- Email: [email protected]
Si substrate under perpendicular magnetic fields
Physica C 426–431 (2005) 340–344 www.elsevier.com/locate/physc
Critical properties of MgB2 thin films on NbN/Si substrate under perpendicular magnetic fields Akihiko Nishida
a,b,*
, Chihiro Taka a, Stefan Chromik c, Rudolf Durny
b,d
a
d
Department of Applied Physics, Faculty of Science, Fukuoka University, 8-19-1, Nanakuma, Jonan-ku, Fukuoka 814-0180, Japan b Advanced Materials Institute, Fukuoka University, 8-19-1, Nanakuma, Jonan-ku, Fukuoka 814-0180, Japan c Institute of Electrical Engineering, Slovak Academy of Sciences, Dubravska cesta 9, 841 04 Bratislava, Slovak Republic Department of Physics, Faculty of Electrical Engineering, Slovak Technical University, Ilkovicova 3, 812 19 Bratislava, Slovak Republic Received 23 November 2004; accepted 7 February 2005 Available online 1 July 2005
Abstract Critical magnetic properties of MgB2 thin films grown on NbN/Si substrate were investigated under magnetic fields perpendicular to the film surface. Polycrystalline MgB2 films were prepared by sequential evaporation of boron and magnesium on NbN buffered Si substrate followed by an in situ annealing. AC and DC magnetizations were measured by the PPMS system. From the onset of AC diamagnetic susceptibilities, the upper critical fields were estimated resulting in the temperature derivative of about 5 kOe/K at lower temperatures. Critical current densities were evaluated from DC magnetization hystereses to be more than 1 MA/cm2 below 14 K (self field). Critical properties including irreversibility fields were examined in comparison with those under parallel magnetic fields. Ó 2005 Elsevier B.V. All rights reserved. PACS: 74.70.Ad; 74.60.Jg; 74.60.Ge; 74.25.Ha Keywords: MgB2 thin film; Upper critical field; Critical current; Irreversibility
1. Introduction Since the discovery of MgB2 [1], many intensive studies have been performed in bulk sintered sam*
Corresponding author. Tel.: +81 92 871 6631; fax: +81 92 865 6030. E-mail address: [email protected] (A. Nishida).
ples [2–4], single crystals [5–7], and thin films [8– 10]. Among these forms of samples, thin films are especially interesting from both basic and practical points of view. In our previous report [11], MgB2 thin films were prepared by sequential evaporation, and the lower critical field Hc1 was studied with the magnetic field parallel to the film surface for avoiding the demagnetization effect. We
0921-4534/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2005.02.044
A. Nishida et al. / Physica C 426–431 (2005) 340–344
2. Experimental The MgB2 thin film studied in this work is the same polycrystalline film as in the previous report [11]. The boron and magnesium layers were sequentially deposited on NbN buffered Si(1 0 0) substrate in vacuum. The thickness of the boron layer in the precursor film was adjusted so as to result in 200 nm stoichiometric MgB2 film after reaction with the excess Mg top layer. The precursor film was in situ annealed in an Ar atmosphere. The obtained film was cut into five pieces and stacked together (putting PTFE film in between them) in order to fit inside the sample holding tube with its axis along the magnetic field. Magnetic measurements were performed with magnetic field perpendicular to the film surface using PPMS magnetometer (Quantum Design). Temperature dependent AC susceptibility was measured with 1 G and 100 Hz excitation field, cooling at respective static fields. From the onset of the AC diamagnetic susceptibility for respective fields H, the transition temperature Tc2(H) was evaluated thus providing the upper critical field Hc2(T). In order to estimate the critical current density Jc, DC magnetization hystereses were measured at variety of constant temperatures. Values
of Jc were estimated on the basis of the Bean model, Jc = 30DM/r, where DM is the height of the magnetization loop and r = 0.13 cm is the sample half-width. Irreversibility field Hirr was also estimated from diminishing magnetization hysteresis.
3. Results and discussion Onsets of the AC diamagnetic susceptibility v 0 under various magnetic fields perpendicular to the film surface are partly shown in Fig. 1. As indicated by arrows, we estimated the transition temperature Tc2(H) at respective magnetic fields H, from which we deduced Hc2(T). In Fig. 2, we indicate thus obtained Hc2 curve under the perpendicular field by the solid circles together with Hc2 under the parallel field (open circles). First we note that Hc2 curves for parallel and perpendicular fields show almost identical variation, and this is consistent with the polycrystalline character of our film. The slight difference may indicate slight tendency of preferred c-axis orientation perpendicular to the film surface. At lower temperatures we estimate the temperature derivative, dHc2/dT, being about 5 kOe/K.
