Magnetic relaxation behavior in the Bi2Sr2Ca2Cu3−xMoxO10+δ system fabricated by glass-ceramic technique

Journal of Magnetism and Magnetic Materials 384 (2015) 186–191

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Magnetic relaxation behavior in the Bi2Sr2Ca2Cu3
xMoxO10 þ δ system fabricated by glass-ceramic technique O. Kizilaslan n, G. Kirat, M.A. Aksan Inonu Universitesi, Fen Edebiyat Fakultesi, Fizik Bolumu, 44280 Malatya, Turkey

art ic l e i nf o

a b s t r a c t

Article history: Received 25 November 2014 Received in revised form 19 February 2015 Accepted 20 February 2015 Available online 21 February 2015

In this paper, we present a study on the critical current density and magnetic relaxation behavior of the Mo-substituted BiSrCaCuO system. The Bi2Sr2Ca2Cu3
xMoxO10 þ y, where x ¼0, 0.5, 1.0 and 1.5 have been fabricated using glass-ceramic technique. It was found that the Mo-substitution to the BiSrCaCuO system lead to the formation of pinning centers and hence an increase of critical current density. The magnetic relaxation experiments were performed to understand physical phenomena behind of flux motion. Time dependence of the magnetization exhibited thermally activation flux motion in the samples. It was found that the normalized magnetic relaxation rate, S, did not vary linearly with temperature, especially at high temperatures, which is attributed to the collective creep theory. In addition, the characteristic pinning energy, Ue/KB, in the samples was calculated using Maley's method. Correlation between the pinning energy and the magnetization showed that obtained experimental results can be fitted to the collective creep theory. & 2015 Elsevier B.V. All rights reserved.

1. Introduction The critical current density (Jc) and upper critical field (Hc2) are two most important parameters of the superconducting systems. High Jc values are required for large scale applications of the superconducting materials. Therefore, extensive efforts have been devoted to increase Jc of the superconducting materials. High temperature (HTc) BiSrCaCuO (BSCCO) system exhibits strong anisotropic behavior with different superconducting properties along and perpendicular to the layers [1]. It was reported that the critical current density on the a–b plane is around Jab  106 A cm
2 at temperatures below 77 K while the critical current density along the c-axis is only Jc 103 A cm
2 below 77 K [2,3]. The Jc value can be found in two different ways: directly by transport measurements [4] or by semi-experimental equations such as Bean's equation using magnetization hysteresis (M–H) data [5,6]. It is very important to enhance the flux pinning properties for practical applications. In recent years, it was showed that stacking faults, dislocations, oxygen defects and twin boundaries act as effective pinning centers which improve Jc in the HTc superconducting systems. The flux pinning can be enhanced by chemical substitution/doping to the materials or by directly embedded of nanosized particles [7,8]. The HTc superconducting systems show a strong magnetic relaxation effect [9]. The relaxation process provides unique n

Corresponding author.

http://dx.doi.org/10.1016/j.jmmm.2015.02.045 0304-8853/& 2015 Elsevier B.V. All rights reserved.

opportunities to understand physical phenomena behind the flux motion. Some effects such as thermal fluctuations, mechanical vibrations can cause non-equilibrium distribution of vortices under magnetic field. Current loops start to redistribute at a constant magnetic field and lead to a decay of the non-equilibrium magnetization with time. Thus, the decay of the magnetization can be thought as a spontaneous motion of the flux lines out of the pinning potential well. Such a motion can be explained by thermally activated flux flow (thermally activated flux-creep) [10–12], which induces the change of the magnetic moment in the superconducting materials. However, the change of the magnetization arises from quantum tunneling at low temperatures, at which thermal energy can be neglected. According to Anderson–Kim model, temperature dependence of the normalized relaxation rate (S–T) is expected to be linear in conventional superconductor. However, the HTc systems show extremely short coherence length and high operation temperatures, vortex motion and fluctuations due to strong anisotropy. The S–T characteristic of the HTc systems, unlike conventional superconductors, is not linear with temperature and a plateau region is observed [13]. Different preparation techniques such as solid-state reaction technique [14], sol–gel technique [15] and glass-ceramic technique [16] have been used to obtain the samples having good superconducting properties. Glass-ceramic technique has been used only for preparation of the HTc BiSrCaCuO system [16]. Glass-ceramic technique possesses some advantages, compared to other preparation techniques; nonporous, high-density and homogeneous

