Enhancement of Magnetic Relaxation Behavior by Texturing in Bi 2212 Superconducting Rods

Transcription

1 Enhancement of Magnetic Relaxation Behavior by Texturing in Bi 2212 Superconducting Rods M.Ozabaci 1 *, O. Kizilaslan 2, G. Kirat 3, M.A. Aksan 3, M.A. Madre 4, A. Sotelo 4, M.E. Yakinci 1,2 1: Scientific and Technological Research Center, SEM/EDX Lab. İnönü University, Malatya, Turkey 2: Faculty of Engineering, Department of Biomedical Engineering, İnönü University, Malatya, Turkey 3: Faculty of Arts and Sciences, Department of Physics, İnönü University, Malatya, 44280, Turkey 4: Instituto de Ciencia de Materiales de Aragón (ICMA), CSIC Universidad de Zaragoza, Maria de Luna, 3, 50018, Zaragoza, Spain Abstract Time decay of magnetization, known as magnetic relaxation, is crucial for both fundamental and applied point of view in bulk high temperature superconductors (HTS) by setting the limits to the HTS devices stability. Melt processed Bi 2 Sr 2 Ca 1 Cu 2 x Ga x O 8+δ rods (Bi 2212, x = 0, 0.1) were used to study the effect of both grain alignment and substitution on the samples critical current density, relaxation and pinning behavior. The magnetic field has been applied both perpendicular and parallel to the rods growth axis to determine the effect of grain alignment. It has been found that Ga substitution reduces grains orientation and sizes leading to lower magnetic properties. The peaks of the curves, which indicate the temperature dependence of the samples magnetic relaxation rate (S), have been observed in the 7 35 K temperature range. Characteristic pinning energy (U e /k B ) of samples was determined using the formalism developed by Maley. The change of pinning energy as a function of magnetization has been found to be exponential between 3 and 60 K, which is in agreement with the collective creep theory. Keywords: BSCCO superconductors, Magnetic relaxation, Pinning energy, Texturing, Laser processing, Alignment. *Corresponding author. Tel.: E mail address: muratozabaci@yahoo.com (M.Ozabaci) 1. Introduction Technological applications of high temperature superconductors (HTSs) require both high critical current density (J c ) and high critical field (H c2 ). These two parameters are closely 1

2 related with the high anisotropic nature of HTSs. HTSs texturing aims to increase J c and H c2 and decrease anisotropy by strengthening the intergranular junctions, leading to the enhancement of superconducting currents [1 4]. Floating zone, melt casting and hot forging routes are among the most applied methods for texturing HTSs [5 7]. Introduction of artificial defects, with sizes matching the coherence length, is another method for raising J c values. These defects can be produced by doping, cation substitution, or ion irradiation, producing nanometric ones in the first two cases or columnar defects in the last approach [8 13]. Taking into account these different possibilities, it has been reported that properly processed HTSs can reach critical current densities as high as 10 6 Acm 2 below 77 K [14]. Magnetic relaxation properties of high temperature superconductors are another critical issue that can significantly affect superconductors magnetic performance envisaging technological applications. The main source of magnetic relaxation is the magnetic flux lines motion by thermal activation or, when they are at ultralow temperatures by quantum tunneling of vortices [15, 16]. Magnetic relaxation studies on HTSs have been mostly performed to elucidate vortex dynamics and related flux pinning parameters in the materials [17 19]. HTSs doping using different types of impurities (magnetic or nonmagnetic), as well as various fabrication and measurement conditions, have often been utilized to tune up their pinning properties. These studies have shown that grains connectivity and alignment influence not only J c, but also the pinning and, in turn, the materials relaxation behavior [20 23]. In this study, the Laser Floating Zone (LFZ) technique was used to fabricate pure and Ga doped Bi 2212 superconducting ceramics in form of long cylindrical rods with 2 mm diameter. The main goal of this work is determining the magnetization decay with time and establishing the magnetic relaxation rate as a function of the applied magnetic field and temperature. Additionally, the dependence of the effective pinning energy, U e, on the magnetization will be studied to understand the vortex dynamics in the LFZ rods. 2. Experimental Bi 2 Sr 2 Ca 1 Cu 2 x Ga x O 8+δ (x = 0, 0.1) powders were prepared by the classical solid state route from commercially available Bi 2 O 3 (Panreac, 98+ %), SrCO 3 (Panreac, 98+ %), CaCO 3 (Panreac, %), CuO (Panreac, 97+ %) and Ga 2 O 3 (Alfa Aesar, %) powders. The 2

