Physica C 495 (2013) 208–212

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Physica C

journal homepage: www.elsevier .com/locate /physc

Enhancement of critical current density for nano (n)-ZnO doped MgB2

superconductor

0921-4534/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.physc.2013.10.001

⇑ Corresponding author. Tel.: +966 1 4676299; fax: +966 1 4673656.E-mail address: intikhabansari@yahoo.com (I.A. Ansari).

Intikhab A. Ansari a,⇑, M. Shahabuddin a, Nasser S. Alzayed a, Khalil A. Ziq b, A.F. Salem b, V.P.S. Awana c,H. Kishan c

a Department of Physics and Astronomy, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabiab Department of Physics, College of Science, King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabiac National Physical Laboratory, Dr. K.S. Krishnan Road, New Delhi 110012, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 15 June 2013Accepted 1 October 2013Available online 10 October 2013

Keywords:Critical current densityVortex-avalanchesResidual resistivity ratioMagnetization

Doping effect of ZnO-nanoparticles on the superconducting properties of MgB2 has been studied. The 2%nano-ZnO doped MgB2 shows the excellent Jc and Hirr at all temperatures and magnetic fields amongstthe doped and undoped sample. The lattice parameter-c shows the higher value for 2% ZnO dopedMgB2 sample, which clearly demonstrates the presence of the lattice strain in doped samples. The resid-ual resistivity ratio was increased as the nano-ZnO doping level increased. Very slight variation in Tc isobserved from the temperature dependence of resistivity plot of nano-ZnO doped MgB2. In M (H) plotat low applied fields, we have observed large vortex instabilities (vortex-avalanches) associated with2% and 4% doped samples. Vortex avalanche effect is diminishes with increasing temperature and disap-pears near 15 K. The results are discussed in terms of local-vortex instabilities caused by doping of ZnOnanoparticles. Scanning electron microscopy studies show that the synthesized samples are well adher-ent and grains are uniformly distributed with an average particle size of �5–10 lm.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Since the discovery of superconductivity at 39 K in layered tran-sition metal boride MgB2 [1], led a rush among the scientific com-munity due to its fundamental properties as well as practicalapplications [2–4]. The two-gap property is one of the most unu-sual aspects of the MgB2 that is very important to its high-field per-formance [5–7]. This superconducting material exposed the simplechemical composition, somewhat high transition temperature (Tc),lower anisotropy, large coherence length and better current flowacross the grain boundaries [8]. However, in bulk MgB2, one ofthe most important challenges is to enhance the upper critical field(Hc2) along with improving the critical current density (Jc). As weknow that MgB2 have poor grain coupling and a lack of pinningcentres, it normally shows a rapid decrease in Jc in high magneticfields [9,10]. Furthermore, the reported result of very high Hc2

and relatively large Jc in dirty thin film grasped the interest ofthe scientists towards its practical use of high field applications[11,12]. Enormous efforts have been made on the way to the fabri-cation of high-quality MgB2 superconductor. In order to enhancethe Jc, Hc2, and the irreversibility field (Hirr), enormous chemicaldoping [13–21], different routes [22–25], and thermo-mechanical

processing techniques [26,27] have been adopted. Remarkably,chemical doping particularly using nano-particle doping has re-vealed the great promise in improving the Jc–H behavior due toform a high density of nano-inclusions in MgB2 matrix and pro-vides a strong flux pinning force (Fp). Amongst nano-dopants, thenano (n)-ZnO has been taken into account because the local struc-ture of MgB2 should be affected by Mg–O interaction on the ZnO. Inthe recent years, various studies have been reported experimen-tally [28–34] and theoretically [35–39] for the enhancement ofsuperconducting properties in Zn-doped MgB2 superconductor.Various attempts have been made for enhancing Jc, Hc2 and Hirr

by doping different organic and inorganic Zn-containing materialsin MgB2 as reported earlier [40,41].

The purpose of the present study is, therefore, to investigateelectrical, structural and magnetic properties of n-ZnO dopedMgB2 superconductor. We ascertain that inclusion of n-ZnO inMgB2 support eloquently improving Jc and Hirr in high appliedfields. This is due to large vortex instabilities at T = 10 K up to10 KOe applied field for all the doped samples. The XRD measure-ments were carried out to evaluate the percentage of Zn doping inMgB2. Here, we observed that Zn atom forms solid solution in theMg site of MgB2. Magnetization measurement as a function of fieldat different temperature from 4 to 30 K have been studied for dif-ferent percentage of n-ZnO doped MgB2 samples. In comparison tothe pure sample, the 2% n-ZnO doped sample show the excellent

I.A. Ansari et al. / Physica C 495 (2013) 208–212 209

Jc–H properties. Irreversibility field Hirr for Mg1�xZnxB2 were ob-tained from the M–H hysteresis plots and found that 2% n-ZnOdoped sample shows the excellent Hirr at all the temperatures.The results were further correlated with the images of scanningelectron microscopy (SEM).

