J Supercond Nov MagnDOI 10.1007/s10948-015-2981-1

ORIGINAL PAPER

Optimized Intergrain Connection and Transport Propertiesof Bi-2212 High-Temperature Superconductors by AgNanoparticles Inclusions

Shengnan Zhang · Chengshan Li · Qingbin Hao ·Jianqing Feng · Tianni Lu · Pingxiang Zhang

Received: 11 November 2014 / Accepted: 20 January 2015© Springer Science+Business Media New York 2015

Abstract Bi2Sr2CaCu2.0O8+δ (Bi-2212) polycrystallinebulks with Ag nanoparticles inclusion content of x = 0,0.125, 0.250 and 0.500 wt% were fabricated by sparkplasma sintering process. Bi-2212 precursor powderswere synthesized by modified coprecipitation process Agnanoparticles with the particle size of 5 ∼ 20 nm weresynthesized by a microplasma reduction method. The influ-ences of Ag inclusions on the textural structures and relatedsuperconducting properties were systematically investi-gated. Both XRD and XPS results revealed the elementalstate of Ag in the bulks. Therefore, no doping effect wasobserved on the lattice parameters, as well as the criticaltemperature of Bi-2212 phase. Obvious enhancements oftexture structures and increase of average grain size wereobtained due to the change of crystallization behavior ofBi-2212 with Ag doping. Thus, intergrain connections wereobviously improved, which contributed to the increase ofself-field critical current density. Besides, the formation ofbetter crystallized grain boundaries increased the numberof effective pinning centers, thus enhanced the flux pinningproperties and the in-field transport properties. Based on theinfluences mentioned above, improvement of critical cur-rent density of over 60 % was obtained with the optimal Agcontent of 0.250 wt %.

S. Zhang (�) · C. Li · Q. Hao · J. Feng · T. Lu · P. ZhangSuperconducting Materials Research CenterNorthwest Institute for Nonferrous Metal ResearchXi’an 710016, Chinae-mail: snzhang@c-nin.com

T. LuCollege of Materials Science and EngineeringXi’an Jiaotong UniversityXi’an 710049, China

Keywords High-temperature superconductor ·Nanoparticles · Intergrain connection · Critical currentdensity

1 Introduction

Bi2Sr2CaCu2O8+δ (Bi-2212) is considered to be one of themost promising high temperature superconductors (HTS)for practical applications under low temperature and highmagnetic field conditions, due to its excellent properties,such as high irreversibility field of nearly 100 T [1] and largein field current capacity exceeding 266 Amm−2 up to 45 T[2]. As the only HTS so far, which can be made into roundwires with isotropic cross sections, it greatly simplifies thewinding process for coils and cables. Therefore, it exhibitsgreat potentials for the applications as insert coils in highfield magnets and Rutherford cables for accelerators [3–9].Recently, the maximum total magnetic field of 33.8 T wassuccessfully achieved under the background field of 31.2 Twith Bi-2212 insert coils, which proved the effectiveness ofBi-2212 in practical applications.

However, there are two crucial factors which still lim-ited the transport properties of Bi-2212 superconductors.One is the intergrain weak links due to the low texturedegree [10–13], high porosity [14, 15] and/or grain bound-aries with secondary phases or amorphous layers [16, 17].The existence of these defects not only blocked the transportof current but also decreased the effective superconductingcrosssections. Therefore, it is very important to fabricatethe superconductors with highly textural structure and largersuperconducting phase content in order to enhance the trans-port properties for practical applications. The other factor isthe weak flux pinning, which can be attributed to its intrin-sic lattice structure [18]. Therefore, more effective pinning

J Supercond Nov Magn

centers were necessary to improve the transport propertiesof Bi-2212.