H
film 0.5T 0.1T
0 8T
χ ’ [10–5emu/G]
observed linear temperature dependence of Hc1 and discussed it in relation to the basic nature of the gap state. On the other hand, from the practical point of view, it is important to investigate the upper critical field Hc2 and the critical current density Jc. In order to examine the rigid current flow throughout the film, it is necessary to measure magnetization with the magnetic field perpendicular to the film surface. Another practical importance should be put on flux pinning and magnetic irreversibility, which actually determines the extent of applicability of superconductors. For example, Jc will be very small under strong magnetic field without effective flux pinning centers, whose density is seriously affected by the sample quality and morphology. In this work, we investigate Hc2, Jc, and irreversibility under the perpendicular magnetic field, comparing them with the parallel field case.
341
5T
4T
3T
2T
1T
0T
–1
–2 0
10
20
30
T [K] Fig. 1. Onsets of the AC diamagnetic susceptibility v 0 upon cooling under various magnetic fields perpendicular to the film surface as indicated by arrows.
342
A. Nishida et al. / Physica C 426–431 (2005) 340–344 100
H film 106
60
Jc [A/cm2]
Hc2 [kOe]
80
H film 40
8K 10K 12K 14K 16K 18K
H
film
20K
105
22K
20 104 101
0 10
20
30
102
103
T [K] Fig. 2. Upper critical field Hc2 as a function of temperature T under perpendicular (solid circles) and parallel (open circles) fields with respect to the film surface.
This value is close to that for H ab c2 in the single crystal [6]. This is not surprising because we estimated Tc2 at the very onset of the superconducting transition (as in Fig. 1), and any nucleation of superconductivity occurs first for those grains oriented with ab-plane parallel to the applied field as soon as the temperature decreases through the H ab c2 line. In order to examine current carrying capability along our film, we next present in Fig. 3 the results of critical current density Jc as a function of the applied field perpendicular to the film surface. It is often said that a common benchmark for practical application is 105 A/cm2 [8]. Our Jc not only exceeds this value but reaches 1 MA/cm2 at lower temperatures (self field). This indicates that wellestablished supercurrent circulates through the entire film. On the other hand, we observe Jc decreases rather fast with applied magnetic field, especially when compared with the cuprate superconductors. This shows that there are not many flux pinning centers sufficiently strong to resist the Lorentz force acting on vortices in the film. Since flux pinning properties are closely related to the magnetic irreversibility behavior, we next estimated the irreversibility field Hirr by taking
104
105
H [Oe]
40
Fig. 3. Critical current density Jc at respective temperatures as a function of the applied field perpendicular to the film surface.
Jc = 8000 A/cm2 in Fig. 3 as the criterion of reversibility. The resultant Hirr is plotted against 1t2 in Fig. 4 with reduced temperature t = T/Tc (Tc = 31 K).
n~ ~3
101
Hirr [kOe]
0
H
film
n~ ~2
H
film
100
1–t 2
100
Fig. 4. Irreversibility field Hirr plotted against 1 t2 with t = T/Tc for the perpendicular (solid circles) and parallel (open circles) fields. The least square fitting to the scaling law: Hirr / (1 t2)n, resulted in the exponent n = 2.92 for the perpendicular field and n = 2.17 for the parallel field.