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structure with strong grain connections can be easily achieved. Additionally, the compositional inhomogeneity and grain boundary effects can be reduced in the HTc materials. In this study, the critical current density properties of the Mosubstituted BiSrCaCuO materials prepared by the glass-ceramic technique were investigated. The main scope of the paper is to investigate the decay of the magnetization with time and to discuss the magnetic relaxation rate as a function of the magnetic field and temperature. Additionally, the dependence of the effective pinning energy, Ue, on critical current, Jc, and hence the magnetization to understand the vortex dynamics in the prepared materials was also examined in details.

2. Experimental Glass samples with nominal compositions of Bi2Sr2Ca2Cu3
xMo where x ¼0.0, 0.5, 1.0 and 1.5, were prepared by glassceramic technique. High purity (Alfa Aesar, 99.99%) powders of Bi2O3, SrCO3, CaCO3, CuO and MoO3 were mixed in an agate mortar for 3–5 h. The mixture was melted in an α-alumina crucible at 1200 °C for 1–3 h, depending on the substitution level. Molten mixture was quickly poured onto a cold copper plate and pressed with another cold plate. Then, rapidly quenched amorphous materials were obtained. The x ¼0.0, 0.5, 1.0 and 1.5 Mo-substituted glass samples were heat treated at 855 °C for 120 h, 855 °C for 120 h, 840 °C for 120 h and 830 °C for 120 h, respectively, in P.I.D. controlled furnace in oxygen atmosphere, which are their optimum heat treatment conditions. The cooling and heating rate during the heat treatments were chosen as 10 °C min
1. Both amorphous and crystalline samples were characterized by X-ray diffraction analysis (XRD). The XRD analyses were performed using Rigaku RadB powder diffractometer and CuKα radiation (λ ¼ 1.5405 Å). All samples were examined with a scan speed of 3° min
1 and 2θ between 3° and 60° at room temperature. Magnetic hysteresis and the decay of the magnetization were measured using a 9 T Quantum Design PPMS system. Jc was determined using Bean's equation from the magnetic hysteresis measurements. Magnetic relaxation experiments were carried out as in the follow: sample was cooled down to the desired temperature under zero magnetic field (ZFC). At that temperature, the magnetic field was applied to the sample and kept constant for 200 s. Simultaneously, the decrease of the magnetization was measured as a function of time. This process was repeated for different magnetic fields from 1.25 kOe to 10 kOe and temperatures from 5 K to 60 K, respectively. xO10 þ y,

3. Result and discussion 3.1. Critical current density Fig. 1a shows magnetic hysteresis (M–H) curves of the samples at 5 K. The magnetization increased with increasing the Mo-concentration in the system. The highest magnetization was obtained for the x¼1.0 Mo-substituted sample. M–H curves of the x¼ 1.0 Mo-substituted sample at four different temperatures are shown in Fig. 1b. At 5 K and 10 K, general M–H characteristic for the HTc superconducting materials was obtained. But, it was seen that the shape of M–H curves changed for T4 20 K. Especially at high magnetic fields, any hysteresis in M–H curves was not observed. Jc of the samples was calculated using Bean's equation [4,5]:

Jc =

20ΔM a ((1 − a/3b))

(1)

where ΔM = M + − M−, a and b (a4 b) is the dimensions of the

Fig. 1. (a) M–H loops of the samples at 5 K and (b) M–H loops of the x ¼ 1.0 Mosubstituted sample at four different temperatures.

