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Peak effect in NdBa2Cu3O7−δ superconductor and related Raman spectra
Journal of Magnetism and Magnetic Materials 262 (2003) 520–525
Peak effect in NdBa2Cu3O7d superconductor and related Raman spectra L.V. Honga,*, D.C. Hiepa, N.X. Phuca, M. Kakihanab, J. Dingc a
Institute of Materials Science, NCST, Nghia-Do, Hoang Quoc Viet Rd., Cau-Giay, Hanoi, Viet Nam b Laboratory of Materials Research, Tokyo Institute of Technology, Japan c Department of Materials Science, National University of Singapore, Lower Kent Ridge Road, Singapore 119260, Singapore
Abstract A peak effect of the critical current density was observed for all NdBa2Cu3O7d (Nd123) samples prepared by the solid-state reaction method in atmospheres containing different oxygen concentrations. The peak position (field value) and its critical current density value measured at 30, 50, and 71 K depend strongly on the sintering time, and on the oxygen concentration in the sintering atmosphere. The synthesized samples have a high critical temperature Tc of about 96 K and a DTc ½¼ Tð0:9ronset Þ Tð0:1ronset Þ; ronset is the normal-state resistivity] lower than 2 K. It implies that the samples are of single superconducting phase without oxygen deficiency. Macro-Raman and micro-Raman spectra were recorded at room temperature for all samples. New modes, supposed to be related with disorder in chain layers, were observed. The disorder is assumed to be induced by a substitution of magnetic Nd3+ ions into the Ba2+ sites in the Nd123 lattice, and is considered to be a main cause of the obtained peak effect. r 2003 Elsevier Science B.V. All rights reserved. PACS: 74.60.J; 74.60.G; 74.72; 78.30.C Keywords: High Tc superconductor; Secondary peak effect; Raman scattering
1. Introduction In NdBa2Cu3O7d (Nd123) materials prepared by the solid-state reaction method, Nd+3 ions can partially substitute into the Ba+2 sites [1,2]. The substituted Nd3+ ions disorder the crystal microstructure and affect several properties of the material, such as a decrease of Tc ; an increase of the phase transition half-width and the occurrence of a maximum in the critical current density as a *Corresponding author. E-mail address: [email protected] (L.V. Hong).
function of the applied field (peak effect) [3]. Nd3+ has a higher valence and smaller ion radius compared with those of the Ba2+ ion. Therefore, the substitution process is effective only if the excess positive charge of Nd3+ ion is compensated by the required amount of the self-overdoped oxygen. Hence, the substitution rate of Nd3+ into Ba2+ sites can be manipulated by controlling the oxygen concentration in the sintering atmosphere and the sintering time. In order to control this process, some authors treated samples in a mixture of argon with a suitable small oxygen amount [4–6], while others adjusted the initial Nd/Ba ratio
0304-8853/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0304-8853(03)00089-1
L.V. Hong et al. / Journal of Magnetism and Magnetic Materials 262 (2003) 520–525
to be higher than 1/2 according to the formula Nd1+xBa2xCu3O7d [6,7]. In this paper, we present the effects of sintering time and sintering atmosphere on structure and properties of Nd123 polycrystalline samples, such as Tc ; DTc ; critical current density Jc in an external magnetic field and related disordered Raman modes of Cu1 and O1 in the chain plane.
2. Experiments Raw Nd2O3, BaCO3, and CuO powders, with a purity of 99.9%, were used as the starting materials for sintering the Nd123 samples by the conventional solid-state reaction method. The constituents were weighed in the stoichiometric ratio of Nd:Ba:Cu=1:2:3.The powder mixture was ground and pressed into pellets of 1.3 cm in diameter, 0.2 cm in thickness, and calcined at 930 C for 40 h in air. The product was then reground and repressed into pellets of the same size. All pellets were sintered at 940 C for three different duration times of 24, 48 or 72 h, in air (E16% oxygen), in flowing oxygen (E100% oxygen) and in flowing argon (E0% oxygen). The samples were marked as K24Nd, K48Nd, K72Nd, O24Nd, O48Nd, O72Nd, Ar24Nd, Ar48Nd and Ar72Nd for air, oxygen, and argon with duration time of 24, 48 and 72 h, respectively. After sintering, all pellets were annealed in flowing oxygen for 1 h at 900 C, then for 24 h at 450 C, and subsequently for 72 h at 350 C. Finally, the samples were cooled down to room temperature by turning off the furnace. A four-probe technique and an inductance bridge were used to measure the sample’s resistance and the magnetic susceptibility in a temperature range from 20 to 300 K. The magnetic hysteresis loops at temperatures of 30, 50, and 71 K were recorded on a commercial SQUID magnetometer. The macro- and microRaman spectra of all the samples were recorded by means of a Jobin Yvon T64000 equipped with optical microscope. For excitation, the 514.5 nm light of the Ar laser with a power of about 20 W was used. Room-temperature Raman spectra were collected in back-scattering geometry with a liquid nitrogen-cooled CCD.