It has been widely accepted that Ag can greatlydecrease the melting temperature of Bi-2212 and Bi-2223(Bi2Sr2Ca2Cu3O10+δ) [19–24]. Therefore highly texturedBi-2212 (or Bi-2223) grains were usually obtained at theinterface between Ag and Bi-2212 (or Bi-2223) [25, 26].It provides us an opportunity that if the interfaces betweenAg and Bi-2212 increased, it would be possible to tune theoverall thermodynamic properties of Bi-2212 and improvethe crystallization behavior of Bi-2212 [27]. Thus theenhancement of the texture structure of final product canbe obtained. Meanwhile, the introduced Ag particles withproper size can also act as pinning centers to enhance theflux pinning properties [28–30]. However, in a previous

Fig. 1 a X-ray diffraction patterns of Bi-2212 bulks with different Agcontent. Degree of preferred orientation were calculated and plotted asa function of Ag content in the inset. b XPS spectrum with the bind-ing energy from 364 to 377 eV, representing the existence of Ag inelementary state

study, the Ag particles adopted were all with the dimen-sion of micrometers, most of which cannot work as effectivepinning centers especially after segregation [30]. Therefore,uniformly distributed Ag particles with proper size couldbe beneficial for both the texture growth of Bi-2212 grainsand the flux pinning properties of final products. In thisstudy, Ag nanoparticles were synthesized and introducedinto Bi-2212 bulks as inclusions. The influences of Agnanoparticles on the texture formation, intergrain connec-tions, as well as the current carrying capacity of Bi-2212bulks were systematically studied.

2 Experimental Details

Silver nanoparticles were prepared by a microplasmamethod with a gas-liquid electrochemical cell [31]. Themicroplasma directed by Argon gas flow was used as cath-ode, and the Ag foil was used as anode. The Ar gas flow wascoupled to a capillary tube and set by a mass flow controller.The electrolytes consisted of 1 mM AgNO3, PVP, andde-ionized water. To ignite the microplasma, high voltage(∼1 kV) was applied, and the discharge current was kept at5 mA. Bi2Sr2CaCu2O8+δ precursor powders were preparedby modified coprecipitation process [32] with the startingmaterials of Bi2O3, SrCO3, CaCO3, and CuO (>99.9 %).Single phased Bi-2212 precursor powders were obtainedafter a series of calcination processes in air at 800 ◦C/12 h,820 ◦C/20 h, and 850 ◦C/ 20 h with intermediate grinding.Ag powders with the nominal weight ratio of , 0.125, 0.250and 0.500 wt% were added respectively after the calcinationprocess and a low temperature annealing with intermedi-ate grinding was adopted to remove the organic compoundsintroduced with Ag particles and make sure the uniform dis-tribution of Ag particles. The precursor powders were thendensely packed into a graphite die and pressed into pelletswith the diameter of ϕ12.7 mm and thickness of 1.5 mmby spark plasma sintering (SPS) technique, with the sinter-ing temperature of 780 ◦C and sintering time of 5 min. Thepost-annealing process was taken under 850 ◦C for 24 h inoxygen atmosphere, in order to optimize the oxygen contentof superconducting phase.

The density of bulks was measured using the stan-dard Archimedes method. Polycrystalline X-ray diffraction(XRD) patterns on both precursor powders and bulks weretaken on an X-ray diffraction (XRD, Bruker D8 Advance)with Cu-Kα radiation (λ = 0.1542 nm). The degree of pre-ferred orientation of (00l) were mostly used to just thequality of texture structures, which were calculated withthe observed diffraction intensity of (0010) and (115) peaksbased on the method provided by Schmahl et al. [33]. X-ray photoelectron spectroscopy (XPS) was performed onthe Bi-2212 bulks by ESCALAB 250Xi Thermo Scientific

J Supercond Nov Magn

Table 1 Microstructural and physical parameters of Ag doped Bi-2212 bulks

Ag content 0 0.125 0.250 0.500

Degree of preferred orientation 9.96 (5) 16.5 (6) 15.7 (6) 15.5 (4)