samples. The calculated Jc values of the samples are listed in Table 1. Fig. 2 shows Jc of the Mo-substituted samples as a function of magnetic field at 5 K. The magnetic field dependence of Jc of the samples displayed similar decreasing trend with increasing the magnetic field. When Mo was substituted for Cu in the BiSrCaCuO system, Jc of the Mo-substituted samples was found to be higher than Jc of the unsusbtituted (x¼0.0) sample. It was revealed that the x¼ 1.0 Mo-substituted sample possessed the highest Jc value (6.96  105 A cm
2). But, when the substitution level was increased to x ¼1.5, it was seen that Jc decreased. The results indicate that the Mo-substitution at low levels caused the formation of pinning centers such as secondary/impurity phases and point defects in the samples. XRD patterns of the samples confirm this situation. XRD patterns of the glass and the glass-ceramic samples heat treated at their optimum conditions are showed in Fig. 3a–e, respectively [17]. Although fully glass materials were obtained at low substitution levels (for x ¼0.0–1.0) as seen in Fig. 3a, the materials in crystalline form were obtained at high substitution level (for x¼1.5), Fig. 3b. It was detected that impurity phases such as SrMoO4, CuMoO4, Cu6Mo5O18 and Bi2MoO6 were formed in the glass matrix. For the x ¼0.5 sample, the material consisted mainly of the Bi-2212 phase, Fig. 3c. However, the Bi-2223, Bi2Sr2.5Ca2.5Cu3O10 þ δ HTc superconducting phases and the Cu3Mo2O9, Bi0.27Mo0.12O5 secondary/impurity phases were also detected. When Mo-substitution level was increased to x¼ 1.0,

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Table 1 Jc value (  105 A cm
2) of the samples depending on the magnetic field and temperature. Material

x ¼0.0

x ¼0.5

x ¼1.0

x ¼1.5

H (kOe)

5K

10 K

20 K

30 K

0 10 20 30 40 50 60 70 80 90 0 10 20 30 40 50 60 70 80 90 0 10 20 30 40 50 60 70 80 90 0 10 20 30 40 50 60 70 80 90

1.57 1.08 0.82 0.68 0.56 0.50 0.44 0.39 0.33 0.20 3.97 2.19 1.65 1.47 1.31 1.18 1.12 1.03 0.96 0.84 6.96 3.40 1.64 2.18 1.91 1.76 1.61 1.46 1.33 1.24 3.90 1.94 1.40 1.15 0.96 0.84 0.74 0.68 0.62 0.49

1.13 0.70 0.52 0.38 0.29 0.22 0.17 0.15 0.12 0.07 2.02 0.86 0.65 0.53 0.41 0.34 0.28 0.21 0.17 0.13 3.67 1.39 0.99 0.76 0.60 0.46 0.38 0.29 0.21 0.17 2.26 0.89 0.60 0.45 0.34 0.26 0.21 0.12 0.09 0.05

0.63 0.29 0.16 0.08 0.04 0.02 0.01 – – – 0.44 0.14 0.09 0.05 0.01 – – – – – 1.04 0.23 0.13 0.07 0.02 – – – – – 0.61 0.20 0.12 0.07 0.02 – – – – –

0.34 0.11 0.06 0.03 0.017 – – – – – 0.22 0.10 0.07 0.04 0.011 – – – – – 0.43 0.17 0.11 0.06 0.023 – – – – – 0.33 0.16 0.11 0.06 0.021 – – – – –

Fig. 3. The XRD patterns of the Bi2Sr2Ca2Cu3
xMoxO10 þ y system: (a) the x ¼0.5 heat treated at 855 °C for 120 h; (b) the x¼ 1.0 sample heat treated at 840 °C for 120 h; (c) the x¼ 1.5 sample heat treated at 830 °C for 120 h [(Ο) Bi-2223 phase; (n) Bi-2212 phase, (♦) Bi2Sr2.5Ca2.5Cu3O10 þ δ phase; (▼) Cu3Mo2O9; (★) Bi0.27Mo0.12O5, (□) SrMoO4; (✪) Cu6Mo5O18; (●) Bi2MoO6; (◘) CuMoO4].

