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Copper rich Y–Ba–Cu–O superconductors
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Journal of Magnetism and Magnetic Materials 226}230 (2001) 309}310
Copper rich Y}Ba}Cu}O superconductors J.J.F. Sousa *, A.M. Luiz , N.V. Vugman , M.R. da Silva, J.M. Neto Instituto de Fn& sica, Universidade Federal do Rio de Janeiro, Fax: 5627368, C.P. 68528, CEP 21945-970, Rio de Janeiro, Brazil Instituto de CieL ncias, EFEI, CP 50, CEP 37500-000, Itajuba& , MG, Brazil
Abstract Copper rich compounds with YBaCu O stoichiometry were synthesized at 9003C under oxygen rich atmosphere and annealed at 3503C in oxygen #ow during 1, 3 and 60 days. Results of AC magnetic susceptibility, thermomagnetic analysis and electron paramagnetic resonance spectroscopy indicate multiphase materials containing the phases YBaCu-123 and YBaCu-247. Critical temperatures were determined by AC magnetic susceptibility and con"rmed by EPR measurements. 2001 Elsevier Science B.V. All rights reserved. Keywords: Superconductors*high-T ; Susceptibility*AC; Thermomagnetic analysis; Electron paramagnetic resonance
It is well established [1] that Cooper pairs are the charge carriers in high-¹ superconductors. However, the mechanisms responsible for pair formation and condensation of the superconducting state are not well understood. It has been suggested that the molar ratio between Cu>/Cu> should be proportional to the critical temperature [2]. In order to work out this hypothesis, a copper rich Y}Ba}Cu}O material was produced with the nominal stoichiometry YBaCu O . The samples were prepared by following three steps. In the "rst step, well mixed powders of the constituent materials (BaO, CuO, Y O ) were submitted to a ther momagnetic analysis in a VSM EGG-PAR model 4500 spectrometer equipped with a 151 oven. The samples were heated up to 10003C at a rate of 4003C/h, maintained at this temperature for 6 h and then cooled down. The experiments were performed with a 2 kG applied magnetic "eld and with no applied "eld. Results are shown in Fig. 1. The cooling data indicate that both treatments produce diamagnetic materials; the one treated with applied "eld exhibits a diamagnetism increase of around 3003C, becoming paramagnetic at about 1403C. Reactions are observed at approximately 3003C, 7503C and 9003C; at 9003C there is a pronounced loss of mass.
* Corresponding author. Fax: #55-56-27-368. E-mail address: [email protected] (J.J.F. Sousa).
In the second step, pellets of the initial reagents were treated at 9003C in oxygen rich air atmosphere during 48 h (sample S0). In the third step, the S0 material was then treated at 3503C in pressurized oxygen #ow during 1, 3 and 60 days (samples S1d, S3d and S60d) and quenched in liquid N . Fig. 2 shows the AC magnetic susceptibility of these samples, measured with a LakeShore model 7500 susceptometer/magnetometer. The S0 material undergoes a transition with an onset temperature about 95 K, due to a YBaCu-123 superconducting phase. As the annealing time is increased (samples S1d, S3d and S60d) the transition temperature decreases, due to the formation of the YBaCuO-247 superconducting phase. Room temperature EPR spectra, taken with a Bruker-ESP 380E spectrometer, are shown in Fig. 3. The CuO "ngerprint is not present in the samples, indicating its complete dissociation. Peak to peak linewidths vary from 187 to 267 G and the intensities indicate a decrease in concentration of paramagnetic ions with increasing annealing time. Samples S0, S1d, S3d and S60d exhibit nearly isotropic powder spectra and g-values in the range 2.071}2.090. EPR experiments at low temperatures reveal a sharp decrease in the spectral signal to noise ratio at a typical temperature. This e!ect is commonly observed in type II superconductors EPR spectroscopy and this temperature is associated with the onset of local superconductivity [3]. The measured onset temperatures are 90 K for S0, 55}60 K for S1d and S3d and 50}55 K for S60d, in agreement with the data displayed in Fig. 2.
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 1 3 5 9 - 7
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J.J.F. Sousa et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 309}310
Fig. 1. Thermomagnetic analysis of the oxide mixture (BaO, CuO, Y O ). Arrows indicate the direction of the temperature change.
