = 10 7 Arcm2 in zero field and 4 = 10 5 Arcm2 under 10 T. The anisotropy w4,5x in these compounds is found to decrease with increase in number of Cu–O planes which means that
0921-4534r99r$ - see front matter q 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 9 9 . 0 0 2 9 4 - 4
40
N.A. Khan et al.r Physica C 320 (1999) 39–44
anisotropy of TlCu-1234 is lower than CuTl-1223 and hence it is capable of carrying higher current densities than CuTl-1223. So there is a dire need to synthesize the TlCu-1234 superconductor thin films for the device and wire applications. In the present study, the results of one such preparation method of predominant single phase of TlCu-1234 is reported.
2. Experimental The stoichiometric target of composition CuBa 2 =Ca 3 Cu 4 O x is prepared by mixing the CuO, BaCO 3 and CaCO 3 followed by firing at 9008C for 15 h. The fired material is ground for about 5 h and is pressed into a 3 inch target which is then annealed for 15 h at 9008C in flowing oxygen. In this process, very slow heating and cooling Ž18Crmin. is followed to avoid cracks in the target. The thin film amorphous phase is deposited by rf-sputtering on the SrTiO 3 Ž100. substrate from this target with rf-power of 100 W in a mixture gas of 15 mTorr ArrO 2 Ž10 mTorr Ar and 5 mTorr O 2 .. The thin film of about 1 mm thickness is achieved after sputtering for 6 h. The amorphous phase epitaxy ŽAPE. method, which is thallium treatment of the amorphous phase deposited on a suitable substrate, is used for the preparation of these films. The films of Cu 1y xTl x Ba 2Ca 3 Cu 4 O x ŽTlCu-1234. phase is prepared by treating the amorphous phase with thallium. This process is accomplished in a Au capsule by treating a thin film amorphous phase with pellets containing thallium in the composition Cu 0.5Tl 0.5 Ba 2 Ca 3 Cu 4 O x . The mouth of the gold capsule is then closed and is heated at 9208C for 60 min followed by quenching to room temperature after the heat treatment. The superconducting TlCu-1234 phase is identified by XRD and composition analysis is carried out by Energy Dispersive X-ray ŽEDX. spectroscopy. The surface of the films is analyzed by scanning electron microscopy; the resistivity of the samples is measured by the four probe method. For the transport critical current density measurements, a bridge is made by lithographic patterning. The current density is measured on patterned samples with bridge of 0.3 mm length and 30 mm width, made from a 1 mm thick film and total dimensions of the sample on which the bridge is patterned is 5 mm = 5 mm. The transport
Jc is determined by following the criterion value of 1 mVrcm, the critical current of post-annealed samples is 0.336 A.
3. Experimental results The XRD of the samples of TlCu-1234 is shown in Fig. 1. This XRD pattern reflects predominantly single phase of TlCu-1234. Most of the peaks are marked as Ž00 l . showing c-axis alignment of the films; impurity peaks unindexed are present in a small amount. The length of the c-axis calculated ˚ The c-axis of Tl-1234 from these peaks is 18.74 A. ˚ which show and Cu-1234 w6x are 19.11 and 17.99 A, that the length of the c-axis of our TlCu-1234 is in between these materials. This is also consistent with the observations that the length of the c-axis is found to increase in the Cu-1234 superconductor with the introduction of the thallium in the charge reservoir layer w6x. In our experiments with different Tl treatment temperatures, we have observed an increase in the length of the c-axis in the samples prepared at relatively low preparation temperatures, but in the samples prepared above 9158C, the c-axis length remains the same, which shows that at lower preparation temperatures, there is more intake of Tl ŽFig. 2, with filled squares.. However, c-axis length is found to decrease by decreasing Tl contents in the
Fig. 1. Typical XRD of Tl 0.8 Cu 0.2 Ba 2 Ca 3 Cu 4 O12yy thin film grown on SrTiO 3 substrate, achieved after heating the amorphous phase at 9208C.
N.A. Khan et al.r Physica C 320 (1999) 39–44
Fig. 2. Variation in Tl contents Ž`., Cu contents in Žv . in the Cu 1y xTl x Ba 2 O4y d charge reservoir layer as a function of Tl treatment temperature of amorphous phase and ŽB. change in the length of c-axis as a function of Tl treatment temperature.
charge reservoir layers of our samples, as shown in Fig. 2 Žwith blank circles.. The concentration of the thallium in the films can be manipulated by changing the treatment time or by varying the concentration of the thallium in the pellet used for the thallium treatment. A f scan of Ž103. reflection also shows preferential a-axis alignment of the films, as shown in Fig. 3. The full width at half maximum D f is 0.88 and reflections appear after every 908 as expected for tetragonal phase of TlCu-1234 films. The surface of our thin film samples is analyzed by electron microscopy ŽFig. 4.. Typical grain size is few micrometers and these grains are well-connected, giving high conductivity to the films. The surface roughness is less than 0.2 mm in the 1 mm thick film. The transport measurements also provide another evidence for the quality of the films ŽFig. 5.. Metallic behavior of resistivity is observed from room temperature down to the onset of superconductivity as far as its variation with temperature is concerned. Superconducting transition temperature Tc is found to be 113 K, as manifested in the inset of Fig. 5. These Tc values in our TlCu-1234 thin films are somewhat lower than previously reported w6x in bulk of TlCu-1234 superconductor. The reasons for this discrepancy are off-stoichiometric composition in the thin film, but not yet clear exactly at the present stage. Preliminary Jc measurements showed a current density of 3.3 = 10 5 Arcm2 at 77 K in 0 T ŽFig. 6a..