Density (g cm−3) 6.26 6.36 6.32 6.38

Average grain radius, μm 5.6 8.5 8.0 8.2

Peak temperature, Tp, K (at HAC = 0.5 Oe) 61 69 66 62

Irreversible field, Birr, T 6.0 ± 0.2 9.3 ± 0.2 11.2 ± 0.3 11.3 ± 0.3

with AlKα radiation. The specimen surface has been care-fully cleaned and swept with Argon gas for 5 min beforeXPS. The total acquisition time was 3 min with the bind-ing energy ranging from 0 to 1350 eV and the energy stepof 0.05 eV. Transmission electron microscopy (TEM) sam-ples were prepared by a grinding dimpling and ion milling(Gatan PIPS). The high-resolution transmission electronmicroscopy (HRTEM) was performed on a JEOL JEM-2010 microscope. The scanning electron microscopy (SEM)

Fig. 2 Typical SEM images of fractural surface of a Ag free bulk andb 0.250 wt % Ag doped Bi-2212 bulk with the pressing direction ofbulks marked in the images correspondingly

images were obtained with JEOL JSM-6700F. The AC sus-ceptibility was measured by the Superconducting QuantumInterference Device (SQUID, MPMS-XL-7) with the ACmagnetic field of 0.1, 0.5, 1.0 and 1.5 Oe and frequency of333 Hz. Meanwhile, SQUID was also used to measure themagnetic susceptibility with the background temperature of4.2 K. The same specimen was used for both the AC sus-ceptibility and magnetization measurements. The specimenswere cut from the pellet with the dimension of ∼ 2 × 2×1.5 mm, with the 2 × 2 surface parallel to the pellet plane.During the measurements, the specimen was put into a cap-sule with the 2 × 2 surface perpendicular to the magneticfield direction and fixed with low-temperature glue to avoidthe movements of specimen in magnetic field or duringcooling. After that, the BEAN model was adopted for thecalculation of the critical current density (Jc) as follows:

Jc = 20�M

b (1 − b/3a)(b < a) (1)

where a and b are the length and the width of thespecimen respectively, which are both perpendicular to theapplied magnetic field, and �M is the difference betweenM+ and M− at same magnetic field.

3 Results and Discussion

The X-ray diffraction patterns of Bi-2212 bulks with dif-ferent Ag content after SPS were obtained, as shown inFig. 1a The major phase can be indexed into Bi-2212 withorthorhombic structures. No obvious change of peak posi-tion can be observed, which suggested that the Ag atomsdid not enter into the lattice of Bi-2212. And in the samplewith large Ag content of 0.500 wt %, the diffraction peakof Ag was observed. Also in the same sample, characteristicXPS peak of Ag element was obtained, which proved thatthe Ag particles remain as elemental state in the Bi-2212bulks. On the other hand, based on the calculation of degreeof preferred orientation as shown in the inset of Fig. 1a, thedegree of preferred orientation of the doped bulks increasedgreatly comparing with that of Ag free bulk. And the den-sity of doped bulks also increased with the introduction of

J Supercond Nov Magn

Fig. 3 a Temperature dependence of the AC susceptibility for Bi-2212 bulks with Ag content of 0, 0.125, 0.25 and 0.5 wt % in the ACfield amplitude of 0.5 Oe at the frequency of 333 Hz; zooming in of bthe real, χ ′, and c imaginary, χ ′′ component; temperature dependence

of the AC susceptibility for Bi-2212 bulks with Ag content of d 0, eAg = 0.125 wt % measured in the variation of AC field, HAC = 0.1,0.5, 1.0, and 1.5 Oe at the frequency of 333 Hz; f AC field dependenceof peak temperature, Tp, with the linear fitting shown as dashed lines

J Supercond Nov Magn

Fig. 4 HRTEM images of a Ag free Bi-2212 bulk and b, cAg = 0.250 wt% doped bulk; typical grain boundary with amorphouscomponent can be observed in a; grain boundary which is well crys-tallized was shown in b; Ag nanoparticles with the radius of 5 nm canbe observed embedded in Bi-2212 matrix in c

Ag particles as listed in Table 1. Both these changes canbe explained by the thermodynamic properties change dueto the introduction of Ag particles. The decrease of meltingtemperature caused by the peritectic reaction between Bi-2212 and Ag lead to the appearance of more liquid phaseduring sintering and decreased the activated energy of crys-tallization, which improved the textural growth of Bi-2212grains.