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3. Results and discussion 3.1. Influence on superconducting phase transition The transition temperature, Tc ; the width of the phase transition DTc ½¼ Tð0:9ronset Þ Tð0:1ronset Þ; and the normal-state resistivity ronset ; of the prepared samples are shown in Table 1. One can see that all the samples have a very high phase transition temperature Tc (about 96 K), a low width (less than 1 K for samples annealed in argon gas), and a small onset normal-state resistivity (about 102 mO cm). It implies that the samples are of single I-orthorhombic superconducting phase without oxygen deficiency. Fig. 1 presents the AC susceptibility versus temperature for the samples annealed in air, argon, and oxygen for 24, 48, and 72 h, respectively. The results coincide with those obtained from transport measurement: the AC susceptibility curves exhibit a single superconducting phase transition having quite the same Tc : It is about 96, 94, and 93 K for samples annealed in argon, air and oxygen, respectively. The samples annealed in air and oxygen gas exhibit wider transitions compared to those of the samples annealed in argon. Because all samples have high transition temperatures, oxygen deficiency or its inhomogeneity in the samples cannot be a the cause of the widening effect. We may assume that the widening is due to local variations in the lattice disorder caused by Nd3+ substitution at Ba2+ sites, as discussed in the Introduction and affirmed
Table 1 Transition temperatures, phase transition widths and normal state onset resistivity of the prepared superconducting NdBa2Cu3O7d samples Sample
Tconset (K)
DTc (K)
ronset (O cm)
Ar24Nd K24Nd O24Nd Ar48Nd K48Nd O48Nd Ar72Nd K72Nd O72Nd
97 94 96 96.5 95 95 97.2 94 95.2
0.8 3.0 1.0 1.0 2.0 3.0 1.0 2.0 2.5
6.0 104 6.5 104 12.0 104 6.0 104 11.0 104 16.0 104 6.5 104 2.0 104 4.0 104
L.V. Hong et al. / Journal of Magnetism and Magnetic Materials 262 (2003) 520–525
Ar24Nd
-0.2
K24Nd O24Nd
-0.4 -0.6 -0.8
0
K48Nd O48Nd
-0.2 -0.4 -0.6 -0.8
-0.2
80 85 90 95 Temperature (K)
100
Ar72Nd K72Nd O72Nd
-0.4 -0.6 -0.8
-1
-1
(a)
Ar48Nd
0 χ' (Arb.units)
χ' (Arb.units)
0
0.2
0.2
0.2
χ' (Arb.units)
522
-1
-1.2 -1.2 70 75 80 85 90 95 100 75 (b) (c) Temperature (K)
80 85 90 95 100 Temperature (K)
Fig. 1. Temperature dependence of the AC susceptibility of the Ar24Nd, K24Nd, O24Nd, Ar48Nd, K48Nd, O48Nd, Ar72Nd, K72Nd, and O72Nd samples.
5 4 3 2
Ar72h_50K O72h_50K
2 1.5 1
2
4 6 H (T)
8
10
0
2
4 6 H (T)
8
10
Ar72h_71K O72h_71K
0.8 0.6 0.4 0.2 0 -0.2
0.5
0
Jc (5×105 A/cm2)
6
1
1
2.5 Ar72h_30K O72h_30K
Jc (5×105 A/cm2)
Jc (5×105 A/cm2)
7
0
2
6 4 H (T)
8
10
Fig. 2. Critical current density versus magnetic field for the Ar72Nd and O72Nd samples, measured at 30, 50, and 71 K.