41
Fig. 3. f scan of the Ž103. reflection of Tl 0.8 Cu 0.2 Ba 2 Ca 3Cu 4 O12yy thin film.
The current density is improved to 1.0 = 10 6 Arcm2 by annealing the samples in oxygen atmosphere at 4508C for 20 h ŽFig. 6b.; however, the Tc is not changed by oxygen annealing. In the Hall effect measurements of Cu-1234 w7x, the full oxidation of CuBa 2 O4y d charge reservoir layers is suggested to result in excellent superconducting properties. In our thin films, the improvement in the current density after annealing may possibly be due to decrease of O 2 and reduction of Tl from Tl 3q to Tl1q in the Cu 1y xTl x Ba 2 O4y d charge reservoir layers of TlCu1234, which results in to the optimum carrier density and hence higher Jc w8x. These Jc measurements are
Fig. 4. Electron micrograph of Tl 0.8 Cu 0.2 Ba 2 Ca 3 Cu 4 O12yy thin film.
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N.A. Khan et al.r Physica C 320 (1999) 39–44
Fig. 5. Temperature dependence of electrical resistivity of Tl 0.8 Cu 0.2 Ba 2 Ca 3 Cu 4 O12yy thin film grown on SrTiO 3 substrate, inset shows the blowup of the transition region.
determined by following the criterion value of 1 mVrcm. Higher Jc values are expected for TlCu1234 compared to CuTl-1223 as the former has lower anisotropy Žg ; 3.2. than later Žg ; 4.0. w9x. These films seem to be the potential candidate for the device and wire applications.
4. Discussion and conclusions
Fig. 6. Jc measurements Žat 77 K, 0 T. of Tl 0.8 Cu 0.2 Ba 2 =Ca 3 Cu 4 O12yy thin films Ža. before annealing, and Žb. after annealing at 4508C for 20 h.
In the previous studies w6x, it is mentioned that coherence length along the c-axis j c depends on the Fermi velocity Õ Fc along the c-axis, the carrier concentration in the CuO 2 planes and the thickness of superconducting layers. If we compare Cu-1234 with Tl-1234, it is observed that the thickness of superconducting layers is the same in both compounds. Then coherence length along the c-axis depends on Õ Fc , which in turn depends on the conductivity along
N.A. Khan et al.r Physica C 320 (1999) 39–44
c-axis in the charge reservoir layers. The conductivity along the c-axis of Cu-1234 is higher than Tl-1234 because of higher conductivity of CuO 2y y than of Tl 2 O 3 . Therefore, a longer coherence length along c-axis j c is inspected in Cu-1234 than Tl-1234; the former compound shows lower anisotropy than the latter, since g s j a brj c . In the TlCu-1234 system, the superconducting anisotropy is intermediate of above two compounds as charge-reservoir-layer Cu 1y xTl x Ba 2 O4y d contains both Cu and Tl atoms. The anisotropy in our Tl 0.8 Cu 0.2 Ba 2 Ca 3 Cu 4 O 12yy compound could be reduced by further reducing the Tl concentration. One of possible ways for reducing the Tl in our TlCu-1234 material is by heating it as high as possible. The reduction of relative amount of Tl results in an increment of Cu in the charge reservoir layer Cu 1y xTl x Ba 2 O4y d which can increase the carrier concentration and decrease the superconducting anisotropy. The high carrier concentration results in better superconducting properties Ž Jc and Hirr .. In the earlier studies on Tl-1223 w10x and Hg-1223 w11x, the transport critical current densities at 77 K under zero magnetic field are G 10 6 Arcm2 . These values are comparable with the current density 1 = 10 6 Arcm2 observed in TlCu-1234 at 77 K and zero magnetic field. The Jc could be increased over 1 = 10 6 Arcm2 up to 2 = 10 7 Arcm2 either by decreasing the Tl contents or by reducing the treatment temperature. The current density of the thin films depends significantly on the surface roughness and intergranular coupling; by improving surface roughness and intergranular coupling, Jc could be improved. In the Hg-system, the high toxicity of mercury makes it difficult to handle and the high volatility coupled with high sensitivity to the air of mercury hinder the growth of its phases like Hg-1223. Many different methods w13–15x have been used for the improvement of the critical temperature and current density in Hg-1223; a Tc, zero s 130 K and transport Jc s 4.4 = 10 5 Arcm2 Žat 77 K, 0 T. is achieved w12x. Krusin-Elbaum et al. w16x observed very high Jc ) 10 7 Arcm2 Ž; 77 K. in the Hg-1212 samples; however, mercury is highly toxic and difficult to handle during preparation stages. Thallium system is preferred over Hg-system because it is less toxic, less volatile, has relatively less sensitivity to the air, and acts as structure stabilizer.