A. Nishida et al. / Physica C 426–431 (2005) 340–344
First we note that values of Hirr are considerably smaller than those of Hc2 shown in Fig. 2. This is related to the matter of flux pinning properties. Since Hirr reflects non-ideal character of superconductivity which is significantly affected by the sample morphology, defects and impurities, the value can be very different from that of Hc2 which reflects more ideal character of superconductivity. Thus, the ideal sample may indicate wide reversibility even with large Hc2 when the sample is ideally pure and clean. Large discrepancy between Hirr and Hc2 is often reported in the high-Tc superconductors [13] and also reported in the MgB2 single crystal [7]. Zehetmayer et al. reported considerably smaller Hirr than Hc2 for the non-irradiated crystal, while found almost identical (improved) values of Hirr and Hc2 after neutron irradiation. Comparing with these results, we infer that our thin film contains not many defects and impurities, resulting in rather wide reversible region. In order to improve Hirr, finding and intentionally introducing defects or impurities effective for flux pinning are necessary. We further examine difference of Hirr between magnetic fields parallel and perpendicular to the film surface as well as their scaling behaviors. The straight lines in Fig. 4 indicate results of least square fitting to the scaling law: Hirr / (1t2)n. By such fitting we obtain n values of about 2 and 3, respectively for parallel and perpendicular fields. The value of n 2 for the parallel field is fairly close to the previously reported value for the powder sample [12] and is consistent with the bulk flux pinning character. On the other hand, the larger n value for the perpendicular field may indicate additional pinning mechanism such as surface or grain boundary pinning. This is also consistent with larger absolute Hirr values for the perpendicular field than those for the parallel field. According to our scanning electron microscope (SEM) observation, we find some grain-like structure in nano-meter scale on the film surface. These results may indicate some existence of perpendicular flux pinning centers in the thin film sample. In summary, AC and DC magnetic measurements were performed in the MgB2 thin film on NbN/Si substrate. The upper critical fields indi-
343
cated similar variations for perpendicular and parallel magnetic fields with temperature derivative of about 5 kOe/K, which is close to that of the single crystal along the ab-plane. Critical current densities were evaluated to be more than 1 MA/cm2 below 14 K (self field), indicating the rigid current flow along the film. Stronger irreversibility and larger n value for the perpendicular field than that for the parallel field suggested additional flux pinning mechanism such as grain boundary effects. Intentional search and incorporation of fine grain structures or impurity phases effective for flux pinning should be pursued to further improve tolerance of superconductivity against higher magnetic fields.
Acknowledgments This work has been supported by the Japanese Ministry of Education, Culture, Sports, Science and Technology (High-Tech Research Center Program) and also supported by the Slovak Grant Agency Vega (projects No. 2/2068/22 and 2/3116/ 23). The financial support of the Center of Excellence ‘‘CENG’’, SAS is acknowledged.
References [1] J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zenitani, J. Akimitsu, Nature 410 (2001) 63. [2] S.L. BudÕko, G. Lapertot, C. Petrovic, C.E. Cunningham, N. Anderson, P.C. Canfield, Phys. Rev. Lett. 86 (2001) 1877. [3] Y. Takano, H. Takeya, H. Fujii, H. Kumakura, T. Hatano, K. Togano, H. Kito, H. Ihara, Appl. Phys. Lett. 78 (2001) 2914. [4] N. Chikumoto, A. Yamamoto, M. Konczykowski, M. Murakami, Physica C 388–389 (2003) 167. [5] S. Lee, H. Mori, T. Masui, Y. Eltsev, A. Yamamoto, S. Tajima, J. Phys. Soc. Jpn. 70 (2001) 2255. [6] Z. Shi, A.K. Pradhan, M. Tokunaga, T. Tamegai, Y. Takano, K. Togano, H. Kito, H. Ihara, Physica C 388– 389 (2003) 157. [7] M. Zehetmayer, M. Eisterer, H.M. Weber, J. Jun, S.M. Kazakov, J. Karpinski, Physica C 388–389 (2003) 159. [8] C.B. Eom, M.K. Lee, J.H. Choi, L.J. Belenky, X. Song, L.D. Cooley, M.T. Naus, S. Patnaik, J. Jiang, M. Rikel, A. Polyanskii, A. Gurevich, X.Y. Cai, S.D. Bu, S.E. Babcock, E.E. Hellstrom, D.C. Larbalestier, N. Rogado,
344
A. Nishida et al. / Physica C 426–431 (2005) 340–344
K.A. Regan, M.A. Hayward, T. He, J.S. Slusky, K. Inumaru, M.K. Haas, R.J. Cava, Nature 411 (2001) 558. [9] W.N. Kang, H.-J. Kim, E.-M. Choi, K.H.P. Kim, S.-I. Lee, Physica C 378–381 (2002) 1246. [10] M.P. Paranthaman, D.K. Christen, H.M. Christen, J.R. Thompson, H.R. Kerchner, C. Cantoni, H.Y. Zhai, Physica C 378–381 (2002) 1252.
[11] A. Nishida, C. Taka, S. Chromik, R. Durny, Physica C 412–414 (2004) 201. [12] T. Akune, H. Abe, N. Sakamoto, Y. Matsumoto, Physica C 388–389 (2003) 169. [13] Y. Nakayama, T. Motohashi, K. Otzschi, J. Shimoyama, K. Kitazawa, K. Kishio, Phys. Rev. B 62 (2000) 1452.
