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Superconducting properties of B on Mg bilayer films sequentially deposited at low substrate temperatures
Physica C 388–389 (2003) 155–156 www.elsevier.com/locate/physc
Superconducting properties of B on Mg bilayer films sequentially deposited at low substrate temperatures S. Yata a
a,*
, G. Shimizu a, Y. Yamada a, S. Kubo a, A. Matsushita
b
Department of Materials Science, Shimane University, 1060 Nishikawatsu, Matsue 690-8504, Japan b National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan
Abstract A simple new method for in situ low temperature synthesis of intermetallic compound superconductor MgB2 thin films is presented. In this method, bilayers are prepared by sequential evaporation of B on Mg film layer at substrate temperature of 225–240 °C. The formation of superconducting thin films with Tc of about 25 K has been confirmed resistively and magnetically by means this simple method. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: MgB2 superconductor; Thin film; Sequential deposition; Low temperature synthesis
1. Introduction MgB2 , the highest critical temperature Tc superconductor of intermetallic compounds [1], has a simple crystal structure and therefore small anisotropic properties as compared with those of cuprate superconductors. From a standpoint of electronic device applications, MgB2 thin film preparations have been extensively examined. In these studies, two groups have reported in situ low temperature syntheses of superconducting MgB2 thin films; one utilized a coevaporation method and the other cosputtering method [2,3]. Here we report a very simple method for synthesizing MgB2 thin films at low substrate temperatures. In this method, superconducting MgB2 thin films can be obtained merely by sequential deposition of B on Mg layer.
2. Experimental Mg and B were evaporated by an effusion cell and EB gun, respectively. The background pressure was *
Corresponding author. E-mail address: [email protected] (S. Yata).
2 107 Torr. The substrates used were mainly sapphire R. Substrate temperatures were set in the range between 200 and 300 °C, because high substrate temperatures cause complete re-evaporation of deposited Mg layer. The deposition sequence in the present study is as follows: The bottom B layer ! Mg layer ! the top B acts layer. The bottom B layer with a thickness of 250 A only as a buffer, which prevents the next Mg layer from peeling-off, and the first B/Mg bilayer never gives rise to superconductivity. Film thickness values of the Mg and , respectop B layers are 4000–6000 and 250–1000 A tively, but these values are nominal ones that are indicated on the film thickness monitor mounted near the substrate holder. Superconductivity was confirmed resistively and magnetically.
3. Results and discussion Superconductivity in the Mg/B bilayers has been obtained only a narrow substrate temperature (Ts ) range of 225–240 °C. When Ts s are lower than this temperature range, samples show only a metallic conductivity, which is assumed to result from an unreacted Mg layer, whereas when Ts s are higher than this temperature
0921-4534/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-4534(02)02703-X
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S. Yata et al. / Physica C 388–389 (2003) 155–156
Fig. 1. Temperature dependence of electrical resistances for different Mg/B layer thickness samples deposited at a substrate temperature of 240 °C. Fig. 3. Temperature dependence of magnetization for Mg 4000 /B 500 A sample deposited at a substrate temperature of A 225 °C.
Fig. 2. Temperature dependence of electrical resistances for samples deposited at different substrate temperatures.
range, they become insulating, suggesting the Mg layer deposited re-evaporate completely. Fig. 1 shows temperature dependence of electrical resistances for different Mg/B layer thickness samples deposited at a Ts of 240 °C. Fig. 2 shows temperature dependence of electrical resistances for samples deposited at different Ts s. From /B 500 A deposited at a Ts of these results, Mg 4000 A 240 °C is the optimum combination in the present experiment. Fig. 3 shows temperature dependence of /B 500 A sample deposited magnetization for Mg 4000 A at a Ts of 225 °C. In this measurement, an external magnetic field of 50 Oe was applied perpendicular to the film surface. From the result of occurrence of super-
conductivity at temperatures above 20 K, superconducting MgB2 layer is believed to be formed in the Mg/B bilayer deposited sequentially. No diffraction lines were, however, detected for any superconducting samples by X-ray diffraction measurement. This fact means that the superconducting MgB2 phase in the bilayer films is assumed to exist in a very thin interfacial layer between the Mg and upper B layers and/or in the form of an amorphous-like structure due to a low temperature deposition. The possibility of the superconducting MgB2 intermatallic compound formation at such a low temperature is now under examination. From a thermodynamical study for the Mg–B system, it is suggested that MgB2 phase may be formed at low temperatures under low pressures [4]. The formation morphology of superconducting MgB2 phase in the present bilayers is also under examination by means of a high-resolution crosssectional electron-microscopic observation.
References [1] J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zentani, J. Akimitsu, Nature (London) 410 (2001) 63. [2] K. Ueda, M. Naito, Appl. Phys. Lett. 79 (2001) 2046. [3] A. Saito, A. Kawakami, H. Shimakage, Z. Wang, Jpn. J. Appl. Phys. 41 (2002) L127. [4] Z.-K. Liu, D. Schlom, Q. Li, X.X. Xi, Appl. Phys. Lett. 78 (2001) 3678.