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In the recent studies on the preparation of Tl-1223 thin films w12x, it is observed that after the film deposition on the substrate, a long annealing time Ž30–70 h. at elevated temperatures Ž8308C. is required for better quality films. In these studies, the longer annealing time is found to improve the surface morphology and produces dense structure. In our TlCu-thin films, we get even better quality thin films with improved phase purity for a short treatment time Ž1 h. employing APE method. In another report w17x, Tl-1223 phase is grown by MOCVD as an intergrowth of Tl-2223 phases which are metastable phases and partially decompose to Tl-1223 after long-time annealing Ž4 h., but these long annealing time are not suitable for the mass production of the wire and device applications. In our APE method, a stable TlCu-1234 phase is produced in which the carrier concentration in the superconducting layers could be controlled by Cu 1y xTl x Ba 2 O4y d charge reservoir layer containing both Cu and Tl atoms. The advantage of the CuTl-system is that it is possible to reduce Tl concentration in the compound, it gives a lower superconducting anisotropy g along c-axis and higher hole concentration. In conclusion, we have synthesized predominant single phase of TlCu-1234 films for the first time. These films are well-oriented and along a and c-axes with D f s 0.88. The critical temperature of these films is 113 K and preliminary Jc values are 1.0 = 10 6 Arcm2 Ž77 K, 0 T..
References w1x H. Ihara, K. Tokiwa, H. Ozawa, M. Hirabayashi, A. Negishi, M. Matuhata, Y.S. Song, Jpn. J. Appl. Phys. 33 Ž1994. 503. w2x H. Ihara, Advances in Superconductivity VII Ž1995. 255. w3x H. Ihara, Y. Sekita, F. Tateai, N.A. Khan, K. Ishida, E. Harashima, T. Kojima, H. Yamamoto, K. Tanaka, Y. Tanaka, N. Terada, H. Obara, IEEE Transactions on Applied Superconductivity, 1999, in press. w4x H. Ihara, A. Iyo, K. Tokiwa, T. Tokamoto, N. Terada, M. Tokumoto, M. Umeda, Czechoslovak Journal of Physics 46 Ž1996. 3185. w5x H. Ihara, A. Iyo, K. Tokiwa, N. Terada, M. Tokumoto, M. Umeda, Advances in Superconductivity VIII Ž1996. 247. w6x H. Ihara, K. Tokiwa, K. Tanaka, T. Tsukamoto, T. Watanabe, H. Yamamoto, A. Iyo, M. Tokumoto, M. Umeda, Physica C 282–287 Ž1997. 957.
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N.A. Khan et al.r Physica C 320 (1999) 39–44
w7x M. Ogino, T. Watanabe, H. Tokiwa, A. Iyo, H. Ihara, Physica C 258 Ž1995. 384. w8x K. Tanaka, A. Iyo, Y. Tanaka, N. Terada, H. Ihara, in preparation. w9x H. Ihara, A. Iyo, K. Tanaka, K. Tokiwa, K. Ishida, N. Terada, M. Tokumoto, Y. Sekita, T. Tsukamoto, T. Watanabe, M. Umeda, Physica C 282–287 Ž1997. 1973. w10x J.Y. Yuang, J.H. Horng, S.P. Chen, C.M. Fu, K.H. Wu, T.M. Uen, Y.S. Gou, Appl. Phys. Lett. 66 Ž1995. 885. w11x S.H. Yun, J.Z. Wu, Appl. Phys. Lett. 68 Ž1996. 862. w12x T. Nabatame, Y. Saito, K. Aihara, T. Kamo, S. Matsuda, Supercond. Sci. Technol. 9 Ž1996. 17.
w13x Y. Moriwaki, T. Sugano, C. Gasser, A. Fukuoka, K. Nakanishi, S. Adachi, K. Tanabe, Appl. Phys. Lett. 69 Ž1996. 3423. w14x S.H. Yun, J.Z. Wu, B.W. Kang, A.N. Ray, A. Gapud, R. Farr, G.F. Sun, S.H. Yoo, Y. Xin, W.S. He, Appl. Phys. Lett. 67 Ž1995. 2866. w15x S.H. Yun, J.Z. Wu, S.C. Tidrow, D.W. Eckart, Appl. Phys. Lett. 68 Ž1996. 2565. w16x L. Krusin-Elbaum, C.C. Tsuei, A. Gupta, Nature 373 Ž1994. 679. w17x G. Malandrino, G.G. Condorelli, I.L. Fragala, F. Miletto Granozio, U. Scotti di Uccio, M. Valentino, Supercond. Sci. Technol. 9 Ž1996. 570.