3 powders were weighed in the appropriate proportions, mixed, and ball milled using agate balls and acetone media for 30 min at 300 rpm to obtain a homogeneous mixture. The obtained suspension was dried using IR radiation in order to totally evaporate the acetone. The resulting mixture was then subjected to a two step thermal treatment consisting in heating at 750 o C and 800 ºC for 12 h, with an intermediate manual milling. After cooling, the powder was ground and isostatically pressed at approximately 200 MPa in form of cylinders (1.5 3 mm diameter and 120 mm long), following the procedure described in previous works [24]. The obtained cylinders were subsequently used both as feed and seed in a LFZ system described elsewhere [25]. The texturing process was performed using a continuous power Nd:YAG laser (λ = 1,064 nm), under air, and 15 mm h 1 growth rate. Due to the fact that Bi 2212 compounds melt incongruently, after laser processing it is necessary to perform a thermal treatment to recover the Bi 2212 superconducting phase. This annealing process consisted in two steps: 60 h at 860 O C to produce the Bi 2212 phase from the secondary ones, followed by 12 h at 800 O C to adjust the oxygen content and maximize electrical properties and a final quench in air to room temperature [26]. The so produced bars were finally cut to obtain samples having the adequate dimensions for their characterization. Phase analysis has been performed by powder XRD utilizing a Rigaku RadB X ray powder diffractometer (Cu Kα radiation) with 2θ between 5 O and 65 O. Magnetic hysteresis and the time decay of magnetization were measured using a 9 T Quantum Design PPMS system. The magnetic hysteresis cycles were determined between ±8 T at 5 and 25 K. Magnetic relaxation experiments were carried out cooling the sample to the desired temperature under zero magnetic field (ZFC). At that temperature, magnetic field was applied to the sample, kept constant for 500 s, and measuring the decrease of magnetization as a function of time. This process was repeated at several temperatures (3, 4, 5, 7, 10, 15, 20, 30, 40, 50, 60 K) at a fixed 1500 Oe applied magnetic field, using both perpendicular and parallel orientations with respect to the growth axis. 3. Results and Discussion 3.1 Critical current density 3

4 Figure 1 displays the magnetic critical current densities, J cmag, of samples as a function of the applied magnetic field at two different temperatures, doping levels and applied magnetic field directions. J cmag values have been calculated by using Bean s critical state model [27]: where ΔM is the width of the magnetization loop at the applied magnetic field and d is the cylindrical sample diameter, assuming supercurrent flow within the whole sample. The highest J cmag values, 4.63x10 5 Acm 2 and 2.19x10 5 Acm 2 have been obtained in the x=0.0 and x=0.1 Ga substituted samples, respectively. The magnetic field dependence of J cmag of all the samples displayed a decreasing trend when the magnetic field was increased, independently of the Ga content and the samples orientation. Moreover, Ga substitution led to J cmag values around one half of the determined in unsubstituted samples. Additionally, J cmag is decreased when the temperature is increased, as expected. The decrease of J cmag with Ga substitution can be explained considering the XRD spectra shown in Figure 2. In this figure, it can be clearly observed that the major peaks correspond to the Bi 2212 phase in all cases. On the other hand, Ga addition promotes the formation of small amounts of Bi 2201 phase. Therefore, it can be concluded that Ga introduction decreases the Bi 2212 phase proportion by promoting the formation of the low T c secondary phase, Bi Since the coupling of superconducting carriers occurs in Cu O planes, a substitution for the Cu directly affects the superconducting correlation in the system. Therefore, it can be deduced that the formation of the Bi 2201 due to Ga substitution might cause the break up of singlet cooper pairs (pair breaking effect). Magnetic anisotropy can be easily observed when the magnetic field is applied in different directions, as it was illustrated in Figure 1. As a consequence, it is clear that J cmag is higher when the field is perpendicularly applied to the growth axis of the rod, compared with the parallel one. The reason of this difference is due to the fact that the LFZ technique provides well aligned grains with ab planes parallel to the growth direction. Since good texture was obtained along the axial direction, J cmag easily flows in this direction while it is decreased when it has to circulate around the rod. Moreover, the behavior of J cmag depends not only on the applied field direction but also on the measurement temperature. The 4