2. Experimental details

In the present report, synthesis of n-ZnO doped MgB2 for nomi-nal composition of x = 0%, 2%, 4% and 6% were attempted by stan-dard solid state reaction method. Highly pure starting materialsMg powder (Sigma Aldrich, >99% purity), B amorphous powder (Sig-ma Aldrich, >99% purity), and n-ZnO powder (Sigma Aldrich, >99%purity) were mixed with stoichiometric ratio, ground in room tem-perature with the agate mortar and pestle up to 1 h and pressedwith hydraulic press in the pellets size of 20 � 10 � 3 mm3. Thesepellets were encapsulated in different soft Fe-tube and its subse-quent heating to 750 �C for two and half hours in an evacuated(10�5 Torr) quartz tube and quenching to liquid nitrogen tempera-ture. Finally, we have to find a bulk polycrystalline and quite poroussample of n-ZnO doped MgB2.

The XRD patterns of the samples were recorded using Cu Karadiation within 2h range of 20–70�. Temperature dependence ofresistivity q(T) from the room temperature to 13 K were investi-gated by standard four-probe method. The magnetizations of thesesamples were measured using PAR-4500 Vibrating Sample Magne-tometer (VSM) at various temperatures in magnetic fields up to70 kOe. The Jc values were deduced from M–H loops using theBean’s Critical State Model. Irreversibility fields (Hirr) were mea-sured from the highest magnetic field where the magnetizationwas irreversible. Additionally, the morphologies were observedby SEM (JEOL, JSM-6360) operating with a 20 kV acceleratingvoltage.

3. Results and discussion

The XRD of the samples were carried out using Cu Ka radiationfor a 2h range of 20–70� with a step time of 1 s/0.02 step each. TheXRD patterns for the samples used in the present study are shownin Fig. 1. The presence of the small amount of MgO and unreactedMg are seen in the pattern near about 2h = 62.2� and 2h = 36.8�,which are marked by the symbol ‘‘⁄’’ and ‘‘#’’, respectively inFig. 1. The impurity phase MgO might arise during the reaction

20 30 40 50 60 700

40

80

120

160

#

#

# Mg

**

*

*

* MgO

(111

)(1

02)(1

10)

(002

)

(101

)

(100

)

(001

)

Inte

nsity

(a.

u.)

2θ (degree)

Pure MgB2

2 % ZnO+MgB2

4 % ZnO+MgB2

6 % ZnO+MgB2

Fig. 1. XRD pattern of 0%, 2%, 4% and 6% n-ZnO doped MgB2.

because Mg is highly reactive with oxygen. No other impurity ex-cept MgO and unreacted Mg are seen in the XRD plots of n-ZnOdoped MgB2 samples. The total XRD plot shows the fine dopingof n-ZnO in the stoichiometry of Mg and B. The MgO impurityphase has been also observed in our earlier reported paper for n-alumina doped MgB2 superconductor [15].

The lattice parameters estimated from the XRD plot show aslight decrease in the a-axis parameter. The lattice parameter a de-creases from 3.0825 Å for the pure sample to 3.0806 Å for the sam-ple with highest concentration, as can be seen in Fig. 2. On thecontrary, quite improvement in the c-axis parameter up to 2% Znconcentration has been observed as shown in Fig. 2. The variationin the lattice parameter-a is negligible with respect to ZnO concen-tration. The (100) peak leads the a-axis parameter and the (002)peak represents the c-axis parameter. Fig. 2 evidently reveals thepresence of the lattice strain in doped samples. The lattice param-eters a and c were manually refined by using the Windows-basedPowderX software.

The Full Widths at Half Maximum (FWHM) were determinedwith the program PowderX by using X-ray diffraction data. TheseFWHM data were used to evaluate the grain size and strain of dif-ferent doped samples using the Williamson and Hall model [42]:

FWHM� cosðhÞ ¼ 0:94kðGrain sizeÞ þ 4� Strain� sinðhÞ

where k is the wavelength of monochromatic Cu Ka radiation(1.540598 Å) and h is the angle of peak position at X-ray plot. Theabove relation encompasses the combination of Scherrer equationfor size broadening and Stokes and Wilson expression for strainbroadening. After plotting the FWHM � cos(h) vs. sin(h) and fittingthe straight line as shown in Fig. 3. Finally, we have calculated thevariation of crystalline size and strain as a function of differentdoped samples of ZnO with MgB2 as indicated in Figs. 4 and 5.The average gain size for the doped samples is found to be between20 nm and 21 nm.