non-superconducting grains. It can be concluded that at high substitution level (x ¼1.5), dense secondary phases and/or defects produced in the sample destroyed the superconducting regions and hence Jc decreased. The increase of Jc in the x ¼1.0 Mosubstituted sample can provide an evidence for the increase of the London penetration depth (λL). However, in the highest substitution level (x ¼1.5), the decrease of Jc suggests that the pinning center density exceeds coherence length. It is worth mentioning that Jc decreased strongly with increasing the applied magnetic field. Especially, at high magnetic fields (460 kOe) and high temperatures (20 K and 30 K), since any hysteresis in the M– H curves was not obtained, the Jc value of the samples was not able to calculate. Jc decreased as the temperature increases. At low temperatures, the flux pinning is strong and the fluxes mostly penetrate to the samples as vortices. Magnetization caused by the pinning decreases as the temperature increases. The vortex motion in the samples fabricated begins with increase of the temperature, which leads to a deterioration in the magnetic configuration of fluxes [18]. In general, it was reported that Jc in the HTc BSCCO system can be obtained around 103–105 A cm
2 at temperatures between 5 K and 77 K, depending on the fabrication technique, stoichiometry, substitution/doping, temperature etc. [2,19–23]. It was found that the BSCCO system fabricated by glass-ceramic technique has Jc less than 104 A cm
2 at temperatures between 5 K and 77 K [24–26]. It should be noted that there is a limited number of studies related to the effects of the Mo-substitution on the BSCCO system fabricated by different techniques in literature [17,27]. The Jc value in the Mosubstituted BSCCO system fabricated by solid-state reaction technique was found to be ∼103 A cm
2 at 10 K, which is approximately hundred times less than the Jc value found for the pure BSCCO system. The Jc value in magnitude of ∼105 A cm
2 obtained in the present study suggests that the Mo substitution to the glassceramic BSCCO system increased significantly the number of the pinning centers and thus strong improvement of Jc was achieved.

Fig. 2. Magnetic field dependence of Jc of the samples.

3.2. Magnetic relaxation investigations similar phases were observed, as seen in Fig. 3d. For x¼ 1.5 sample, the intensities of both superconducting and secondary/impurity phases decreased significantly, Fig. 3e. Multiphase and complex crystallized material was obtained. This suggests that the excess Mo content in the system largely deformed the phase coordination of the BSCCO system and caused the growth of

Fig. 4a and b shows time dependence of non-equilibrium magnetization, (−M , t ), of the x ¼0.0 and x ¼1.0 Mo-substituted BSCCO samples at different temperatures. The other materials also exhibited similar (−M , t ) trend. It was seen that the magnetization changed almost linear with ln (t), as expected.

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information about motion of vortices in the samples. Fig. 5 shows temperature dependence of the magnetic relaxation rate, S, at different substitution levels. S(T) showed exponential dependence on temperature. Faster relaxation at high temperatures than the relaxation at low temperatures was observed in the samples. Due to exponential behavior, the relaxation cannot be explained in framework of thermally activated flux-creep theory proposed by Kim–Anderson, but with collective creep theory [13,28,29]. The critical density, J, is far below Jc in this region and J decays nonlogarithmically in time, suggesting that thermally activated vortices creep as a bundle. Similar results for the BSCCO system are also obtained by other researchers [30]. Another important parameter which characterizes vortex dynamics in the HTc superconductors is the dependence of the pinning energy, U, on J and M. According to the collective creep theory, the pinning energy, U, changes nonlinearly with J [9]. U as a function of J is given as bellow:

U (J ) =

μ ⎤ U0 ⎡ ⎛ Jc 0 ⎞ ⎢ ⎜ ⎟ − 1⎥ ⎥⎦ μ ⎢⎣ ⎝ J ⎠

(2)

where U0 is the characteristic pinning energy, J current density, m the glassy exponent, which determines the nature of the pinning barrier and Jc0 the critical current density at which the barrier vanishes. The m value gives information about size of the vortex bundle. If m is
1, Eq. (2) is reduced to Anderson–Kim model which describes linear dependence of U on J. For m 40, Eq. (2) describes a nonlinear U–J variation. In order to determine nature of flux creep, the pinning energy, U(J), should be calculated. Maley's method is used to calculate experimentally the effective pinning energy Ue [31]:

Ue dM = − T ln + TA kB dt

Fig. 4. Time dependence of the non-equilibrium magnetization
M at different temperatures: (a) x ¼0.0 and (b) x ¼ 1.0 Mo-substituted sample.