Superconducting properties of B on Mg bilayer films sequentially deposited at low substrate temperatures S. Yata a
a,*
, G. Shimizu a, Y. Yamada a, S. Kubo a, A. Matsushita
b
Department of Materials Science, Shimane University, 1060 Nishikawatsu, Matsue 690-8504, Japan b National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan
Abstract A simple new method for in situ low temperature synthesis of intermetallic compound superconductor MgB2 thin films is presented. In this method, bilayers are prepared by sequential evaporation of B on Mg film layer at substrate temperature of 225–240 °C. The formation of superconducting thin films with Tc of about 25 K has been confirmed resistively and magnetically by means this simple method. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: MgB2 superconductor; Thin film; Sequential deposition; Low temperature synthesis
1. Introduction MgB2 , the highest critical temperature Tc superconductor of intermetallic compounds [1], has a simple crystal structure and therefore small anisotropic properties as compared with those of cuprate superconductors. From a standpoint of electronic device applications, MgB2 thin film preparations have been extensively examined. In these studies, two groups have reported in situ low temperature syntheses of superconducting MgB2 thin films; one utilized a coevaporation method and the other cosputtering method [2,3]. Here we report a very simple method for synthesizing MgB2 thin films at low substrate temperatures. In this method, superconducting MgB2 thin films can be obtained merely by sequential deposition of B on Mg layer.
2. Experimental Mg and B were evaporated by an effusion cell and EB gun, respectively. The background pressure was *
Corresponding author. E-mail address: [email protected] (S. Yata).
2 107 Torr. The substrates used were mainly sapphire R. Substrate temperatures were set in the range between 200 and 300 °C, because high substrate temperatures cause complete re-evaporation of deposited Mg layer. The deposition sequence in the present study is as follows: The bottom B layer ! Mg layer ! the top B acts layer. The bottom B layer with a thickness of 250 A only as a buffer, which prevents the next Mg layer from peeling-off, and the first B/Mg bilayer never gives rise to superconductivity. Film thickness values of the Mg and , respectop B layers are 4000–6000 and 250–1000 A tively, but these values are nominal ones that are indicated on the film thickness monitor mounted near the substrate holder. Superconductivity was confirmed resistively and magnetically.
3. Results and discussion Superconductivity in the Mg/B bilayers has been obtained only a narrow substrate temperature (Ts ) range of 225–240 °C. When Ts s are lower than this temperature range, samples show only a metallic conductivity, which is assumed to result from an unreacted Mg layer, whereas when Ts s are higher than this temperature
0921-4534/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-4534(02)02703-X
156
S. Yata et al. / Physica C 388–389 (2003) 155–156
Fig. 1. Temperature dependence of electrical resistances for different Mg/B layer thickness samples deposited at a substrate temperature of 240 °C. Fig. 3. Temperature dependence of magnetization for Mg 4000 /B 500 A sample deposited at a substrate temperature of A 225 °C.
Fig. 2. Temperature dependence of electrical resistances for samples deposited at different substrate temperatures.
range, they become insulating, suggesting the Mg layer deposited re-evaporate completely. Fig. 1 shows temperature dependence of electrical resistances for different Mg/B layer thickness samples deposited at a Ts of 240 °C. Fig. 2 shows temperature dependence of electrical resistances for samples deposited at different Ts s. From /B 500 A deposited at a Ts of these results, Mg 4000 A 240 °C is the optimum combination in the present experiment. Fig. 3 shows temperature dependence of /B 500 A sample deposited magnetization for Mg 4000 A at a Ts of 225 °C. In this measurement, an external magnetic field of 50 Oe was applied perpendicular to the film surface. From the result of occurrence of super-
conductivity at temperatures above 20 K, superconducting MgB2 layer is believed to be formed in the Mg/B bilayer deposited sequentially. No diffraction lines were, however, detected for any superconducting samples by X-ray diffraction measurement. This fact means that the superconducting MgB2 phase in the bilayer films is assumed to exist in a very thin interfacial layer between the Mg and upper B layers and/or in the form of an amorphous-like structure due to a low temperature deposition. The possibility of the superconducting MgB2 intermatallic compound formation at such a low temperature is now under examination. From a thermodynamical study for the Mg–B system, it is suggested that MgB2 phase may be formed at low temperatures under low pressures [4]. The formation morphology of superconducting MgB2 phase in the present bilayers is also under examination by means of a high-resolution crosssectional electron-microscopic observation.
References [1] J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zentani, J. Akimitsu, Nature (London) 410 (2001) 63. [2] K. Ueda, M. Naito, Appl. Phys. Lett. 79 (2001) 2046. [3] A. Saito, A. Kawakami, H. Shimakage, Z. Wang, Jpn. J. Appl. Phys. 41 (2002) L127. [4] Z.-K. Liu, D. Schlom, Q. Li, X.X. Xi, Appl. Phys. Lett. 78 (2001) 3678.