in Ref. [8]. Superconductivity is suppressed by the reduction of the hole concentration [9]. Because of the necessary compensation of the excess charge by interstitial excess oxygen, the number of substituted Nd3+ ions depends on the oxygen concentration in the sintering atmosphere. Consequently, the samples annealed in air and oxygen gas should have a higher number of Nd3+ substituted into Ba2+ sites compared to the sample annealed in argon. The local lower-Tc regions act as pinning centers, having a main contribution to the peak effect, which will be discussed in details in the next section. 3.2. Influence on Jc —the peak effect Fig. 2 presents the magnetic field dependence of the critical current density for Ar72Nd and O72Nd samples estimated from the hysteresis curves measured by a commercial SQUID magnetometer in magnetic fields in the range 0–10 T, at 30, 50,
and 71 K. One can see that all the samples have a quite high critical current density (about 2 105, 6 105, and 1.5 106 A/cm2 measured at 30, 50, and 71 K, respectively at the peak point, i.e. at the magnetic field at which the critical current density has a maximum value). The sample annealed in oxygen gas has a peak field of 1, 3, and 6 T, measured at 71, 50, and 30 K, respectively. The sample annealed in argon gas has a higher peak field. This result could be related with the number of the Nd3+ substituted into Ba2+ sites and the coupling between them. Following Blatter et al. [10], there are two fundamentally different pinning mechanisms describing the interaction of a vortex core with a pinning site, the dlpinning and the dTc -pinning mechanism. As reported by Ref. [11], the peak effect in Jc (H) for the 123 type bulk samples could be due to the dTc -pinning mechanism, which is effective when submicron-sized pinning sites are present.
L.V. Hong et al. / Journal of Magnetism and Magnetic Materials 262 (2003) 520–525 18
16
509 641 230
14
316
Intensity (Arb. Units)
Intensity (Arb. Units)
510 611
16 14
523
Air72h
433
12 10
O72h
611 316
12 Ar24h
433 10
Ar48h 8
8
Ar72h Ar72h
6 200
400
(a)
Intensity (Arb. Units)
145
316 433
12
611
193
641
14
12
106
506
597
O72h Air72h Ar72h 8
8 200
(c)
1000
10
O72h Ar72h Air72h
10
600 800 Raman shift (cm-1)
308
16
16
14
400
200
(b)
510
18
Intensity (Arb. Units)
6
1000
800 600 Raman shift (cm-1)
400
200
1000
800 600 Raman shift (cm-1)
400
(d)
16
600 Raman shift (cm-1)
800
1000
510 610 641
Intensity (Arb. Units)
14 316
O72h
433
12
O48h 10
8
6
O24h
200
400
(e)
800 600 Raman shift (cm-1)
1000
Fig. 3. Raman spectra of the Nd123 samples. (a) 72 h in air, argon and oxygen flowing gas, (b) 24, 48, and 72 h in argon flowing gas, (c) 72 h in air, argon and oxygen flowing gas (micro-Raman with xðzzÞ x polarization), (d) 72 h in air, argon, and oxygen flowing gas (micro-Raman with zðxxÞ z polarization), (e) 24, 48, and 72 h in oxygen flowing gas.
3.3. Influence on structure—Raman spectra Nd123 has the same structure and crystalline symmetry as Y123, and then it exhibits similar Raman spectra. Fig. 3a presents the macro-Raman spectra of the Nd123 samples synthesized in air, argon, and in an oxygen flow, for 72 h. Besides the
vibration modes of the apical oxygen O4, the inphase and out-of phase of the pair O2–O3 with Raman shifts of 509, 433, and 316 cm1, we observe additional Raman lines at 641, 611 cm1, and a very weak one at 230 cm1. In the previous papers [12,13], the 230 cm1 mode was observed for the sample doped with more than 25%
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L.V. Hong et al. / Journal of Magnetism and Magnetic Materials 262 (2003) 520–525
vanadium. This mode was also observed and when the exciting laser power was increased. The process is reversible, it means that it was not observed when the laser power was decreased. Therefore, following Thomsen et al. [14], we had identified [13] it to a vibration mode related with the disorder in the Cu1 chain plane. As known, the impurity phases, which could be created in Nd123 superconducting materials are BaCuOx with x changing from 2 to 3, and the green phases 211 or 422. According to the results reported by Kakihana et al. [15] and Chang et al. [16], the additional modes 611 and 641 cm1 do not belong to these impurity phases. Fig. 3b presents Raman spectra of the sample synthesized in argon flowing gas for 24, 48, and 72 h. In these spectra the vibration of 