The change of crystallization properties, density, as wellas degree of preferred orientation can also be observed inSEM images. The SEM images of the fractured surface ofthe Ag = 0 and 0.250 wt % bulk are shown in Fig. 2a, b.Platelike grains with smaller average grain size and less ori-ented structures are obtained on the Ag free bulk. Whileafter Ag doping, obvious increase of grain size can beobserved along the radial direction. And the texture withthe plate surface perpendicular to the pressing direction hasbeen formed. Based on the SEM observation, the averagegrain radius of Ag = 0 and 0.250 wt % bulks has beenestimated to be 5.6 and 8.0 μm, respectively. This obviousincrease of average grain size proved the enhancement ofcrystallization properties due to Ag addition.

AC susceptibility is widely used as a nondestructivemethod for the determination and characterization of theintergrain component in the polycrystalline hightemperaturesuperconductors. In particular the imaginary component ofthe AC susceptibility has been widely used to probe thenature of weak links in polycrystalline superconductors[33–36]. Both the temperature dependencies of the real, χ ′,and imaginary, χ ′′, parts of AC susceptibility for the Bi-2212 bulks with different Ag content in AC field of 0.5 Oewith the frequency of 333 Hz are plotted in Fig. 3a. And thereal component and imaginary component were zoomed inin Fig. 3b c, respectively It can be observed that for thesesamples, the real part of AC susceptibility, χ ′, shows twodrops as the temperature decreased. The first drop at Tc isdue to the transition within grains. Since Ag atoms did notenter into the lattice of Bi-2212, the critical temperature Tc

of 85 K was kept constant. And the second drop at Tp is dueto the occurrence of the superconducting coupling betweengrains, which became slightly sharper for Ag doped samplethan that of Ag free sample. While on each of the imaginarypart curve, χ ′′, a peak which is a measure of the dissipationin the sample, can be observed correspondingly. The peakappears at higher temperatures with proper amount of Agdoping. The result suggests that the intergranular couplingbetween grains in Bi-2212 sample has been enhanced afterAg doping.

AC susceptibility measurement is also a probe for theintragranular pinning force of Abrikosov vortices. The tem-perature dependence of AC susceptibility of Bi-2212 bulkswith the Ag doping content 0 and 0.125 wt % have been

J Supercond Nov Magn

measured and plotted in Fig. 3d, e under different ACfieldsHAC of 0.1, 0.5, 1.0, and 1.5 Oe as examples. It canbe observed that with the increase of HAC, the peak of χ ′′shifts towards lower temperature and broadened. The peaktemperatures, Tp, of each bulk have been plotted with thechange of HAC in Fig. 3f. It can be observed that the Tp

values of the doped sample are higher than those of the Agfree sample regardless of the applied AC field. By fitting theTp − HAC curves with the linear dependence from Mullermodel [37], the intragranular pinning force can be estimatedwith (2).

Tp = Tc

(1 −

(μ0

8Rgαg(0)

) 12

HAC

)(2)

where, the values of Tc is 85 K for all these samples andTp are taken from AC susceptibility measurement, Rg isthe average grain radius, which is estimated from the SEMimages as discussed above and listed in Table 1, αg(0) is theintragranular pinning force density for Abrikosov vortices.

