BZO composite superconductors

Accepted Manuscript

The magnetoresistance of YBCO/BZO composite superconductors Bilal A. Malik , K. Asokan , V Ganesan , Durgesh Singh , Manzoor A. Malik PII: DOI: Reference:

S0921-4534(16)30170-8 10.1016/j.physc.2016.11.004 PHYSC 1253104

To appear in:

Physica C: Superconductivity and its applications

Received date: Revised date: Accepted date:

20 June 2016 25 October 2016 4 November 2016

Please cite this article as: Bilal A. Malik , K. Asokan , V Ganesan , Durgesh Singh , Manzoor A. Malik , The magnetoresistance of YBCO/BZO composite superconductors, Physica C: Superconductivity and its applications (2016), doi: 10.1016/j.physc.2016.11.004

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Highlights

Limited addition of BZO in YBCO shows low resistive tailing behavior. Limited addition of BZO in YBCO increases the activation energy of flux lines. Vortex glass transition temperature increases with the limited addition of BZO. Significant enhancement of is observed up to 4% BZO addition.

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The magnetoresistance of YBCO/BZO composite superconductors Bilal A. Malik1, K. Asokan2, V Ganesan3, Durgesh Singh3 , and Manzoor A. Malik1* 1

Department of Physics, University of Kashmir, Srinagar- 190006, India.

2

Materials Science Division, Inter University Accelerator Centre, New Delhi -110067, India.

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UGC-DAE Consortium for Scientific Research, Khandwa Road, Indore (MP) 452 001, India. Email address: *[email protected]

Abstract

We study the effect of addition of BaZrO3 (BZO) on normal and superconducting state of YBa2Cu3O7-δ (YBCO). We find that in general both room temperature and residual resistivity increase with the addition

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of BZO except at low concentration of BZO. The temperature dependence of resistivity in presence of magnetic field also shows less resistivity broadening in composites containing low concentration of BZO below transition temperature (TC). The zero temperature upper critical field (

), estimated by

using Werthamer, Helfand and Hohenberg theory and Ginzburg Landau theory, shows an increase by the finite addition of BZO in YBCO. Further, the activation energy (U0) determined from Arrhenius plots enhancement in

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and vortex glass transition temperature (Tg) also increase with the limited addition of BZO. Such an , Uo and Tg has been attributed to the increase in grain connectivity of YBCO .

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We conclude that the limited addition of BZO in YBCO significantly improves its superconducting performance in magnetic environment.

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Key words: Superconductivity, YBCO, Inter grain connectivity, Activation energy, Upper critical field Introduction: The relatively high transition temperature and high critical magnetic field make the cuprate

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oxide superconductors ideally suited for high current applications. However, these materials fail to deliver in magnetic environment due to poor flux pinning and grain boundaries which restrict their current carrying capabilities [1-5]. The addition of composites like oxides [6-8], metals [ 9-12] and other

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ceramics [13-17] in cuprates has dramatically enhanced their prospectus for promising applications. Among them BZO has been one of the most important material for making YBCO attractive for high current applications [18, 19]. The addition of BZO in YBCO improves its critical current density [20, 21, 22]. Significant improvement in the dc properties of YBCO has been found with the addition of BZO particles over the dimensions of 10-100nm [23, 24]. In this study, we are looking at parameters other than critical current density (J C) which are important in their own right. Specifically, we here study the effect of secondary phase addition on the vortex glass 2

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transition temperature, zero temperature upper critical field and

activation energy. Study of these

parameters is important to determine the superconducting performance of a material in high magnetic field. These parameters are sensitive to the type and density of secondary phase which alters the inter grain coupling of YBCO. Such studies have been carried out for Ag as a secondary phase [12, 25, 26]. Here we choose BZO as a secondary phase because of its excellent lattice mismatch with YBCO. Also the sub- micron sized BZO particles help in improving the inter grain coupling of YBCO in high magnetic

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field. Further, since optimization of these properties requires a wide choice of the concentration of BZO, we have carried out these studies for YBCO+xBZO composites with x= 0.0%, 2.0%, 4.0%, 6.0% and 10.0 wt.%. Experimental Details:

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All the samples reported in this study were prepared by solid state reaction method. YBCO was prepared by thoroughly mixing the high purity powders of Y2O3 (99.99%), BaCO3 (99.99%) and CuO (99.99%) according to exact stoichiometric ratio Y:Ba:Cu=1:2:3. After initial grinding, this mixture of powders was calcined at temperatures 900⁰C, 915⁰C and 930⁰C

in air for 12h with intermediate grinding. The

superconducting composites of YBCO+ dielectric BZO were obtained by adding BZO in the pre-reacted YBCO. A series of polycrystalline composite samples YBCO+xBZO with nominal composition x = 0, 2,