641 cm1 was not observed. Figs. 3c and d present micro-Raman spectra, collected in the xðzzÞ x and the zðxxÞ z configuration, for the O72Nd, K72Nd, and Ar72Nd samples. The micro-Raman spectrum recorded on one grain in xðzzÞ x polarization exhibits the same spectrum with an additional vibration mode of 193 cm1. It implies that the modes of 611 and 641 cm1 are intrinsic modes of the Nd123 phase. Recently, Petrykin et al. observed Raman spectra of Nd2BaCu3Ox [17]. On the basis of their reported results, we are sure that there was no Nd213 phase in our samples. According to the results reported by the authors [18,19], the vibration mode of 193 cm1 is related with activating the IR vibration mode. Comparing the Raman spectra recorded in two polarizations, xðzzÞ x and zðxxÞ z; one can see that the vibration mode of the oxygen pair O2–O3 is anisotropic, and that the Raman lines measured in polarization zðxxÞ z shift down about 10 cm1 compared to those measured in xðzzÞ x polarization. As pointed out by Iliev [20], in the R123 superconductor there are many different possible disorder configurations in the Cu1 chain plane, and every disorder configuration can give characteristic Raman modes. Therefore, we can receive different Raman lines related with the Cu1 chain plane disorder induced by oxygen deficiency and/ or by substitution of Nd for Ba. As seen in Figs. 3a and e, the O72Nd sample prepared in oxygen flowing gas exhibits the strongest Raman lines of 641 and 611 cm1. The intensity of these lines
considerably decreases in spectra of the K72Nd sample, and the 641 cm1 line is almost not observed for all the Ar72Nd samples. These results corroborate the conclusions derived above with respect to the Nd3+ substitution into Ba2+ sites. The Nd3+ ions more easily substitute into Ba sites in the samples synthesized in air and oxygen flowing gas, and consequently in those samples the chain plane has been distorted most. Fig. 3e presents Raman spectra of the samples synthesized in oxygen flowing gas for sintering times of 24, 48, and 72 h. It is clear that the lines 611 and 641 cm1 increase in intensity with increasing sintering time. Also in agreement with this picture is that the 641 cm1 line was not seen in Raman spectra of the samples synthesized in argon flowing gas, and that the 611 cm1 line decreases in intensity with increasing sintering time.
4. Conclusions We have successfully synthesized NdBa2Cu3O7d superconducting samples having a high purity and very good quality with a Tc around 97 K and a narrow half-width. The level of Nd substitution into the Ba sites in Nd123 structure could be controlled by an adjustment of the oxygen content in the sintering atmosphere. Pure argon gas constitutes a reasonable sintering atmosphere for a successful preparation of Nd123 superconducting samples of high quality. Additional Raman modes, supposed to be related with the distortion in the Cu1 chain plane of the Nd123 superconductor, were observed. A peak effect, supposed to be due to the dTc –pinning mechanism, was observed for all the prepared samples. The peak field of the critical current density strongly depends on the oxygen content of the sintering atmosphere.
Acknowledgements The paper was partially supported by the National Basic Research Program in Natural Sciences, and JSPS program.
L.V. Hong et al. / Journal of Magnetism and Magnetic Materials 262 (2003) 520–525
References [1] S. Tanaka, Physica C 282–287 (1997) 31. [2] P. Schatzle, G. Ebbing, W. Bieger, G. Krabber, Physica C 330 (2000) 19. [3] Xin Yao, Makoto Kambara, Takater Umeda, Yuh Shiohara, Physica C 296 (1998) 69. [4] W. Bieger, G. Krebber, P. Schatzle, L. Zelwnina, U. Wiesner, P. Verges, Physica C 257 (1996) 46. [5] A.W. Kaizer, W. Hennig, H.J. Bornemann, Physica C 299 (1998) 185. [6] N. Chikumoto, S. Ozawa, S.I. Yoo, N. Hayashi, M. Mukarami, Physica C 27 (1997) 187. [7] N. Chikumoto, J. Yoshioka, M. Otsuka, N. Hayashi, M. Mukarami, Physica C 281 (1997) 253. [8] G. Osabe, S.I. Yoo, N. Sakai, T. Higuchi, T. Takizawa, K. Yashohama, M. Murakami, Supercond. Sci. Technol. 13 (2000) 637. [9] S. Banzai, G. Osabe, N. Sakai, M. Otsuka, M. Murakami, Supercond. Sci. Technol. 13 (2000) 633. [10] G. Blatter, M.V. Feigel’man, V.B. Geshkenbein, A.I. Larkin, V.M. Vinokor, Rev. Mod. Phys. 66 (1994) 1125. [11] M.R. Koblischka, M. Murakami, Supercond. Sci. Technol. 13 (2000) 738.