Critical properties of MgB2 thin films on NbN/Si substrate under perpendicular magnetic fields Akihiko Nishida
a,b,*
, Chihiro Taka a, Stefan Chromik c, Rudolf Durny
b,d
a
d
Department of Applied Physics, Faculty of Science, Fukuoka University, 8-19-1, Nanakuma, Jonan-ku, Fukuoka 814-0180, Japan b Advanced Materials Institute, Fukuoka University, 8-19-1, Nanakuma, Jonan-ku, Fukuoka 814-0180, Japan c Institute of Electrical Engineering, Slovak Academy of Sciences, Dubravska cesta 9, 841 04 Bratislava, Slovak Republic Department of Physics, Faculty of Electrical Engineering, Slovak Technical University, Ilkovicova 3, 812 19 Bratislava, Slovak Republic Received 23 November 2004; accepted 7 February 2005 Available online 1 July 2005
Abstract Critical magnetic properties of MgB2 thin films grown on NbN/Si substrate were investigated under magnetic fields perpendicular to the film surface. Polycrystalline MgB2 films were prepared by sequential evaporation of boron and magnesium on NbN buffered Si substrate followed by an in situ annealing. AC and DC magnetizations were measured by the PPMS system. From the onset of AC diamagnetic susceptibilities, the upper critical fields were estimated resulting in the temperature derivative of about 5 kOe/K at lower temperatures. Critical current densities were evaluated from DC magnetization hystereses to be more than 1 MA/cm2 below 14 K (self field). Critical properties including irreversibility fields were examined in comparison with those under parallel magnetic fields. Ó 2005 Elsevier B.V. All rights reserved. PACS: 74.70.Ad; 74.60.Jg; 74.60.Ge; 74.25.Ha Keywords: MgB2 thin film; Upper critical field; Critical current; Irreversibility
1. Introduction Since the discovery of MgB2 [1], many intensive studies have been performed in bulk sintered sam*
Corresponding author. Tel.: +81 92 871 6631; fax: +81 92 865 6030. E-mail address: [email protected] (A. Nishida).
ples [2–4], single crystals [5–7], and thin films [8– 10]. Among these forms of samples, thin films are especially interesting from both basic and practical points of view. In our previous report [11], MgB2 thin films were prepared by sequential evaporation, and the lower critical field Hc1 was studied with the magnetic field parallel to the film surface for avoiding the demagnetization effect. We
0921-4534/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2005.02.044
A. Nishida et al. / Physica C 426–431 (2005) 340–344
2. Experimental The MgB2 thin film studied in this work is the same polycrystalline film as in the previous report [11]. The boron and magnesium layers were sequentially deposited on NbN buffered Si(1 0 0) substrate in vacuum. The thickness of the boron layer in the precursor film was adjusted so as to result in 200 nm stoichiometric MgB2 film after reaction with the excess Mg top layer. The precursor film was in situ annealed in an Ar atmosphere. The obtained film was cut into five pieces and stacked together (putting PTFE film in between them) in order to fit inside the sample holding tube with its axis along the magnetic field. Magnetic measurements were performed with magnetic field perpendicular to the film surface using PPMS magnetometer (Quantum Design). Temperature dependent AC susceptibility was measured with 1 G and 100 Hz excitation field, cooling at respective static fields. From the onset of the AC diamagnetic susceptibility for respective fields H, the transition temperature Tc2(H) was evaluated thus providing the upper critical field Hc2(T). In order to estimate the critical current density Jc, DC magnetization hystereses were measured at variety of constant temperatures. Values
of Jc were estimated on the basis of the Bean model, Jc = 30DM/r, where DM is the height of the magnetization loop and r = 0.13 cm is the sample half-width. Irreversibility field Hirr was also estimated from diminishing magnetization hysteresis.
3. Results and discussion Onsets of the AC diamagnetic susceptibility v 0 under various magnetic fields perpendicular to the film surface are partly shown in Fig. 1. As indicated by arrows, we estimated the transition temperature Tc2(H) at respective magnetic fields H, from which we deduced Hc2(T). In Fig. 2, we indicate thus obtained Hc2 curve under the perpendicular field by the solid circles together with Hc2 under the parallel field (open circles). First we note that Hc2 curves for parallel and perpendicular fields show almost identical variation, and this is consistent with the polycrystalline character of our film. The slight difference may indicate slight tendency of preferred c-axis orientation perpendicular to the film surface. At lower temperatures we estimate the temperature derivative, dHc2/dT, being about 5 kOe/K.