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4, 6 and 10 wt.% were obtained by calcining the thoroughly mixed powders at 950⁰C for 12 h. Finally, the products were again reground and pressed into pellets of size 1cm and sintered at 950⁰C for 16h in

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presence of oxygen for achieving proper density. In order to get exact oxygen stoichiometry the process was followed by oxygen annealing at 650⁰C, 550⁰C and 450⁰C for 12h at each temperature and cooled to room temperature for another 6h. The phase purity of these pure and composite samples was determined

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by using Bruker D8 Advance X-ray diffractometer with Cu Kα radiation ( data was collected over diffraction angle range

⁰

). The diffraction

⁰ with setting of 40mA current and 40 kV

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voltage. The surface morphological studies were carried out using scanning electron microscope (SEM) MIRA II LMH, TESCAN. Precise electrical resistivity measurements at close intervals of temperature

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were carried out using Quantum Design Physical Property Measurement system (PPMS) in four probe configuration down to 2K with magnetic field up to 13T. The resistivity in the superconducting state was measured by sweeping temperature at fixed fields. Results and Discussion: The Rietveld refinement of the XRD reveals that all samples crystallize in orthorhombic structure with P mmm space group at room temperature. It has been observed from the diffraction pattern that BZO remains a separate phase within YBCO which was further confirmed from SEM images. SEM images 3

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suggest grain connectivity of YBCO is improved by the addition of BZO at low concentrations while as the addition of higher concentration of BZO results in decoupling of grain due to the porous nature of BZO. Both XRD and SEM results of YBCO+ xBZO (x=0.0, 2.0, 4.0, 6.0 and 10.0 wt. %) samples are discussed in Malik et al [22]. These results motivate us to probe the effects on grain coupling for YBCO with the addition of BZO in terms of resistivity measurements under high magnetic fields. Fig. 1 shows ⁄

) of YBCO+ xBZO

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the temperature dependence of the electrical resistivity ρ(T) and its derivative (

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(x=0.0, 2.0, 4.0, 6.0 and 10.0 wt. %) composites in zero magnetic field. It is clear that BZO effects

Figure.1: (a) Temperature dependence of the resistivity for YBCO+ xBZO composites (x=0.0, 2.0, 4.0,

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6.0 and 10.0 wt. %). The linear fitting of the resistivity in the temperature range 150–250K, extrapolated to 0K gives resistivity slope (d𝞀/dT) and residual resistivity (𝞀0) (b) Temperature derivative of resistivity

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for YBCO+ xBZO composites (x=0.0, 2.0, 4.0, 6.0 and 10.0 wt. %) . both normal and superconducting state of composite samples. The room temperature resistivity as well as residual resistivity in general increases on addition of BZO in YBCO. However, the room temperature resistivity is found to be less than pristine YBCO up to 4%BZO while as the residual resistivity is found low in case of 2%BZO than pristine YBCO. The increase in room temperature resistivity in nearly 2.3 times and residual resistivity is nearly 4.4 times in case of 10% addition of BZO in YBCO. This implies that the addition of BZO in YBCO fills the voids and cracks up to 4%BZO thereby increase its transport properties. At higher concentration, BZO gets segregated on grain boundaries that increase residual as 4

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well as room temperature resistivity. Further, all the samples were found superconducting around 90K that shows the onset of superconductivity in YBCO has not been hampered with the addition of BZO. Fig 1(b) shows that transition width as well as two stage transition decreases with the addition of BZO in YBCO up to 4wt.% signifying a better transport channels between grains. The whole resistive plot (𝞀 vs T) can be divided into two different regimes: one corresponding to the normal state behavior (above 2Tc) which follows Anderson and Zou relation

[27], where

is normal state resistivity,

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B is slope and A is intercept. The other region is superconducting characterized by non linearity and is due to the formation of bound states of electrons. The slope of resistivity (d

/dT=B) has been calculated

by linear fitting of resistivity within temperature range 150–250 K and extrapolation to 0K determines residual resistivity (

= A). On applying Matheissen’s rule to the normal state, the total electrical

resistivity can be written as

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(1)

The extrinsic term

is insensitive to temperature and arises from grain boundaries, dislocations,

vacancies etc. while as the temperature dependent intrinsic term

arises from electron-electron

scattering and scattering from elementary excitations such as lattice viberations (phonons). Since the residual resistivity is temperature independent, therefore, effect on electron scattering with the addition of

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BZO can be evaluated over a range of temperatures. The last column of Table (1) last column shows change in intrinsic resistivity (electron scattering ) from temperature 300K to 100K with the addition of

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BZO by using Matheissen’s rule. The change in intrinsic resistivity for pure sample is nearly three times that of 10% BZO composite sample. Hence, with the addition of BZO in YBCO, the contribution from

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residual resistivity increases more appreciably as compared to intrinsic resistivity.