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[12] L.V. Hong, N.H. Dan, D.T. Dat, N.X. Phuc, Proc. NCST Vietnam 8 (1996) 75. [13] Duong Cong Hiep, Vu Dinh Lam, Le Van Hong, J. Raman Spectrosc. 32 (2001) 827. [14] C. Thomsen, M. Cardona, B. Gegenheimer, R. Liu, A. Simon, Phys. Rev. B 37 (1988) 284. [15] M. Kakihana, L. Bojesson, S. Eriksson, Physica C 162–164 (1989) 1253. [16] H. Chang, Y.T. Ren, Y.Y. Sun, Y.Q. Wang, Y.Y. Xue, C.W. Chu, Physica C 228 (1994) 383; H. Chang, Y.T. Ren, Y.Y. Sun, Y.Q. Wang, Y.Y. Xue, C.W. Chu, Physica C 252 (1995) 333. [17] V. Petrykin, M. Kakihana, P. Berastegui, Physica C 338 (2000) 151. [18] M. Stavola, D.M. Krol, W. Weber, S.A. Sunshine, A. Jayaraman, G.A. Kourouklis, R.J. Cava, E.A. Rietman, Phys. Rev. B 36 (1987) 850. [19] J. Klamut, T. Glowiak, J. Hanuza, Z. Henkie, R. Horyn, B. Jezowska-Trzebiatowska, R. Kubiak, K. Lukaszewicz, J. Stepien-Damm, A. Zygmunt, Physica C 153–155 (1988) 288. [20] M.N. Iliev, Spectroscopy of superconducting materials, ASC Symposium Series 730, 1999 (Chapter 7).
Peak effect in NdBa2Cu3O7d superconductor and related Raman spectra L.V. Honga,*, D.C. Hiepa, N.X. Phuca, M. Kakihanab, J. Dingc a
Institute of Materials Science, NCST, Nghia-Do, Hoang Quoc Viet Rd., Cau-Giay, Hanoi, Viet Nam b Laboratory of Materials Research, Tokyo Institute of Technology, Japan c Department of Materials Science, National University of Singapore, Lower Kent Ridge Road, Singapore 119260, Singapore
Abstract A peak effect of the critical current density was observed for all NdBa2Cu3O7d (Nd123) samples prepared by the solid-state reaction method in atmospheres containing different oxygen concentrations. The peak position (field value) and its critical current density value measured at 30, 50, and 71 K depend strongly on the sintering time, and on the oxygen concentration in the sintering atmosphere. The synthesized samples have a high critical temperature Tc of about 96 K and a DTc ½¼ Tð0:9ronset Þ Tð0:1ronset Þ; ronset is the normal-state resistivity] lower than 2 K. It implies that the samples are of single superconducting phase without oxygen deficiency. Macro-Raman and micro-Raman spectra were recorded at room temperature for all samples. New modes, supposed to be related with disorder in chain layers, were observed. The disorder is assumed to be induced by a substitution of magnetic Nd3+ ions into the Ba2+ sites in the Nd123 lattice, and is considered to be a main cause of the obtained peak effect. r 2003 Elsevier Science B.V. All rights reserved. PACS: 74.60.J; 74.60.G; 74.72; 78.30.C Keywords: High Tc superconductor; Secondary peak effect; Raman scattering
1. Introduction In NdBa2Cu3O7d (Nd123) materials prepared by the solid-state reaction method, Nd+3 ions can partially substitute into the Ba+2 sites [1,2]. The substituted Nd3+ ions disorder the crystal microstructure and affect several properties of the material, such as a decrease of Tc ; an increase of the phase transition half-width and the occurrence of a maximum in the critical current density as a *Corresponding author. E-mail address: [email protected] (L.V. Hong).
function of the applied field (peak effect) [3]. Nd3+ has a higher valence and smaller ion radius compared with those of the Ba2+ ion. Therefore, the substitution process is effective only if the excess positive charge of Nd3+ ion is compensated by the required amount of the self-overdoped oxygen. Hence, the substitution rate of Nd3+ into Ba2+ sites can be manipulated by controlling the oxygen concentration in the sintering atmosphere and the sintering time. In order to control this process, some authors treated samples in a mixture of argon with a suitable small oxygen amount [4–6], while others adjusted the initial Nd/Ba ratio
0304-8853/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0304-8853(03)00089-1
L.V. Hong et al. / Journal of Magnetism and Magnetic Materials 262 (2003) 520–525
to be higher than 1/2 according to the formula Nd1+xBa2xCu3O7d [6,7]. In this paper, we present the effects of sintering time and sintering atmosphere on structure and properties of Nd123 polycrystalline samples, such as Tc ; DTc ; critical current density Jc in an external magnetic field and related disordered Raman modes of Cu1 and O1 in the chain plane.
