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The effects of growth conditions on superconducting properties of Nd1Ba2Cu3O7−δ single crystals
Physica C 387 (2003) 198–202 www.elsevier.com/locate/physc
The effects of growth conditions on superconducting properties of Nd1Ba2Cu3O7d single crystals B. Koscielska a
a,*
, B. Andrzejewski b, B. Susła c, S. Szuba c, W. Sadowski
a
Faculty of Applied Physics and Mathematics, Gda nsk University of Technology, Ulica Narutowicza 11/12, 80-952 Gda nsk, Poland b Institute of Molecular Physics of the Polish Academy of Sciences, Ulica Smoluchowskiego 17, 60-179 Pozna n, Poland c Institute of Physics, Pozna n University of Technology, Ulica Nieszawska 13A, 60-965 Pozna n, Poland
Abstract The influence of thermal treatment conditions on the structure and superconducting properties of single crystal Nd1 Ba2 Cu3 O7d growths has been studied. The crystals were grown in air by the flux method. The microstructure of the crystals was studied using scanning electron microscopy and scanning tunnelling microscopy. To examine the superconducting properties of the crystals measurements of the DC magnetic moment in applied magnetic fields were performed. A superconducting transition was not observed in any of the samples. This may have been a result of Nd/Ba or Cu/Al substitution. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Crystal growth; DC magnetisation; Scanning electron microscopy; Scanning tunnelling microscopy
1. Introduction The Nd–Ba–Cu–O system is one of the most promising for practical applications. This is because Nd1 Ba2 Cu3 O7d (Nd123) demonstrates higher Tc in comparison to Y1 Ba2 Cu3 O7d and a higher critical current density in high magnetic fields [1,2]. This may be related to its crystalline structure. The crystallisation processes in Nd123 have been widely discussed by many researchers [3–5] and it seems to be difficult to grow only the pure Nd123 phase. A very important feature of Nd123 is its wide region of solubility in solid state, Nd1þx Ba2x Cu3 O7d , originating in the similar sizes of Ba2þ and Nd3þ . It is believable that the *
Corresponding author. Fax: +48-58-347-28-21. E-mail address: [email protected] (B. Kos´cielska).
presence of effective pinning centres, such as Nd/ Ba substitution sites and oxygen vacancy clusters, may play an important role in the explanation of advanced physical properties. It is, therefore, important to investigate the surface topography and local electronic properties of the Nd123 system. In this paper we report the experimental results of scanning electron microscopy (SEM), of scanning tunnelling microscopy (STM), and of DC magnetisation measurements in applied magnetic fields of the samples at different stages of the crystallisation process.
2. Experimental The crystals were grown in air, in alumina crucibles by spontaneous crystal growth from the
0921-4534/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-4534(03)00669-5
B. Koscielska et al. / Physica C 387 (2003) 198–202
flux method [6]. The commercial powders of Nd2 O3 , BaCO3 and CuO were mixed together in appropriate ratios, were subsequently decomposed at 600 °C, and were calcined at 900 °C. Four sets of crystals were produced in the following way: the calcined mixture was melted at 1100 °C for 2 h and slowly cooled (1 °C/1 h) to 1050, 1010, 985 or 955 °C respectively. The hot crucible was quickly removed from the furnace and the grown crystals were separated from the residual flux by decanting. The crucible was then cooled to room temperature. The crystals obtained in this method were rather small, about 1:5 1:0 mm. As grown crystals were finally annealed in oxygen at 430 °C for 90 h. The SEM images were taken on the crystals annealed in oxygen using a Philips-FEI XL 30 ESEM (Environmental Scanning Electron Microscope) with an embedded EDX (Energy Dispersive X-ray Analysis) detector. Scanning tunnelling microscope (STM) images of the crystal surface (after annealing in oxygen) were provided with a homemade STM operating at ambient conditions. Imaging was performed in constant current mode (0.1 nA) at sample bias +1.5 V, using a tungsten tip. No particular cleaning of the sample was made, except by scanning several times prior to recording the image, which was initially very noisy but eventually improved. Relatively high bias and noise resulted from the rather high electrical resistivity of the sample studied––on the order of 1 MX. The magnetic measurements were performed using a MagLab 2000 System AC susceptometer/ DC magnetometer (Oxford Instruments Ltd.). Each set of measurements was preceded by calibration of the magnetometer. The DC magnetic moment dependence in magnetic fields in a range from 0–8 104 Am1 was measured at the temperature T ¼ 5 K. The measurements were taken on as grown crystals and crystals oxygenated at T ¼ 430 °C for 90 h.