5 difference on parallel and perpendicular J cmag at 5 K significantly decreases at 25 K, due to the increased thermal energy in the structure. This energy degrades the pinning ability of samples and decreases the alignment effect on J cmag values. The samples texture is illustrated in Figure 3, where SEM images of the different samples are shown. These images show very similar microstructure to the previously obtained in similar systems [28]. Moreover, when comparing both micrographs, it can be easily seen an increased grain misorientation together with a decrease in their sizes when Ga is added. This result is in agreement with the decrease of the J cmag upon Ga substitution. 3.2 Magnetic relaxation investigations Figure 4 shows the time dependence of non equilibrium magnetization in the two different samples orientations. In the figure, it can be observed that magnetization is changed almost linearly with ln, as expected. The slope of ln vs ln curve allows determining the normalized magnetic relaxation rate: /, which lets understanding the vortices motion in these samples. The temperature dependence of magnetic relaxation for 1500 Oe applied magnetic field, as a function of samples orientation, is displayed in Figure 5. Due to the nonlinear behavior of S, the relaxation cannot be explained using the thermally activated fluxcreep theory proposed by Kim Anderson. A detailed analysis of Bi 2212 single crystals relaxation behavior can be found in Ref. [29]. The sharp peak observed in the S T curve corresponds to the transition between two different pinning regimes. The pinning of individual flux lines takes place below the temperature at which the S T curve reaches the maximum value. A single vortex line pins to point defects with a certain characteristic energy. Above the temperature of this maximum, the collective pinning of small flux bundles begins and gradually grows into large bundles. Moreover, in the figure it can be observed that the temperature at which this maximum value appears is modified with sample orientation with respect to the applied magnetic field. It has been found that perpendicular orientation shifts this maximum from 7 K (for the parallel one) to around 35 K. This difference is due to fact that the ab planes of grains allow larger shielding currents than the ones in the c direction (see Figure 6a). As mentioned previously, LFZ texturing causes an important grain growth and their alignment along the 5

6 growth axis. As a consequence, the number of grain boundaries along this plane is greatly reduced, leading to the increase of J c along the growth axis and thus producing higher screening currents (electrical current whirling around each flux line). In the case of the field parallelly applied with respect to the ab plane, the current mostly flows along the c axis and encounters a higher number of grain boundaries due to the intrinsic nature of Bi 2212 phase (see Figure 6b). The preferential growth of this phase is along the ab plane, which allows the production of grains easily exceeding 100 m in the a, or b direction using the LFZ technique [30]. On the other hand, they are in the range of tenths of nm in the c direction producing a drastic raise on the number of grain boundaries which cause a decrease of J c in this direction. These intrinsic properties of Bi 2212 phase clearly explain the higher peak value when the field is applied perpendicular to the growth direction. The J dependence as a function of pinning energy has been determined in previous works [21]. It is well known from collective creep theory that the pinning energy is nonlinearly proportional to J. In this case, it can be deduced that the grain alignment causes stronger pinning energy, leading to an increase of temperature at which the peak in S T curve appears. In order to investigate the effect of grain alignment on the pinning energy, U(J), it has been calculated using Maley's method, which has been demonstrated to be adequate to calculate the effective pinning energy, U e, from the experimental data [31]: ln 2 where A is a time independent constant which is a function of the average hopping velocity, k B is Boltzman constant and T is the measurement temperature. In this study, the A value has been taken as 18 in agreement with the data obtained in previous works [32,33]. A correlation between the characteristic pinning energy, U e /k B, and magnetization, M, is shown in Figure 7. In this figure, it can be easily seen that the pinning energy is increased when the magnetization is decreased. The decay of / with increasing magnetization is almost exponential and it is in good agreement with the collective creep theory [15]. It is worth mentioning that U e /k B is higher when the applied field is perpendicular to the rod growth axis, which is consistent with the previous discussions. On the other hand, in order to determine if these results are reproducible, a second set of data are presented in Figure 7b 6