2. Experiments Raw Nd2O3, BaCO3, and CuO powders, with a purity of 99.9%, were used as the starting materials for sintering the Nd123 samples by the conventional solid-state reaction method. The constituents were weighed in the stoichiometric ratio of Nd:Ba:Cu=1:2:3.The powder mixture was ground and pressed into pellets of 1.3 cm in diameter, 0.2 cm in thickness, and calcined at 930 C for 40 h in air. The product was then reground and repressed into pellets of the same size. All pellets were sintered at 940 C for three different duration times of 24, 48 or 72 h, in air (E16% oxygen), in flowing oxygen (E100% oxygen) and in flowing argon (E0% oxygen). The samples were marked as K24Nd, K48Nd, K72Nd, O24Nd, O48Nd, O72Nd, Ar24Nd, Ar48Nd and Ar72Nd for air, oxygen, and argon with duration time of 24, 48 and 72 h, respectively. After sintering, all pellets were annealed in flowing oxygen for 1 h at 900 C, then for 24 h at 450 C, and subsequently for 72 h at 350 C. Finally, the samples were cooled down to room temperature by turning off the furnace. A four-probe technique and an inductance bridge were used to measure the sample’s resistance and the magnetic susceptibility in a temperature range from 20 to 300 K. The magnetic hysteresis loops at temperatures of 30, 50, and 71 K were recorded on a commercial SQUID magnetometer. The macro- and microRaman spectra of all the samples were recorded by means of a Jobin Yvon T64000 equipped with optical microscope. For excitation, the 514.5 nm light of the Ar laser with a power of about 20 W was used. Room-temperature Raman spectra were collected in back-scattering geometry with a liquid nitrogen-cooled CCD.
521
3. Results and discussion 3.1. Influence on superconducting phase transition The transition temperature, Tc ; the width of the phase transition DTc ½¼ Tð0:9ronset Þ Tð0:1ronset Þ; and the normal-state resistivity ronset ; of the prepared samples are shown in Table 1. One can see that all the samples have a very high phase transition temperature Tc (about 96 K), a low width (less than 1 K for samples annealed in argon gas), and a small onset normal-state resistivity (about 102 mO cm). It implies that the samples are of single I-orthorhombic superconducting phase without oxygen deficiency. Fig. 1 presents the AC susceptibility versus temperature for the samples annealed in air, argon, and oxygen for 24, 48, and 72 h, respectively. The results coincide with those obtained from transport measurement: the AC susceptibility curves exhibit a single superconducting phase transition having quite the same Tc : It is about 96, 94, and 93 K for samples annealed in argon, air and oxygen, respectively. The samples annealed in air and oxygen gas exhibit wider transitions compared to those of the samples annealed in argon. Because all samples have high transition temperatures, oxygen deficiency or its inhomogeneity in the samples cannot be a the cause of the widening effect. We may assume that the widening is due to local variations in the lattice disorder caused by Nd3+ substitution at Ba2+ sites, as discussed in the Introduction and affirmed
Table 1 Transition temperatures, phase transition widths and normal state onset resistivity of the prepared superconducting NdBa2Cu3O7d samples Sample
Tconset (K)
DTc (K)
ronset (O cm)
Ar24Nd K24Nd O24Nd Ar48Nd K48Nd O48Nd Ar72Nd K72Nd O72Nd
97 94 96 96.5 95 95 97.2 94 95.2
0.8 3.0 1.0 1.0 2.0 3.0 1.0 2.0 2.5
6.0 104 6.5 104 12.0 104 6.0 104 11.0 104 16.0 104 6.5 104 2.0 104 4.0 104
L.V. Hong et al. / Journal of Magnetism and Magnetic Materials 262 (2003) 520–525
Ar24Nd
-0.2
K24Nd O24Nd
-0.4 -0.6 -0.8
0
K48Nd O48Nd
-0.2 -0.4 -0.6 -0.8
-0.2
80 85 90 95 Temperature (K)
100
Ar72Nd K72Nd O72Nd
-0.4 -0.6 -0.8
-1
-1
(a)
Ar48Nd
0 χ' (Arb.units)
χ' (Arb.units)
0
0.2
0.2
0.2
χ' (Arb.units)
522
-1
-1.2 -1.2 70 75 80 85 90 95 100 75 (b) (c) Temperature (K)
80 85 90 95 100 Temperature (K)
Fig. 1. Temperature dependence of the AC susceptibility of the Ar24Nd, K24Nd, O24Nd, Ar48Nd, K48Nd, O48Nd, Ar72Nd, K72Nd, and O72Nd samples.