3. Results and discussion SEM micrographs of the crystals obtained in the crystallisation process stopped at 1010 and 955 °C are presented in Fig. 1. On the surface of the
199
Fig. 1. SEM micrographs of the crystal obtained in crystallisation process stopped at 1010 and 955 °C.
crystal obtained at 1010 °C some irregular areas surrounded by white points with a great number of these white points dispersed on the whole surface are observed. The images of the crystal grown to 955 °C were different. Regular, parallel stripes are present, this may suggest that the crystals have grown as elongated terraces. There are only a few white points on this surface. EDX results show that the terraces and white points both have the same composition. Structures similar to these seen by the SEM method are observed in STM images. STM images of the crystals grown in the crystallisation process stopped at 1010 °C are presented in Fig. 2. Image (a) shows terraces separated by nearly parallel steps with several randomly dispersed hills (present in SEM images as white points). The height of these steps and hills may be calculated from images (b) and (c). The height of each step is 1.2 nm, which corresponds to the caxis lattice parameter of the Nd123 system [7]. The
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Fig. 3. STM image of the crystal obtained in crystallisation process stopped at 955 °C (a) and height profile (b) along the dashed line. The image area is 300 300 nm2 . Tunneling conditions are as in Fig. 2. Note lack of hills and higher filling of the terraces as compared with the Fig. 2. Fig. 2. STM image of the crystal obtained in crystallisation processes stopped at 1010 °C (a) and height profiles taken along A–A (b) and B–B (c) lines. The image area is 600 600 nm2 , tunneling voltage )1.5 V, tunneling current )0.1 nA. Grey scale resolves the lower lying details, while large bright spots represent the higher areas. Terraces separated by nearly parallel, 1.2 nm high steps are visible along with several randomly dispersed hills of height varying from few to 30 nm.
hill height varies from a few nm to 30 nm. It may be that the growth of these hills is caused by surface defects. The STM images of the crystals grown in the crystallisation process stopped at 955 °C (Fig. 3(a) and (b)) shows elongated terraces also about 1.2 nm high. In both cases the terraces consist of a great number of small islands, more or less separated. If the images observed by the SEM and STM methods are compared it may be sug-
gested that the growth of the crystals begins with crystal nucleation on surface defects, then continues by their broadening, and finally connecting into irregular, flat terraces. Subsequently, when the temperature of growth decreases (the growth process is longer), the terraces become more regular and narrow but their height is the same. All as grown crystals obtained in crystallisation processes stopped at 1050, 1010, 985 and 955 °C respectively showed no indication of superconductivity. The results of DC magnetic moment versus magnetic field MðH Þ for the crystals grown to 1050 °C are presented in Fig. 4. It may be seen that annealing in oxygen does not change the magnetic properties of the samples. Fig. 5 shows MðH Þ results of the crystals obtained at 1010, 985 and 955 °C, after annealing in oxygen. These
B. Koscielska et al. / Physica C 387 (2003) 198–202
Fig. 4. The results of DC magnetic moment versus magnetic field MðH Þ of the crystal obtained in crystallisation process stopped at 1050 °C (as grown and annealed in oxygen at 430 °C for 90 h). The crystals do not indicate superconductivity even after annealing in oxygen.
Fig. 5. The results of DC magnetic moment versus magnetic field MðH Þ results of the crystals obtained in crystallisation process stopped at 1010, 985 and 955 °C, after annealing in oxygen. No crystal exhibits diamagnetic properties.
crystals still do not exhibit diamagnetic properties. This behaviour may be caused by deviations in the Nd/Ba stoichiometry. In the Nd123 system a wide region of solubility in a solid state, Nd1þx Ba2x Cu3 O7d (originating from the similar sizes of Ba2þ and Nd3þ ), is observed and it is difficult to grow only the pure Nd123 phase. The Nd/Ba substitution changes the electron configuration and depresses the Tc [8]. A high Tc Nd1þx Ba2x Cu3 O7d system, with Tc near or above 90 K can only be
201
obtained when 0 6 x 6 0:1 [9]. Other defects can occur during the crystallisation process beside Nd/ Ba substitution. The crystals were grown in an alumina crucible, so Cu ions could be substituted (in a very small range) by Al3þ ions. This would influence any superconducting properties of the crystals and leads to reduction of the Tc [10]. It is seen from EDX results, that both kinds of substitution (Nd/Ba and Cu/Al) are present in Nd123 crystals obtained in crystallisation processes stopped at 1050, 1010, 985 and 955 °C. The x parameter describing Nd/Ba substitution varied much more than the range needed for superconducting behaviour. A small amount of Al ions was also observed.