7 for the x=0.1 Ga substituted sample. As it can be easily observed, Ga substitution is detrimental for the magnetic properties and the pinning energy, as expected from the previous results. Although Ga substitution was expected to produce better pinning properties (by formation of small, non superconducting phases which could act as effective pinning centers), when compared to undoped Bi 2212 [34,35], it has been found that the texture degradation and the formation of higher amount of low temperature Bi 2201 phase avoid the possible pinning improvements promoted by Ga doping. 4. Conclusion In this work, magnetic relaxation experiments were carried out to investigate the vortex dynamics in Bi 2212 textured rods. The J c dependence with the grain alignment was also investigated in detail. A clear magnetic anisotropy has been observed in all the samples, which show different properties as a function of the applied field direction (parallel or perpendicular to the growth axis). This effect can be clearly observed due to the good grain orientation (ab plane parallel to the growth direction) obtained when the samples are processed through the LFZ method. These characteristics are clearly reflected in the microstructural observations. On the other hand, it has been found that Ga substitution leads to the decrease of the superconducting characteristics of samples, inducing grains misalignment, smaller sizes and higher amount of secondary phases. In spite of these issues, the texturing process has been shown to be a crucial tool to improve the magnetic relaxation properties. The peak in S T curves has been shifted from 7 K to 35 K, for parallel and perpendicular orientations, respectively. In addition, it was found that pinning energy follows a nonlinear J dependence, as expected from the collective creep theory, and that the characteristic pinning energy was increased by texturing. References [1] S.N. Zhang, C.S. Li, Q.B. Hao, X.B. Ma, T.N. Lu, P.X. Zhang, Optimization of Bi 2212 high temperature superconductors by potassium substitution, Superconductor Science & Technology, 28 (2015) [2] P. Xie, T.M. Qu, K.T. Huang, F. Feng, Z.H. Han, Critical current density improvement by intermediate deformation for the fabrication of Bi 2 Sr 2 Ca 2 Cu 3 O 10+δ /Ag round wires, Materials Express, 4 (2014) [3] M. Tepe, Y. Uzun, U.S. Gokay, Superconducting Properties of Melt Textured Growth YBa 2 Cu 3 O 7 x Materials with Y211 Addition, Journal of Superconductivity and Novel Magnetism, 28 (2015)

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10 Figure 1. Magnetic field dependence of magnetic critical current density, J cmag, in Bi 2 Sr 2 Ca 1 Cu 2 x Ga x O 8+δ textured rods, for x a) 0; and b) 0.1, at 5 and 25 K between 0 8 T applied field. Figure 2. Powder XRD patterns of Bi 2 Sr 2 Ca 1 Cu 2 x Ga x O 8+δ textured rods, for x a) 0; and b) 0.1. Figure 3. SEM images of longitudinal polished cross sections performed on Bi 2 Sr 2 Ca 1 Cu 2 xga x O 8+δ textured rods, for x a) 0; and b)

11 Figure 4. Time dependence of the non equilibrium magnetization measured under H=0.15 T at different temperatures. Magnetic field was applied both a) perpendicular ( ) and b) parallel (//) to the growth axis of the undoped textured rods. Figure 5. Temperature dependence of the normalized magnetic relaxation rate, S, under H=0.15 T applied magnetic field along the two different directions of the undoped textured rods. 11

12 Figure 6. Schematic representation of the grain orientation, applied magnetic field and critical current flowing through the textured rods. Figure 7. Characteristic pinning energies, /, of Bi 2 Sr 2 Ca 1 Cu 2 x Ga x O 8+δ textured rods, for a) 0; and b) 0.1, as a function of magnetization along the two different applied magnetic field directions at several temperatures. 12