Fig. 5 a Critical current density and b reduced flux pinning force ofBi-2212 bulks with different Ag content in the magnetic field of 0 to6 T at 4.2 K

In order to compare the pinning force density of these sam-ples, we assume the αg(0) of Ag free sample to be α′

g(0),and the obtained αg(0) of Ag doped samples are 1.33α′

g(0),1.05α′

g(0) and 0.51α′g(0) for the samples with Ag doping

content of 0.125, 0.250, and 0.500 wt %, respectively. The30 % enhancement of intragranular pinning force densitysuggested that the Ag nanoparticles embedded in Bi-2212matrix can act as effective pinning centers. While, with theincrease of Ag content, Ag particles segregated and the sizeof particles became much larger than the coherent lengthof Bi-2212. The decreasing number of effective pinningcenters finally led to the decrease of pinning force.

HRTEM images of Ag free and 0.250 wt % Ag dopedbulks have been obtained to observe the microstructureschange with Ag doping. During the observation, it is noticedthat in the Ag free bulks, amorphous components withthe thickness of over ∼3 nm, as shown in Fig. 4a can beobserved at many grain boundaries. While in Ag dopedsamples, most of the grain boundaries are well crystal-lized, even the large angle grain boundaries as shown inFig. 4b. The different grain boundaries structures can beattributed to thermodynamic properties change caused byAg doping, which not only improve the crystallization ofBi-2212 grains, but also the crystallization along grainboundaries. Meanwhile, the Ag nanoparticles can also beobserved embedded in Bi-2212 matrix as shown in Fig. 4c.No severely segregation can be observed in Ag = 0.250 wt% bulks and the average size of Ag particles kept about5 nm. Thus, the enhancement of intergrain connections andflux pinning properties caused by Ag doping as discussedabove are both proved by the microstructure observation.

The critical current densities of Ag doped Bi-2212 bulkshave been calculated based on the magnetization measure-ment within the magnetic field of 0 to 6 T at 4.2 K andplotted in the logarithmic axis in Fig. 5a With the increaseof Ag doping content, the self-field Jc increases to themaximum value, then decreased. An obvious increase ofself field critical current density, Jc of over 60 % hasbeen obtained on Ag = 0.250 wt % doped sample due tothe enhancement of intergrain connections comparing withthe Ag free sample. And the Jc values at high magneticfield also increased, which proved the enhancement of fluxpinning properties.

In order to further investigate the flux pinning mecha-nism variation of these samples, the pinning force densitiesof Bi-2212 bulks doped with different Ag content have beencalculated with the equation Fp = Jc × B. The normal-ized pinning force density (f = Fp/Fpmax) as a functionof reduced magnetic fields (b = B/Birr) are plotted inFig. 5b, where the irreversible field Birr represents the mag-netic field value at which Jc fell down to zero. The valuesof Birr are estimated from Fig. 5a using the extrapolationmethod and listed in Table 1. It is known that the pinning

J Supercond Nov Magn

mechanism can be deduced based on the analysis of f vs b

curves [38]. As shown in Fig. 5b, the major pinning mech-anism of the pure sample can be recognized as the pointpinning of normal phase, with thefmax value obtained atb ∼ 0.33. After Ag doping, the peak shifted towards lowerreduced magnetic field to b ∼ 0.20. Generally speaking, theshift of peaks suggests the change of flux pinning mecha-nism. The f vs b curves with the peak obtained at b ∼ 0.20after Ag doping could be fitted to be the surface pinningof normal phase [39]. And the pinning centers for this sur-face pinning mechanism should be the grain boundarieswith highly crystallized structures and small grain boundaryangles. Therefore, it can be concluded that with the dopingof Ag nanoparticles, the thermodynamic properties of Bi-2212 were changed, which lead to the highly crystallizedgrain boundaries instead of the amorphous grain boundaries.Both the proper thickness of grain boundaries and greatnumber of small angle grain boundaries, which is causedby the enhancement of texture structure, contributed to theenhancement of surface pinning properties and changed thepinning mechanism in this Bi-2212 system.