4. Conclusions Our SEM and STM investigations of the crystals obtained in crystallisation processes stopped at 1050, 1010, 985 and 955 °C respectively reveals that the growth of layers of the crystals takes place by broadening of the small crystal nucleus until they connect into irregular, flat terraces. The terraces consist of a great number of small islands, more or less separated. Subsequently, when the temperature of growth decreases (and the growth process increases) the terraces become more regular and narrow. The height of a terrace is about 1.2 nm, which corresponds to a caxis lattice parameter of the Nd123 system [7]. It is seen from measurements of the DC magnetic moment versus magnetic field MðH Þ, that none of the crystals indicate superconducting properties. Annealing in oxygen does not change the magnetic properties of the samples. Nd/Ba and Cu/Al substitutions present in the crystals measured may have influenced the presence of superconducting properties.
References [1] M. Murakami, S.I. Yoo, T. Higuchi, N. Sakai, J. Weltz, N. Koshizuka, S. Tanaka, Jpn. J. Appl. Phys. 33 (1994) L715. [2] M. Murakami, N. Sakai, T. Higuchi, S.I. Yoo, Supercond. Sci. Technol. 9 (1996) 1015.
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[3] M.J. Kramer, H. Wu, K.W. Dennis, B.I. Polzin, D.K. Falzgraf, R.W. McCallum, Adv. Supercond. 95 (1995) 385. [4] H. Wu, K.W. Dennis, R.W. McCallum, Physica C 290 (1997) 252. [5] G. Osabe, S.I. Yoo, N. Sakai, T. Higuchi, T. Takizawa, K. Yasohama, M. Murakami, Supercond. Sci. Technol. 13 (2000) 637. [6] W. Sadowski, H.J. Scheel, J. Less-Common Metals 150 (1989) 219.
[7] R. Gladyszewski, in: C.P. Poole (Ed.), Handbook of Superconductivity: Crystal Structures of High-Tc Superconductivity Cuprates, Academic Press, 2000, p. 267. [8] I.G. Chen, H.J. Liu, Supercond. Sci. Technol. 13 (2000) 665. [9] E. Goodilin, M. Limonov, A. Panfilov, N. Khasanova, A. Oka, S. Tajima, Y. Shiohara, Physica C 300 (1998) 250. [10] J.T. Markert, B.D. Dunlap, M.B. Maple, MRS Bull. 14 (1989) 37.
The effects of growth conditions on superconducting properties of Nd1Ba2Cu3O7d single crystals B. Koscielska a
a,*
, B. Andrzejewski b, B. Susła c, S. Szuba c, W. Sadowski
a
Faculty of Applied Physics and Mathematics, Gda nsk University of Technology, Ulica Narutowicza 11/12, 80-952 Gda nsk, Poland b Institute of Molecular Physics of the Polish Academy of Sciences, Ulica Smoluchowskiego 17, 60-179 Pozna n, Poland c Institute of Physics, Pozna n University of Technology, Ulica Nieszawska 13A, 60-965 Pozna n, Poland
Abstract The influence of thermal treatment conditions on the structure and superconducting properties of single crystal Nd1 Ba2 Cu3 O7d growths has been studied. The crystals were grown in air by the flux method. The microstructure of the crystals was studied using scanning electron microscopy and scanning tunnelling microscopy. To examine the superconducting properties of the crystals measurements of the DC magnetic moment in applied magnetic fields were performed. A superconducting transition was not observed in any of the samples. This may have been a result of Nd/Ba or Cu/Al substitution. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Crystal growth; DC magnetisation; Scanning electron microscopy; Scanning tunnelling microscopy
1. Introduction The Nd–Ba–Cu–O system is one of the most promising for practical applications. This is because Nd1 Ba2 Cu3 O7d (Nd123) demonstrates higher Tc in comparison to Y1 Ba2 Cu3 O7d and a higher critical current density in high magnetic fields [1,2]. This may be related to its crystalline structure. The crystallisation processes in Nd123 have been widely discussed by many researchers [3–5] and it seems to be difficult to grow only the pure Nd123 phase. A very important feature of Nd123 is its wide region of solubility in solid state, Nd1þx Ba2x Cu3 O7d , originating in the similar sizes of Ba2þ and Nd3þ . It is believable that the *
Corresponding author. Fax: +48-58-347-28-21. E-mail address: [email protected] (B. Kos´cielska).
presence of effective pinning centres, such as Nd/ Ba substitution sites and oxygen vacancy clusters, may play an important role in the explanation of advanced physical properties. It is, therefore, important to investigate the surface topography and local electronic properties of the Nd123 system. In this paper we report the experimental results of scanning electron microscopy (SEM), of scanning tunnelling microscopy (STM), and of DC magnetisation measurements in applied magnetic fields of the samples at different stages of the crystallisation process.