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Table 1 Variation of parameters associated with resistivity for different concentrations of BZO wt.% added. ⁄

(µΩcm)

(µΩcm)

(µΩcmK )

0.0

3130

5.23

1540

1990

2.0

2540

3.56

1460

1740

4.0

2860

3.43

1810

2090

6.0

3920

2.26

3230

3460

10.0

7270

1.89

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(µΩcm)

BZO (wt.%)

⁄

6750

6880

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-1

1.57

1140

1.45

800

1.36

770

1.13

460

1.06

390

The investigation of resistivity transition under magnetic field is important for probing the intergranular medium of cuprates and in calculation of parameters like T g,

and U0. Because of the granular

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nature of cuprates the resistivity transitions under magnetic field mainly breaks into two steps: i) a steep transition indicating onset of superconductivity in individual grains and ii) a long transition tail due to The temperature derivative of resistivity

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connective nature of grains [28, 29].

informative in probing the two peak behavior of resistivity transition in cuprates.

) is more ) gives narrow

intense peak near onset and broad peak at low temperatures signifying the characteristics of intergranular

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medium [28, 30]. The two peak behavior has been observed even in single crystal of YBCO with high Oxygen content [31]. Further, one can study the thermally activated flux flow behavior by investigating for vortex dynamics [32-34]. The upper critical magnetic field

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resistivity transitions in magnetic field

derived from resistivity measurements gives us information about the applicability of material and also

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allows us to calculate the microscopic parameters like coherence length of superconducting state. To study the role of BZO as an intergranular medium of YBCO we have measured resistivity as a function of temperature for composite samples under applied magnetic field up to 13T. The plots are shown in Fig. 2 from 20 to 100K.The transitions are relatively sharp in zero field but broadening takes

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Fig 2: Temperature dependence of electrical resistivity of YBCO+ xBZO (x=0.0, 2.0, 4.0, 6.0 and 10.0

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wt. %) composites for different applied magnetic fields up to 13T. place with application of magnetic field. Once the applied magnetic field exceeds lower critical magnetic

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field, the vortex formation takes place. These vortices with sufficient thermal energy are responsible for broadening of resistive transitions. The resistive transition occurs in two steps which is consistent with earlier results [26, 28, 35]. In the first stage, the individual grains become superconducting that show sudden drop in resistivity signifying onset of superconductivity (T on). Due to short coherence length of cuprate and iron based superconductors the onset of superconductivity is field independent. On lowering the temperature further, intergranular links become activated and the long range ordered superconducting state with zero resistance at global superconductivity temperature (TC0) is established [36]. These inter 7

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grain links play important role in applicability of superconducting materials and are extremely sensitive to impurities as well as applied magnetic field [37, 38]. The observed tailing behavior of composite samples can be attributed to the disturbance of percolation path between grains due to misorientation of grains [39].

Thus unlike onset temperature (Ton),

TC0 shifts to lower temperatures. This shifting of TC0

towards lower temperature is more in case of pristine and in composite samples containing 6%BZO and 10%BZO. On the other hand, the tailing behavior is found less significant in case of 2wt.%BZO and

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4wt.%BZO composite samples. This shows that improvement in coupling of grains in YBCO takes place up to addition of 4 wt.% BZO, beyond which decoupling takes place due to porous nature of BZO. Similar results have been reported by several authors [35, 40]. These effects by the addition of BZO on superconducting transition of YBCO in presence of magnetic field are further analyzed from temperature derivative of resistivity. The evolution of the derivative of resistivity with respect to temperature under

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various magnetic fields is shown in Fig. 3 which shows that the grains maintain good connectivity between them in absence of magnetic field. Accordingly single peak dominance appears in the derivative curve. However, with the application of magnetic field, the grain coupling is significantly influenced and in the transition two peaks appears which correspond to two different regimes. The first peak appearing on higher temperature belongs to individual grains which does not shift but diminishes with the application of magnetic field due to flux penetration into grains. The second peak, belonging to

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intergranular network, shifts towards low temperature and is boosted by the applied magnetic field. Hence, in presence of applied magnetic field, these peaks get separated from each other at different rates.