5 4 3 2
Ar72h_50K O72h_50K
2 1.5 1
2
4 6 H (T)
8
10
0
2
4 6 H (T)
8
10
Ar72h_71K O72h_71K
0.8 0.6 0.4 0.2 0 -0.2
0.5
0
Jc (5×105 A/cm2)
6
1
1
2.5 Ar72h_30K O72h_30K
Jc (5×105 A/cm2)
Jc (5×105 A/cm2)
7
0
2
6 4 H (T)
8
10
Fig. 2. Critical current density versus magnetic field for the Ar72Nd and O72Nd samples, measured at 30, 50, and 71 K.
in Ref. [8]. Superconductivity is suppressed by the reduction of the hole concentration [9]. Because of the necessary compensation of the excess charge by interstitial excess oxygen, the number of substituted Nd3+ ions depends on the oxygen concentration in the sintering atmosphere. Consequently, the samples annealed in air and oxygen gas should have a higher number of Nd3+ substituted into Ba2+ sites compared to the sample annealed in argon. The local lower-Tc regions act as pinning centers, having a main contribution to the peak effect, which will be discussed in details in the next section. 3.2. Influence on Jc —the peak effect Fig. 2 presents the magnetic field dependence of the critical current density for Ar72Nd and O72Nd samples estimated from the hysteresis curves measured by a commercial SQUID magnetometer in magnetic fields in the range 0–10 T, at 30, 50,
and 71 K. One can see that all the samples have a quite high critical current density (about 2 105, 6 105, and 1.5 106 A/cm2 measured at 30, 50, and 71 K, respectively at the peak point, i.e. at the magnetic field at which the critical current density has a maximum value). The sample annealed in oxygen gas has a peak field of 1, 3, and 6 T, measured at 71, 50, and 30 K, respectively. The sample annealed in argon gas has a higher peak field. This result could be related with the number of the Nd3+ substituted into Ba2+ sites and the coupling between them. Following Blatter et al. [10], there are two fundamentally different pinning mechanisms describing the interaction of a vortex core with a pinning site, the dlpinning and the dTc -pinning mechanism. As reported by Ref. [11], the peak effect in Jc (H) for the 123 type bulk samples could be due to the dTc -pinning mechanism, which is effective when submicron-sized pinning sites are present.
L.V. Hong et al. / Journal of Magnetism and Magnetic Materials 262 (2003) 520–525 18
16
509 641 230
14
316
Intensity (Arb. Units)
Intensity (Arb. Units)
510 611
16 14
523
Air72h
433
12 10
O72h
611 316
12 Ar24h
433 10
Ar48h 8
8
Ar72h Ar72h
6 200
400
(a)
Intensity (Arb. Units)
145
316 433
12
611
193
641
14
12
106
506
597
O72h Air72h Ar72h 8
8 200
(c)
1000
10
O72h Ar72h Air72h
10
600 800 Raman shift (cm-1)
308
16
16
14
400
200
(b)
510
18
Intensity (Arb. Units)
6
1000
800 600 Raman shift (cm-1)
400
200
1000
800 600 Raman shift (cm-1)
400
(d)
16
600 Raman shift (cm-1)
800
1000
510 610 641
Intensity (Arb. Units)
14 316
O72h
433
12
O48h 10
8
6
O24h
200
400
(e)
800 600 Raman shift (cm-1)
1000
Fig. 3. Raman spectra of the Nd123 samples. (a) 72 h in air, argon and oxygen flowing gas, (b) 24, 48, and 72 h in argon flowing gas, (c) 72 h in air, argon and oxygen flowing gas (micro-Raman with xðzzÞ x polarization), (d) 72 h in air, argon, and oxygen flowing gas (micro-Raman with zðxxÞ z polarization), (e) 24, 48, and 72 h in oxygen flowing gas.