H
film 0.5T 0.1T
0 8T
χ ’ [10–5emu/G]
observed linear temperature dependence of Hc1 and discussed it in relation to the basic nature of the gap state. On the other hand, from the practical point of view, it is important to investigate the upper critical field Hc2 and the critical current density Jc. In order to examine the rigid current flow throughout the film, it is necessary to measure magnetization with the magnetic field perpendicular to the film surface. Another practical importance should be put on flux pinning and magnetic irreversibility, which actually determines the extent of applicability of superconductors. For example, Jc will be very small under strong magnetic field without effective flux pinning centers, whose density is seriously affected by the sample quality and morphology. In this work, we investigate Hc2, Jc, and irreversibility under the perpendicular magnetic field, comparing them with the parallel field case.
341
5T
4T
3T
2T
1T
0T
–1
–2 0
10
20
30
T [K] Fig. 1. Onsets of the AC diamagnetic susceptibility v 0 upon cooling under various magnetic fields perpendicular to the film surface as indicated by arrows.
342
A. Nishida et al. / Physica C 426–431 (2005) 340–344 100
H film 106
60
Jc [A/cm2]
Hc2 [kOe]
80
H film 40
8K 10K 12K 14K 16K 18K
H
film
20K
105
22K
20 104 101
0 10
20
30
102
103
T [K] Fig. 2. Upper critical field Hc2 as a function of temperature T under perpendicular (solid circles) and parallel (open circles) fields with respect to the film surface.
This value is close to that for H ab c2 in the single crystal [6]. This is not surprising because we estimated Tc2 at the very onset of the superconducting transition (as in Fig. 1), and any nucleation of superconductivity occurs first for those grains oriented with ab-plane parallel to the applied field as soon as the temperature decreases through the H ab c2 line. In order to examine current carrying capability along our film, we next present in Fig. 3 the results of critical current density Jc as a function of the applied field perpendicular to the film surface. It is often said that a common benchmark for practical application is 105 A/cm2 [8]. Our Jc not only exceeds this value but reaches 1 MA/cm2 at lower temperatures (self field). This indicates that wellestablished supercurrent circulates through the entire film. On the other hand, we observe Jc decreases rather fast with applied magnetic field, especially when compared with the cuprate superconductors. This shows that there are not many flux pinning centers sufficiently strong to resist the Lorentz force acting on vortices in the film. Since flux pinning properties are closely related to the magnetic irreversibility behavior, we next estimated the irreversibility field Hirr by taking
104
105
H [Oe]
40
Fig. 3. Critical current density Jc at respective temperatures as a function of the applied field perpendicular to the film surface.
Jc = 8000 A/cm2 in Fig. 3 as the criterion of reversibility. The resultant Hirr is plotted against 1t2 in Fig. 4 with reduced temperature t = T/Tc (Tc = 31 K).
n~ ~3
101
Hirr [kOe]
0
H
film
n~ ~2
H
film
100
1–t 2
100
Fig. 4. Irreversibility field Hirr plotted against 1 t2 with t = T/Tc for the perpendicular (solid circles) and parallel (open circles) fields. The least square fitting to the scaling law: Hirr / (1 t2)n, resulted in the exponent n = 2.92 for the perpendicular field and n = 2.17 for the parallel field.