4 Conclusions

Bi-2212 superconducting bulks were prepared with differ-ent content of Ag nanoparticles inclusions. The introductionof Ag particles changed the thermodynamic properties ofBi-2212 obviously. Therefore, the bulks with denser struc-ture, higher degree of preferred orientation and larger grainsize have been obtained with proper amount of Ag dop-ing. The intergrain connections thus been greatly enhanced.Meanwhile, the Ag nanoparticles which still kept the origi-nal size of ∼5 nm were observed embedded in the Bi-2212matrix. Thus the intragrain flux pinning force density ofAbrikosov vortices was enhanced. Due to the improvementof intergrain connections, the self-field critical current den-sity of 0.250 wt % Ag doped sample increased for over60 %. And the in-field Jc was also enhanced due to theenhancement of flux pinning properties. The change of fluxpinning mechanism was discussed based on the measure-ment of critical current density. It can be concluded thatwith Ag doping, large number of small angle grain bound-aries with proper thickness appeared, which can work asflux pinning centers. Therefore, the surface pinning becamethe major pinning mechanism after Ag doping. The resultsof this study provide a new opportunity for the furtherenhancement of the current carrying capacity of Bi-2212wires and tapes for practical applications.

Acknowledgments This research was financially supported byNational “973” Project, under contract no. 2011CBA00104, NationalNatural Science Foundation of China under contract no. 51102198

and 51472206, the International Scientific and Technological Cooper-ation Projects of China no. S2010GR0518, the national ITER programof China no. 2013GB110001, and the Innovative Research Team ofShaanxi province no. 2013KCT-07.

References

1. Trociewitz, U.P., Schwartz, J., Marken, K., Miao, H., Meinesz, M.,Czabaj, B.: NHMFL Reports 13, 31 (2006)

2. Miao, H., Marken, K.R., Meinesz, M., Czabaj, B., Hong, S.: IEEETrans. Appl. Supercond. 15, 2554 (2005)

3. Larbalestier, D.C., Jiang, J., Trociewitz, U.P., Kametani, F.,Scheuerlein, C., Dalban-Canassy, M., Matras, M., Chen, P., Craig,N.C., Lee, P.J., Hellstrom, E.E.: Nat. Mater. 13, 375 (2014)

4. Weijers, H.M., Trociewitz, U.P., Markiewicz, W.D., Jiang, J.,Myers, D., Hellstrom, E.E., Xu, A., Jaroszynski, J., Noyes, P.,Viouchkov, Y., Larbalestier, D.C.: IEEE Trans. Appl. Supercond.20, 576 (2010)

5. Kiyoshi, T., Sato, A., Wada, H., Hatashi, S., Shimada, M., Kawate,Y.: IEEE Trans. Appl. Supercond. 9, 472 (1999)

6. Melhem, Z., Ball, S., Chappell, S.: Phys. Procedia 36, 805 (2012)7. Dalban-Canassy, M., Myers, D.A., Trociewitz, U.P., Jiang, J.,

Hellstrom, E.E., Viouchkov, Y., Larbalestier, D.C.: Supercond.Sci. Technol. 25, 115015 (2012)

8. Bock, J., Breuer, F., Bruzek, C.E., Lallouet, N., Rikel, M.O.,Walter, H.: Nexans activities and plans on HTS material. High pre-formance Bi2212 tape and bulk conductors for magnet technology(2004)

9. Ohsemochi, K., Koyanagi, K., Kurusu, T., Tosaka, T., Tasaki,K., Ono, M., Ishii, Y., Shimada, K., Nomura, S., Kidoguchi, K.,Onoda, H., Hirano, N., Nagaya, S.: J. Phys. Conf. Ser. 43, 825(2006)

10. Vinu, S., Sarun, P.M., Biju, A., Shabna, R., Guruswamy, P.,Syamaprasad, U.: Supercond. Sci. Technol. 21, 045001 (2008)

11. Bulaevskii, L.N., Daemen, L.L., Maley, M.P., Coulter, J.Y.: Phys.Rev. B 48, 13798 (1993)

12. Buhl, D., Lang, T., Gauckler, L.J.: Appl. Supercond. 4, 299 (1997)13. Stassen, S., Vanderschueren, A., Cloots, R., Rulmont, A., Ausloos,