(3)

where A is time independent constant depending on the average hopping velocity, kB Boltzman constant and T the measurement temperature. In this study, the constant A was chosen to be 18 from [32,33]. A correlation between characteristic pinning energy, Ue/KB, and magnetization, M, is given in Fig. 6. Magnetic relaxation data at different temperatures was scaled to a single curve. Experimental data was fitted to collective creep theory. The decay of characteristic pinning energy, Ue/KB, with increasing the magnetization was fitted to Eq. (2), dashed lines in Fig. 6. It was found that Ue/KB relaxed according to collective creep theory as the

Fig. 5. Temperature dependence of the normalized magnetic relaxation rate, S, at an applied magnetic field of H¼1.25 kOe.

The slope of ln (
M) vs ln (t) curve gives the normalized magnetic relaxation rate; S = d ln ( − M)/d ln (t) S= dln ( − M)/dln (t) . The magnetic relaxation rate presents important

Fig. 6. Magnetization dependence of characteristic pinning energy, Ue/kB. The curves are scaled to single curve at 1.25 kOe. The dashed lines give fitting curves.

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Table 2 The obtained m and Ue/KB values. Material x

Ue/kB (K)

l

0.0 0.5 1.0 1.5

13.24 16.16 18.40 15.37

0.771 0.773 0.812 0.797

obtained for the x ¼ 0.5 and 1.0 Mo-substitution samples. The highest Jc value was found to be 6.96  105 A cm
2 at 0 T and 5 K for the x¼ 1.0 sample, suggesting that the Mo-substitution caused good pinning centers until x ¼1.0. But, at high substitution level (x ¼1.5), many number of the secondary/impurity phases and/or defects produced in the sample destroyed the superconducting regions and hence Jc decreased significantly. Magnetic relaxation experiments were carried out to investigate the vortex dynamics of the Bi2Sr2Ca2Cu3
xMoxO10 þ y system. It was found that the pinning energy follows a nonlinear J dependence. The Ue/KB value increased with increasing the Mocontent (for x ¼0.5 and 1.0) in the system. The m values obtained near 0.8 indicates the presence of large vortex creep at 1.25 kOe. Normalized magnetic relaxation rate, S, was investigated as a function of the magnetic field. The magnetic field at which S reaches saturation did not change for all the substitution levels of Mo. The obtained results suggest that the high Mo-substitution level does not lead to further enhancement of pinning. In particular, the centers such as secondary/impurity phases and/or defects do not act as additional pinning centers in the system. This can be ascribed to the large size of these centers compared to the dimensions of the vortex bundles introduced in collective creep theory.

Acknowledgment

Fig. 7. Magnetic field dependence of the normalized magnetic relaxation time, S, at different Mo-substitution levels.

magnetization increased. The m and Ue/KB values obtained by fitting are listed in Table 2. The Ue/KB value increased with increasing the Mo-content in the system, suggesting that the secondary/impurity phases and/or defects generated by the Mosubstitution act as good pinning centers. The obtained m values are very close to m ¼0.78 value given in collective creep theory [9,33], indicating that the flux creep is caused by the large vortex bundle (Rb) in all the samples. In collective creep theory, it is assumed that bundle size is much larger than London penetration length (λl). In addition, it can be easily seen from Fig. 6 that the pinning energy increased with decreasing the magnetization. On the other hand, the highest Ue/KB was obtained to be 18.4 K (Ue ¼1.58 meV) at 1.25 kOe for the x¼ 1.0 sample. However, the pinning energy decreased at highest substitution level (x ¼ 1.5). This indicates that the Mo-concentrations higher than x¼ 1.0 are detrimental for the pinning forces. Eq. (2) gives J-dependence of the pinning energy. It was found that the pinning energy is nonlinearly proportional to J. Thus, the decrease of the pinning energy for the x ¼1.5 sample can be attributed to the decrease of J, as expected from Fig. 2. Magnetic field dependence of S is shown in Fig. 7 for the different substitution levels. It was found that S possessed a small initial value (E0.013) at low magnetic fields. It increased with the applied magnetic field and then reached almost saturation. This means that vortices reaches equilibrium state much more quickly with increasing the magnetic field. However, the magnetic field at which S saturates did not change with the Mo-concentration in the system and stayed constant at ∼10 kOe.

4. Conclusion In this study, dependence of Jc on the Mo-substitution to the BSCCO system was investigated in details. An increase in Jc was

This work was supported by the Research Fund of Inonu University, Turkey under Grant Contract no. 2013-46.

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