A. Nishida et al. / Physica C 426–431 (2005) 340–344
First we note that values of Hirr are considerably smaller than those of Hc2 shown in Fig. 2. This is related to the matter of flux pinning properties. Since Hirr reflects non-ideal character of superconductivity which is significantly affected by the sample morphology, defects and impurities, the value can be very different from that of Hc2 which reflects more ideal character of superconductivity. Thus, the ideal sample may indicate wide reversibility even with large Hc2 when the sample is ideally pure and clean. Large discrepancy between Hirr and Hc2 is often reported in the high-Tc superconductors [13] and also reported in the MgB2 single crystal [7]. Zehetmayer et al. reported considerably smaller Hirr than Hc2 for the non-irradiated crystal, while found almost identical (improved) values of Hirr and Hc2 after neutron irradiation. Comparing with these results, we infer that our thin film contains not many defects and impurities, resulting in rather wide reversible region. In order to improve Hirr, finding and intentionally introducing defects or impurities effective for flux pinning are necessary. We further examine difference of Hirr between magnetic fields parallel and perpendicular to the film surface as well as their scaling behaviors. The straight lines in Fig. 4 indicate results of least square fitting to the scaling law: Hirr / (1t2)n. By such fitting we obtain n values of about 2 and 3, respectively for parallel and perpendicular fields. The value of n 2 for the parallel field is fairly close to the previously reported value for the powder sample [12] and is consistent with the bulk flux pinning character. On the other hand, the larger n value for the perpendicular field may indicate additional pinning mechanism such as surface or grain boundary pinning. This is also consistent with larger absolute Hirr values for the perpendicular field than those for the parallel field. According to our scanning electron microscope (SEM) observation, we find some grain-like structure in nano-meter scale on the film surface. These results may indicate some existence of perpendicular flux pinning centers in the thin film sample. In summary, AC and DC magnetic measurements were performed in the MgB2 thin film on NbN/Si substrate. The upper critical fields indi-
343
cated similar variations for perpendicular and parallel magnetic fields with temperature derivative of about 5 kOe/K, which is close to that of the single crystal along the ab-plane. Critical current densities were evaluated to be more than 1 MA/cm2 below 14 K (self field), indicating the rigid current flow along the film. Stronger irreversibility and larger n value for the perpendicular field than that for the parallel field suggested additional flux pinning mechanism such as grain boundary effects. Intentional search and incorporation of fine grain structures or impurity phases effective for flux pinning should be pursued to further improve tolerance of superconductivity against higher magnetic fields.
Acknowledgments This work has been supported by the Japanese Ministry of Education, Culture, Sports, Science and Technology (High-Tech Research Center Program) and also supported by the Slovak Grant Agency Vega (projects No. 2/2068/22 and 2/3116/ 23). The financial support of the Center of Excellence ‘‘CENG’’, SAS is acknowledged.
References [1] J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zenitani, J. Akimitsu, Nature 410 (2001) 63. [2] S.L. BudÕko, G. Lapertot, C. Petrovic, C.E. Cunningham, N. Anderson, P.C. Canfield, Phys. Rev. Lett. 86 (2001) 1877. [3] Y. Takano, H. Takeya, H. Fujii, H. Kumakura, T. Hatano, K. Togano, H. Kito, H. Ihara, Appl. Phys. Lett. 78 (2001) 2914. [4] N. Chikumoto, A. Yamamoto, M. Konczykowski, M. Murakami, Physica C 388–389 (2003) 167. [5] S. Lee, H. Mori, T. Masui, Y. Eltsev, A. Yamamoto, S. Tajima, J. Phys. Soc. Jpn. 70 (2001) 2255. [6] Z. Shi, A.K. Pradhan, M. Tokunaga, T. Tamegai, Y. Takano, K. Togano, H. Kito, H. Ihara, Physica C 388– 389 (2003) 157. [7] M. Zehetmayer, M. Eisterer, H.M. Weber, J. Jun, S.M. Kazakov, J. Karpinski, Physica C 388–389 (2003) 159. [8] C.B. Eom, M.K. Lee, J.H. Choi, L.J. Belenky, X. Song, L.D. Cooley, M.T. Naus, S. Patnaik, J. Jiang, M. Rikel, A. Polyanskii, A. Gurevich, X.Y. Cai, S.D. Bu, S.E. Babcock, E.E. Hellstrom, D.C. Larbalestier, N. Rogado,
344
A. Nishida et al. / Physica C 426–431 (2005) 340–344
K.A. Regan, M.A. Hayward, T. He, J.S. Slusky, K. Inumaru, M.K. Haas, R.J. Cava, Nature 411 (2001) 558. [9] W.N. Kang, H.-J. Kim, E.-M. Choi, K.H.P. Kim, S.-I. Lee, Physica C 378–381 (2002) 1246. [10] M.P. Paranthaman, D.K. Christen, H.M. Christen, J.R. Thompson, H.R. Kerchner, C. Cantoni, H.Y. Zhai, Physica C 378–381 (2002) 1252.
[11] A. Nishida, C. Taka, S. Chromik, R. Durny, Physica C 412–414 (2004) 201. [12] T. Akune, H. Abe, N. Sakamoto, Y. Matsumoto, Physica C 388–389 (2003) 169. [13] Y. Nakayama, T. Motohashi, K. Otzschi, J. Shimoyama, K. Kitazawa, K. Kishio, Phys. Rev. B 62 (2000) 1452.