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In case of composites of 2 wt.% and 4 wt. % BZO the shift in second peak is found less as compared to

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rest of the samples. This is in agreement with the resistivity curves. Therefore, the analysis of resistivity

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Fig 3: Temperature derivative of electrical resistivity of YBCO+ xBZO (x=0.0, 2.0, 4.0, 6.0 and 10.0 wt.

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%) composites for different applied magnetic fields up to 13T.

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and its derivative under magnetic field reveal that limited addition of BZO in YBCO improves its superconducting capability. The broadening of resistivity curves of composite sample in presence of applied magnetic field can be understood in terms of formation of vortices and their thermally activated motion. The thermally activated motion of vortices can be expressed by TAFF theory [4, 41, 42]; ⁄ )exp(

⁄ )

exp(

⁄ )

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(

(2)

where U is thermal activation energy (TAE) which is temperature and magnetic field dependent while as ⁄

the prefactor

eq. (2) becomes the Arrhenius relation,

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⁄

is assumed to be constant for cuprates in general. Further, assuming

⁄ ,

where

(3)

⁄

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⁄

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is the apparent activation energy which must be overcome to allow flux motion and is given vs. ⁄ should be linear with slope equal to apparent

. Hence in the TAFF region

activation energy

and intercept equal to

.

Further,

vs.

gives

.

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slope equal to ⁄

and intercept represented by

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and by

(4)

Fig 4(a-e) shows Arrhenius plots in resistivity of composite samples under different applied magnetic fields. The plots suggest that resistivity is thermally activated over several orders of 𝞀 below

which is

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expected for cuprate superconductor [42, 43]. The solid lines indicate the results of linear regression analysis in the TAFF region where inset of fig 4(a-e) we have plotted values of

and vs

have been calculated according to eq. (3). In the ) for estimating

by using eq. (4). The observed

for composite samples are consistent within the range of error. It is obvious that

is well described by the Arrhenius relation. Fig 4(f) shows the variation of field for composite samples. Our data best fits the Farazdaghi-Harris model

vs. ⁄

with applied magnetic with

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fitting parameters

to

to

and

. The fitting has been done using the Origin 8 software. The plot suggests

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to

,

Fig 4: Arrhenius plots of the resistivity for different magnetic fields of (a) Pure YBCO (b) YBCO+2%BZO, (c) YBCO+4%BZO, (d) YBCO+6%BZO and (e) YBCO+10%BZO. The solid lines

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show linear fitting in the TAFF region. In the inset, the intercept is plotted with activation energy (in K) for calculating

. (f) Activation energy

in presence of magnetic field for YBCO+BZO composites

(red solid lines are fitted data). that the value of

is more in case of composites containing BZO up to 4%. This means that the

free motion of vortices gets reduced in YBCO with the limited addition of BZO that results in pinning of flux lines which is consistent with the results of Anderson and Kim [44, 45]. These results yet again prove

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that addition of BZO up to 4 wt.% in YBCO matrix improves its superconducting behavior in a magnetic environment. The evolution of

with respect to temperature of composite samples is shown in Fig. 5. At low

temperatures, the slope of each curve starts increasing from a particular temperature which marks the entry into vortex glass state as previously seen in high

faster in composites of 2%BZO and 4%BZO with decreasing temperatures. This is

consistent with the behavior of

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slope increases

cuprates [46, 47]. It is clear from Fig. 5 that the

of composites. Thus, the limited addition of BZO in YBCO helps

in the occurrence of vortex glass state. At higher temperatures, Fig. 5 does not reflect the true evolution of

. This contradiction originates from the two basic assumptions introduced in Arrhenius

relation that the pre-factor dependence of pre-factor

is constant and and nonlinear relation of

varies linearly with T.

The temperature

vs T is considered elsewhere [32, 33].

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Further, the study of vortex dynamics within the frame work of flux creep model is valid only when and in low current limit. At high temperatures, thermal energy becomes comparable to pinning energy barrier and hence a diffusion model, corresponding to flux flow is more appropriate than a hopping model for flux creep. It has been argued [48] that it is meaningless to give interpretation of slope [49, 50].

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in terms of activation energy as the different processes dominate near

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⁄

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Fig 5: The temperature dependence of

⁄

of YBCO+ xBZO (x=0.0, 2.0, 4.0, 6.0 and

10.0 wt. %) samples for different applied magnetic fields.