2. Experimental The crystals were grown in air, in alumina crucibles by spontaneous crystal growth from the
0921-4534/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-4534(03)00669-5
B. Koscielska et al. / Physica C 387 (2003) 198–202
flux method [6]. The commercial powders of Nd2 O3 , BaCO3 and CuO were mixed together in appropriate ratios, were subsequently decomposed at 600 °C, and were calcined at 900 °C. Four sets of crystals were produced in the following way: the calcined mixture was melted at 1100 °C for 2 h and slowly cooled (1 °C/1 h) to 1050, 1010, 985 or 955 °C respectively. The hot crucible was quickly removed from the furnace and the grown crystals were separated from the residual flux by decanting. The crucible was then cooled to room temperature. The crystals obtained in this method were rather small, about 1:5 1:0 mm. As grown crystals were finally annealed in oxygen at 430 °C for 90 h. The SEM images were taken on the crystals annealed in oxygen using a Philips-FEI XL 30 ESEM (Environmental Scanning Electron Microscope) with an embedded EDX (Energy Dispersive X-ray Analysis) detector. Scanning tunnelling microscope (STM) images of the crystal surface (after annealing in oxygen) were provided with a homemade STM operating at ambient conditions. Imaging was performed in constant current mode (0.1 nA) at sample bias +1.5 V, using a tungsten tip. No particular cleaning of the sample was made, except by scanning several times prior to recording the image, which was initially very noisy but eventually improved. Relatively high bias and noise resulted from the rather high electrical resistivity of the sample studied––on the order of 1 MX. The magnetic measurements were performed using a MagLab 2000 System AC susceptometer/ DC magnetometer (Oxford Instruments Ltd.). Each set of measurements was preceded by calibration of the magnetometer. The DC magnetic moment dependence in magnetic fields in a range from 0–8 104 Am1 was measured at the temperature T ¼ 5 K. The measurements were taken on as grown crystals and crystals oxygenated at T ¼ 430 °C for 90 h.
3. Results and discussion SEM micrographs of the crystals obtained in the crystallisation process stopped at 1010 and 955 °C are presented in Fig. 1. On the surface of the
199
Fig. 1. SEM micrographs of the crystal obtained in crystallisation process stopped at 1010 and 955 °C.
crystal obtained at 1010 °C some irregular areas surrounded by white points with a great number of these white points dispersed on the whole surface are observed. The images of the crystal grown to 955 °C were different. Regular, parallel stripes are present, this may suggest that the crystals have grown as elongated terraces. There are only a few white points on this surface. EDX results show that the terraces and white points both have the same composition. Structures similar to these seen by the SEM method are observed in STM images. STM images of the crystals grown in the crystallisation process stopped at 1010 °C are presented in Fig. 2. Image (a) shows terraces separated by nearly parallel steps with several randomly dispersed hills (present in SEM images as white points). The height of these steps and hills may be calculated from images (b) and (c). The height of each step is 1.2 nm, which corresponds to the caxis lattice parameter of the Nd123 system [7]. The
200
B. Koscielska et al. / Physica C 387 (2003) 198–202
Fig. 3. STM image of the crystal obtained in crystallisation process stopped at 955 °C (a) and height profile (b) along the dashed line. The image area is 300 300 nm2 . Tunneling conditions are as in Fig. 2. Note lack of hills and higher filling of the terraces as compared with the Fig. 2. Fig. 2. STM image of the crystal obtained in crystallisation processes stopped at 1010 °C (a) and height profiles taken along A–A (b) and B–B (c) lines. The image area is 600 600 nm2 , tunneling voltage )1.5 V, tunneling current )0.1 nA. Grey scale resolves the lower lying details, while large bright spots represent the higher areas. Terraces separated by nearly parallel, 1.2 nm high steps are visible along with several randomly dispersed hills of height varying from few to 30 nm.