Fig. 3. EPR spectra of samples at room temperature. The sharp peak, coming from a MgO : Cr> standard sample, is taken as a normalization signal. A CuO powder spectrum is also shown in the "gure. Fig. 2. AC magnetic susceptibility (real and imaginary) of samples.
The authors are indebted to FINEP and FUJB for funding. NVV thanks CNPq for a research fellowship. From Fig. 2 we conclude that the critical temperature decreases with increasing annealing time (at 3503C). This conclusion is similar to the results reported in a recent work [4], which shows that the increase of the annealing time in a crystal of YBaCu-123 in a reducing atmosphere (at 4503C) produces a decrease in ¹ from 60 to 45 K. According to literature, the Cu> detected in the `123a powders comes from impurity phases [5}7]. However, the properties of EPR spectra shown in Fig. 3, such as line shape and g-values at room temperature, and their small variation when lowering the temperature, are different from those reported in multiphase ceramic samples [7]. Further investigation on materials with high copper content may contribute to throw some light on the mechanisms governing the superconducting phase formation.
References [1] D. Este`ve, J.M. Martinis, C. Urbina, M.H. Devoret, G. Collin, P. Monod, M. Ribault, A. Revcolevschi, Europhys. Lett. 3 (11) (1987) 1237. [2] B. Raveau, C. Michel, A. Maignan, M. Hervieu, J. Provost, in: High Temperature Superconductors, J. Heiras, R.A. Bario, T. Akachi, J. Taguena (Eds.), World Scienti"c, Singapore, 1988, p. 5. [3] F. Mehran, S.E. Barnes, T.R. McGuire, T.R. Dinger, D.L. Kaiser, F. Holtzberg, Solid State Commun. 66 (1988) 299. [4] R.P. Sharma, S.B. Ogale, Z.H. Zhang, J.R. Liu, W.K. Chu, B. Veal, A. Paulikas, H. Zheng, Nature 404 (2000) 736. [5] J.A.O. Aguiar, A.A. Menovsky, J. van den Berg, H.B. Brom, J. Phys. C 21 (1988) L237. [6] F. Mehran, P.W. Anderson, Solid State Commun. 71 (1989) 29. [7] A. Punnoose, R.J. Singh, Int. J. Mod. Phys. B (1995) 1123.
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Journal of Magnetism and Magnetic Materials 226}230 (2001) 309}310
Copper rich Y}Ba}Cu}O superconductors J.J.F. Sousa *, A.M. Luiz , N.V. Vugman , M.R. da Silva, J.M. Neto Instituto de Fn& sica, Universidade Federal do Rio de Janeiro, Fax: 5627368, C.P. 68528, CEP 21945-970, Rio de Janeiro, Brazil Instituto de CieL ncias, EFEI, CP 50, CEP 37500-000, Itajuba& , MG, Brazil
Abstract Copper rich compounds with YBaCu O stoichiometry were synthesized at 9003C under oxygen rich atmosphere and annealed at 3503C in oxygen #ow during 1, 3 and 60 days. Results of AC magnetic susceptibility, thermomagnetic analysis and electron paramagnetic resonance spectroscopy indicate multiphase materials containing the phases YBaCu-123 and YBaCu-247. Critical temperatures were determined by AC magnetic susceptibility and con"rmed by EPR measurements. 2001 Elsevier Science B.V. All rights reserved. Keywords: Superconductors*high-T ; Susceptibility*AC; Thermomagnetic analysis; Electron paramagnetic resonance
It is well established [1] that Cooper pairs are the charge carriers in high-¹ superconductors. However, the mechanisms responsible for pair formation and condensation of the superconducting state are not well understood. It has been suggested that the molar ratio between Cu>/Cu> should be proportional to the critical temperature [2]. In order to work out this hypothesis, a copper rich Y}Ba}Cu}O material was produced with the nominal stoichiometry YBaCu O . The samples were prepared by following three steps. In the "rst step, well mixed powders of the constituent materials (BaO, CuO, Y O ) were submitted to a ther momagnetic analysis in a VSM EGG-PAR model 4500 spectrometer equipped with a 151 oven. The samples were heated up to 10003C at a rate of 4003C/h, maintained at this temperature for 6 h and then cooled down. The experiments were performed with a 2 kG applied magnetic "eld and with no applied "eld. Results are shown in Fig. 1. The cooling data indicate that both treatments produce diamagnetic materials; the one treated with applied "eld exhibits a diamagnetism increase of around 3003C, becoming paramagnetic at about 1403C. Reactions are observed at approximately 3003C, 7503C and 9003C; at 9003C there is a pronounced loss of mass.