M.: J. Cryst. Growth 166, 281 (1996)14. Jiang, J.Y., Miao, H.P., Huang, Y.B., Hong, S., Parrell, J.A.,

Scheuerlein, C., Michiel, M.D., Ghosh, A.K., Trocitwitz, U.P.,Hellstrom, E.E., Larbalestier, D.C.: IEEE Trans. Appl. Super-cond. 23, 6400206 (2013)

15. Kametani, F., Lee, E.G., Shen, T., Lee, P.J., Jiang, J., Hellstrom,E.E., Larbalestier, D.C.: Supercond. Sci. Technol. 27, 055004(2014)

16. Eastell, C.J., Henry, B.M., Morgan, C.G., Grovenor, C.R.M.,Goringe, M.J.: IEEE Trans. Appl. Supercond. 7, 2083 (1997)

17. Tsay, Y.N., Li, Q., Zhu, Y., Suenaga, M., Shibutani, K., Shigaki,I., Ogawa, R.: IEEE Trans. Appl. Supercond. 9, 1662 (1999)

18. Nakayama, Y., Motohashi, T., Otzschi, K., Shimoyama, J.,Kitazawa, K., Kishio, K.: Phys. Rev. B 62, 1452 (2000)

19. Matsushita, T., Suzuki, A., Kishida, T., Okuda, M., Naito, H.:Supercond. Sci. Technol. 7, 222 (1994)

20. Grivel, J.C., Flukiger, R.: Physica C 229, 177 (1994)21. Liu, H.B., Ferreira, P.J., Sande, J.B.V.: Physica C 303, 161 (1998)22. Majewski, P.: Supercond. Sci. Technol. 10, 453 (1997)23. Majewski, P., Aubele, A., Aldinger, F.: Physica C 341–348, 517

(2000)24. Majewski, P., Sotelo, A., Szillat, H., Kaesche, S., Aldinger, F.:

Physica C 275, 47 (1997)25. Garnier, V., Passerini, R., Giannini, E., Flukiger, R.: Supercond.

Sci. Technol. 16, 820 (2003)

J Supercond Nov Magn

26. Song, Y., Zhao, L., Li, P., Qu, T., Huang, Y., Han, Z.H.: PhysicaC 442, 134 (2006)

27. Ozkurt, B., Madre, M.A., Sotelo, A., Yakinci, M.E., Ozcelik, B.:J. Supercond. Nov. Magn. 25, 799 (2012)

28. Song, K.Y., Lee, M.S.: Supercond. Sci. Technol. 19, 1253(2006)

29. Sobha, A., Aloysius, R.P., Guruswamy, P., Warrier, K.G.K.,Syamaprasad, U.: Physica C 307, 277 (1998)

30. Zhou, R., Hults, W.L., Sebring, R.J., Bingert, J.F., Coulter, J.Y.,Willis, J.O.: Physica C 255, 275 (1995)

31. Richmonds, C., Sankaran, R.M.: Appl. Phys. Lett. 93, 131501(2008)

32. Mao, C.B., Du, Z.H., Zhou, L.: Sci. China E2, 181 (1996)33. Salamati, H., Kameli, P.: Physica C 403, 60 (2004)34. Gul, I.H., Amin, F., Abbasi, A.Z., Anis-ur-Rehman, M., Maqsood,

A.: Physica C 449, 139 (2006)35. Mihalache, V., Deac, I.G., Pop, A.V., Miu, L.: Curr. Appl. Phys.

11, 1010 (2011)36. Kameli, P., Salamati, H., Eslami, M.: Solid State Commun. 137,

30 (2006)37. Muller, K.-H.: Physica C 159, 717 (1989)38. Hamshire, R.G., Taylor, M.T.: J. Phys. F 2, 89 (1972)39. Vinu, S., Sarun, P.M., Shabna, R., Aswathy, P.M., Anooja, J.B.,

Syamaprasad, U.: Physica B 405, 4355 (2010)