3.3. Influence on structure—Raman spectra Nd123 has the same structure and crystalline symmetry as Y123, and then it exhibits similar Raman spectra. Fig. 3a presents the macro-Raman spectra of the Nd123 samples synthesized in air, argon, and in an oxygen flow, for 72 h. Besides the
vibration modes of the apical oxygen O4, the inphase and out-of phase of the pair O2–O3 with Raman shifts of 509, 433, and 316 cm1, we observe additional Raman lines at 641, 611 cm1, and a very weak one at 230 cm1. In the previous papers [12,13], the 230 cm1 mode was observed for the sample doped with more than 25%
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vanadium. This mode was also observed and when the exciting laser power was increased. The process is reversible, it means that it was not observed when the laser power was decreased. Therefore, following Thomsen et al. [14], we had identified [13] it to a vibration mode related with the disorder in the Cu1 chain plane. As known, the impurity phases, which could be created in Nd123 superconducting materials are BaCuOx with x changing from 2 to 3, and the green phases 211 or 422. According to the results reported by Kakihana et al. [15] and Chang et al. [16], the additional modes 611 and 641 cm1 do not belong to these impurity phases. Fig. 3b presents Raman spectra of the sample synthesized in argon flowing gas for 24, 48, and 72 h. In these spectra the vibration of 641 cm1 was not observed. Figs. 3c and d present micro-Raman spectra, collected in the xðzzÞ x and the zðxxÞ z configuration, for the O72Nd, K72Nd, and Ar72Nd samples. The micro-Raman spectrum recorded on one grain in xðzzÞ x polarization exhibits the same spectrum with an additional vibration mode of 193 cm1. It implies that the modes of 611 and 641 cm1 are intrinsic modes of the Nd123 phase. Recently, Petrykin et al. observed Raman spectra of Nd2BaCu3Ox [17]. On the basis of their reported results, we are sure that there was no Nd213 phase in our samples. According to the results reported by the authors [18,19], the vibration mode of 193 cm1 is related with activating the IR vibration mode. Comparing the Raman spectra recorded in two polarizations, xðzzÞ x and zðxxÞ z; one can see that the vibration mode of the oxygen pair O2–O3 is anisotropic, and that the Raman lines measured in polarization zðxxÞ z shift down about 10 cm1 compared to those measured in xðzzÞ x polarization. As pointed out by Iliev [20], in the R123 superconductor there are many different possible disorder configurations in the Cu1 chain plane, and every disorder configuration can give characteristic Raman modes. Therefore, we can receive different Raman lines related with the Cu1 chain plane disorder induced by oxygen deficiency and/ or by substitution of Nd for Ba. As seen in Figs. 3a and e, the O72Nd sample prepared in oxygen flowing gas exhibits the strongest Raman lines of 641 and 611 cm1. The intensity of these lines
considerably decreases in spectra of the K72Nd sample, and the 641 cm1 line is almost not observed for all the Ar72Nd samples. These results corroborate the conclusions derived above with respect to the Nd3+ substitution into Ba2+ sites. The Nd3+ ions more easily substitute into Ba sites in the samples synthesized in air and oxygen flowing gas, and consequently in those samples the chain plane has been distorted most. Fig. 3e presents Raman spectra of the samples synthesized in oxygen flowing gas for sintering times of 24, 48, and 72 h. It is clear that the lines 611 and 641 cm1 increase in intensity with increasing sintering time. Also in agreement with this picture is that the 641 cm1 line was not seen in Raman spectra of the samples synthesized in argon flowing gas, and that the 611 cm1 line decreases in intensity with increasing sintering time.
4. Conclusions We have successfully synthesized NdBa2Cu3O7d superconducting samples having a high purity and very good quality with a Tc around 97 K and a narrow half-width. The level of Nd substitution into the Ba sites in Nd123 structure could be controlled by an adjustment of the oxygen content in the sintering atmosphere. Pure argon gas constitutes a reasonable sintering atmosphere for a successful preparation of Nd123 superconducting samples of high quality. Additional Raman modes, supposed to be related with the distortion in the Cu1 chain plane of the Nd123 superconductor, were observed. A peak effect, supposed to be due to the dTc –pinning mechanism, was observed for all the prepared samples. The peak field of the critical current density strongly depends on the oxygen content of the sintering atmosphere.
Acknowledgements The paper was partially supported by the National Basic Research Program in Natural Sciences, and JSPS program.
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