hill height varies from a few nm to 30 nm. It may be that the growth of these hills is caused by surface defects. The STM images of the crystals grown in the crystallisation process stopped at 955 °C (Fig. 3(a) and (b)) shows elongated terraces also about 1.2 nm high. In both cases the terraces consist of a great number of small islands, more or less separated. If the images observed by the SEM and STM methods are compared it may be sug-
gested that the growth of the crystals begins with crystal nucleation on surface defects, then continues by their broadening, and finally connecting into irregular, flat terraces. Subsequently, when the temperature of growth decreases (the growth process is longer), the terraces become more regular and narrow but their height is the same. All as grown crystals obtained in crystallisation processes stopped at 1050, 1010, 985 and 955 °C respectively showed no indication of superconductivity. The results of DC magnetic moment versus magnetic field MðH Þ for the crystals grown to 1050 °C are presented in Fig. 4. It may be seen that annealing in oxygen does not change the magnetic properties of the samples. Fig. 5 shows MðH Þ results of the crystals obtained at 1010, 985 and 955 °C, after annealing in oxygen. These
B. Koscielska et al. / Physica C 387 (2003) 198–202
Fig. 4. The results of DC magnetic moment versus magnetic field MðH Þ of the crystal obtained in crystallisation process stopped at 1050 °C (as grown and annealed in oxygen at 430 °C for 90 h). The crystals do not indicate superconductivity even after annealing in oxygen.
Fig. 5. The results of DC magnetic moment versus magnetic field MðH Þ results of the crystals obtained in crystallisation process stopped at 1010, 985 and 955 °C, after annealing in oxygen. No crystal exhibits diamagnetic properties.
crystals still do not exhibit diamagnetic properties. This behaviour may be caused by deviations in the Nd/Ba stoichiometry. In the Nd123 system a wide region of solubility in a solid state, Nd1þx Ba2x Cu3 O7d (originating from the similar sizes of Ba2þ and Nd3þ ), is observed and it is difficult to grow only the pure Nd123 phase. The Nd/Ba substitution changes the electron configuration and depresses the Tc [8]. A high Tc Nd1þx Ba2x Cu3 O7d system, with Tc near or above 90 K can only be
201
obtained when 0 6 x 6 0:1 [9]. Other defects can occur during the crystallisation process beside Nd/ Ba substitution. The crystals were grown in an alumina crucible, so Cu ions could be substituted (in a very small range) by Al3þ ions. This would influence any superconducting properties of the crystals and leads to reduction of the Tc [10]. It is seen from EDX results, that both kinds of substitution (Nd/Ba and Cu/Al) are present in Nd123 crystals obtained in crystallisation processes stopped at 1050, 1010, 985 and 955 °C. The x parameter describing Nd/Ba substitution varied much more than the range needed for superconducting behaviour. A small amount of Al ions was also observed.
4. Conclusions Our SEM and STM investigations of the crystals obtained in crystallisation processes stopped at 1050, 1010, 985 and 955 °C respectively reveals that the growth of layers of the crystals takes place by broadening of the small crystal nucleus until they connect into irregular, flat terraces. The terraces consist of a great number of small islands, more or less separated. Subsequently, when the temperature of growth decreases (and the growth process increases) the terraces become more regular and narrow. The height of a terrace is about 1.2 nm, which corresponds to a caxis lattice parameter of the Nd123 system [7]. It is seen from measurements of the DC magnetic moment versus magnetic field MðH Þ, that none of the crystals indicate superconducting properties. Annealing in oxygen does not change the magnetic properties of the samples. Nd/Ba and Cu/Al substitutions present in the crystals measured may have influenced the presence of superconducting properties.
References [1] M. Murakami, S.I. Yoo, T. Higuchi, N. Sakai, J. Weltz, N. Koshizuka, S. Tanaka, Jpn. J. Appl. Phys. 33 (1994) L715. [2] M. Murakami, N. Sakai, T. Higuchi, S.I. Yoo, Supercond. Sci. Technol. 9 (1996) 1015.
202
B. Koscielska et al. / Physica C 387 (2003) 198–202
[3] M.J. Kramer, H. Wu, K.W. Dennis, B.I. Polzin, D.K. Falzgraf, R.W. McCallum, Adv. Supercond. 95 (1995) 385. [4] H. Wu, K.W. Dennis, R.W. McCallum, Physica C 290 (1997) 252. [5] G. Osabe, S.I. Yoo, N. Sakai, T. Higuchi, T. Takizawa, K. Yasohama, M. Murakami, Supercond. Sci. Technol. 13 (2000) 637. [6] W. Sadowski, H.J. Scheel, J. Less-Common Metals 150 (1989) 219.
[7] R. Gladyszewski, in: C.P. Poole (Ed.), Handbook of Superconductivity: Crystal Structures of High-Tc Superconductivity Cuprates, Academic Press, 2000, p. 267. [8] I.G. Chen, H.J. Liu, Supercond. Sci. Technol. 13 (2000) 665. [9] E. Goodilin, M. Limonov, A. Panfilov, N. Khasanova, A. Oka, S. Tajima, Y. Shiohara, Physica C 300 (1998) 250. [10] J.T. Markert, B.D. Dunlap, M.B. Maple, MRS Bull. 14 (1989) 37.