* Corresponding author. Fax: #55-56-27-368. E-mail address: [email protected] (J.J.F. Sousa).
In the second step, pellets of the initial reagents were treated at 9003C in oxygen rich air atmosphere during 48 h (sample S0). In the third step, the S0 material was then treated at 3503C in pressurized oxygen #ow during 1, 3 and 60 days (samples S1d, S3d and S60d) and quenched in liquid N . Fig. 2 shows the AC magnetic susceptibility of these samples, measured with a LakeShore model 7500 susceptometer/magnetometer. The S0 material undergoes a transition with an onset temperature about 95 K, due to a YBaCu-123 superconducting phase. As the annealing time is increased (samples S1d, S3d and S60d) the transition temperature decreases, due to the formation of the YBaCuO-247 superconducting phase. Room temperature EPR spectra, taken with a Bruker-ESP 380E spectrometer, are shown in Fig. 3. The CuO "ngerprint is not present in the samples, indicating its complete dissociation. Peak to peak linewidths vary from 187 to 267 G and the intensities indicate a decrease in concentration of paramagnetic ions with increasing annealing time. Samples S0, S1d, S3d and S60d exhibit nearly isotropic powder spectra and g-values in the range 2.071}2.090. EPR experiments at low temperatures reveal a sharp decrease in the spectral signal to noise ratio at a typical temperature. This e!ect is commonly observed in type II superconductors EPR spectroscopy and this temperature is associated with the onset of local superconductivity [3]. The measured onset temperatures are 90 K for S0, 55}60 K for S1d and S3d and 50}55 K for S60d, in agreement with the data displayed in Fig. 2.
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 1 3 5 9 - 7
59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92
310
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56
J.J.F. Sousa et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 309}310
Fig. 1. Thermomagnetic analysis of the oxide mixture (BaO, CuO, Y O ). Arrows indicate the direction of the temperature change.
Fig. 3. EPR spectra of samples at room temperature. The sharp peak, coming from a MgO : Cr> standard sample, is taken as a normalization signal. A CuO powder spectrum is also shown in the "gure. Fig. 2. AC magnetic susceptibility (real and imaginary) of samples.
The authors are indebted to FINEP and FUJB for funding. NVV thanks CNPq for a research fellowship. From Fig. 2 we conclude that the critical temperature decreases with increasing annealing time (at 3503C). This conclusion is similar to the results reported in a recent work [4], which shows that the increase of the annealing time in a crystal of YBaCu-123 in a reducing atmosphere (at 4503C) produces a decrease in ¹ from 60 to 45 K. According to literature, the Cu> detected in the `123a powders comes from impurity phases [5}7]. However, the properties of EPR spectra shown in Fig. 3, such as line shape and g-values at room temperature, and their small variation when lowering the temperature, are different from those reported in multiphase ceramic samples [7]. Further investigation on materials with high copper content may contribute to throw some light on the mechanisms governing the superconducting phase formation.
References [1] D. Este`ve, J.M. Martinis, C. Urbina, M.H. Devoret, G. Collin, P. Monod, M. Ribault, A. Revcolevschi, Europhys. Lett. 3 (11) (1987) 1237. [2] B. Raveau, C. Michel, A. Maignan, M. Hervieu, J. Provost, in: High Temperature Superconductors, J. Heiras, R.A. Bario, T. Akachi, J. Taguena (Eds.), World Scienti"c, Singapore, 1988, p. 5. [3] F. Mehran, S.E. Barnes, T.R. McGuire, T.R. Dinger, D.L. Kaiser, F. Holtzberg, Solid State Commun. 66 (1988) 299. [4] R.P. Sharma, S.B. Ogale, Z.H. Zhang, J.R. Liu, W.K. Chu, B. Veal, A. Paulikas, H. Zheng, Nature 404 (2000) 736. [5] J.A.O. Aguiar, A.A. Menovsky, J. van den Berg, H.B. Brom, J. Phys. C 21 (1988) L237. [6] F. Mehran, P.W. Anderson, Solid State Commun. 71 (1989) 29. [7] A. Punnoose, R.J. Singh, Int. J. Mod. Phys. B (1995) 1123.
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