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The estimation of the temperature dependence of upper critical magnetic field ( samples is determined from 90% used

⁄

, 50%

and 10%

) of composite

criteria and is shown in Fig. 6 (a - e). We have

under various different magnetic field to analyse the vortex glass transition. According to

the vortex glass transition theory [51, 52], resistivity vanishes at vortex glass transition temperature following the relation length

(

)

(

)

, where

is static exponent of the vortex glass correlation

and z is the dynamic exponent for correlation time

. Therefore, the 13

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versus T as determined from the 90%, 50% and 10% criteria of the

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Figure 6: Upper critical fields

normal state resistivity for (a) Pure YBCO (b) YBCO+2%BZO, (c) YBCO+4%BZO, (d) YBCO+6%BZO is the vortex glass transition temperature. (f) Variation of

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and (e) YBCO+10%BZO.

with BZO

wt.% estimated by using WHH and GL theory.

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logarithmic derivative of the resistivity is linearly dependent on T as inverse of

and

are estimated from the linear region of

calculated from the slope of plot

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of

.

⁄

⁄

with slope ⁄

vs T. The value

vs T is found to vary from 5.2 to 3.13 for pristine

YBCO, 4.5 to 2.3 for YBCO+2%BZO, 3.5 to 2.8 for YBCO+4%BZO, 6.1 to 2.0 for YBCO+6%BZO and 5.0 to 1.3 for YBCO+10%BZO as the magnetic field changes from 0T to 13T. These values are consistent with what exist in the literature for different materials [53, 54]. fields of composite samples are plotted in Fig. 6 (a - e). It is clear that

under various magnetic is more in the composites

containing BZO up to 4wt.%. This means that thermally activated motion of vortices is significantly affected by BZO particles. Further

of all composites show concave curvature (upward curvature) in 14

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the vicinity of

and a linear trend in low temperatures which is in agreement with earlier reports [12, 40,

55]. One can estimate

of composite sample by employing standard theory of Werthamer, Helfand

and Hohenberg (WHH) [56]:

⁄

For the calculation of

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⁄

we have ignored the initial curvature of experimental data. Thus

using the slope of linear portion of experimental data and corresponding extrapolated , for 50%

criterion

6(f). It is clear from figure that

is determined for various composite samples and is shown in Fig. is 71T in case of pure YBCO and gets enhanced up to 79T for

can also be estimated from the Ginzburg-Landau (GL) mean field theory

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YBCO+2%BZO.

where

values to

is reduced temperature. The value of

(using GL fitting) is shown in fig. 7. The value of

for 50%

criterion of composite samples

comes out to be 86T for pure YBCO and

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attains maximum value of 101T for YBCO+2%BZO. The estimation of

GL method is together plotted with the estimated values of

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shown in figure 6(f). Both methods reveal that YBCO due to increase in grain connectivity.

for composites by

from WHH method and is

is enhanced by adding 2 to 4 wt.% of BZO in

On further addition of BZO,

decreases due to

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accumulation of BZO on grain boundaries thereby degrades the grain connectivity. Numerous measurements of

have been carried out earlier on single crystals and thin films of

YBCO [57-60]. These studies reveal that due to the anisotropic nature of coherence length

also

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shows different behavior along parallel and perpendicular direction of Cu-O2 planes. Single crystals of YBCO show

values of 120T and 250T along perpendicular and parallel directions of the Cu-O2 in this study are consistent with

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planes respectively [59]. The findings related to estimation of the recent reported articles on bulk YBCO composites [12, 26, 40, 55].

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Fig 7: GL fitting of composite samples for 50% criteria of the normal state resistivity.

Conclusion: We have studied the normal state and vortex dynamics of YBCO/BZO composites in presence of magnetic field. The study of superconducting state in presence of magnetic field of

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composites shows that less resistivity broadening takes place by the addition of BZO up to 4wt.%. This has been further supported by the increase in activation energy determined from Arrhenius relation. Both the less resistivity broadening and increase in activation energy of YBCO/BZO composites under

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magnetic field has been attributed to restriction of motion of vortices. We also observe that vortex glass transition temperature is optimized at 4%BZO concentration. The estimation of

by WHH and GL

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theory also reveal enhancement by the addition of BZO up to 4 wt.% due to better grain connectivity. However, the study of normal state reveals that the addition of BZO beyond 4%wt. in YBCO increases its

reveals

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residual as well room temperature resistivity due to accumulation of insulating BZO grains. This study that the limited addition of BZO in YBCO improves its performance as a superconducting

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material in presence of magnetic field. Acknowledgement: The work was supported by IUAC, New Delhi with research fellowship UFR-52307. Authors acknowledge the use of magneto resistance measurement facility at UGC-DAE, Indore. Authors would also like to thank the anonymous reviewers for useful comments that helped improve clarity of the manuscript.

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