The temperature dependence of resistivity, q(T) of n-ZnO dopedMgB2 is shown in Fig. 6 at different doping level. It is evident fromthe q(T) plot that the all samples shows the sudden transition up toq = 0. This study endorsed the slight variation of Tc with the n-ZnOdoping. Residual resistivity ratio (RRR) values varies from 1.65 to3.67 with the increase of the doping level from pure sample to6% doped n-ZnO. The Tc of n-ZnO doped MgB2 varies very slightlywith increasing doping level as shown in the inset of Fig. 6. Thevariation of Tc (onset), Tc (q = 0) and DTc are described in Table 1

0 1 2 3 4 5 63.070

3.080

3.090

3.510

3.520

3.530

3.540

3.550

latti

ce p

aram

eter

(A0 )

conc. (%)

a-axis

c-axis

Fig. 2. Variation of a-axis and c-axis lattice parameter with different Zn-dopingconcentration.

0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.550.0070

0.0075

0.0080

0.0085

0.0090 Pure MgB2

2 % doped ZnO 4 % doped ZnO 6 % doped ZnO

FWH

M *

cos

(θ)

sin (θ)

Fig. 3. The fitting of FWHM � cos(h) vs. sin(h) plot.

0 1 2 3 4 5 60.0003

0.0004

0.0005

Stra

in

Conc. (%)

Fig. 4. Variations of strain as a function of different doped samples of ZnO withMgB2.

0 1 2 3 4 5 618

19

20

21

Cry

stal

line

size

(nm

)

Conc. (%)

Fig. 5. Variations of grain size as a function of different doped samples of ZnO withMgB2.

0 50 100 150 200 250 300

0

40

80

120

160

35 36 37 38 39 40 41

0

20

40

60n-ZnO doped MgB2

(-c

m)

T (K)

0% ZnO 2% ZnO 4% ZnO 6% ZnO

n-ZnO doped MgB2

(-c

m)

T (K)

0% ZnO

2% ZnO

4% ZnO

6% ZnO

Fig. 6. Temperature dependence of the resistivity of n-ZnO doped MgB2 system.Inset shows the temperature dependence of resistivity curve in the vicinity of 35–45 K.

Table 1Variation of Tc (onset), Tc (q = 0) and DTc for all the n-ZnO doped MgB2.

x (%) Tc (onset) (K) Tc (q = 0) (K) DTc (K)

0 38.9 37.9 1.02 37.6 36.0 1.64 39.1 36.9 2.26 39.0 37.7 1.3

0 40000 80000

-15

-10

-5

0

5

10

15

0 40000 80000

-18

-12

-6

0

6

12

18 T = 10 K

x = 0 %x = 2 %x = 4 %x = 6 %

M (

emu/

cm3 )

H (Oe)

H (Oe)

x = 0 %

x = 2 %

x = 4 %

x = 6 %

T = 20 K

M (

emu/

cm3 )

Fig. 7. Magnetic hysteresis M (H) loop for Mg1�xZnxB2 sample at T = 20 K withdoping level 0% 6 x 6 6%. Inset shows the M (H) loop for the same samples atT = 10 K.

210 I.A. Ansari et al. / Physica C 495 (2013) 208–212

for 0%, 2%, 4% and 6% n-ZnO doped MgB2 sample. The 6% ZnO dopedsample shows increased resistivity than undoped; 2% and 4%

shows reduced resistivity. The reason is that the voids have createin the doped sample due to the doping of ZnO, as we can see theSEM image of Figs. 10a–10c.

Fig. 7 depicts the magnetic hysteresis loop for all the n-ZnOdoped samples up to applied fields of 70 kOe at T = 10 and 20 K.The weight of all the samples was �45 to �55 mg. It is observedthat magnetization width suppresses and the areas of the M (H)loop become narrower with the increase of the doping level inaddition with temperature. The peculiar point with regard to theM (H) plot at T = 10 K is that the vortex-avalanches region are seenbelow the applied field of 10 kOe for all the doped samples. ZnOdoped Mg1�xZnxB2 sample shows the less vortex-avalanche in the

0 20000 40000 60000

103

104

105

106

20 K

J c (A

/cm

2 )

H (Oe)

Pure MgB2

2% n-ZnO + MgB2

4% n-ZnO + MgB2

6% n-ZnO + MgB2

Fig. 8. Jc (H) plots for n-ZnO doped MgB2 sample along with pure sample at 20 K.

10 15 20 25 3010000

20000

30000

40000

50000

60000

70000

MgB2 + nano ZnO

x = 0 %

x = 2 %

x = 4 %

x = 6 %

Hir

r (O

e)

T (K)

Fig. 9. The temperature dependence of irreversibility field Hirr for Mg1�xZnxB2

sample.

Fig. 10a. SEM micrographs of the 2% n-ZnO doped MgB2 samples operating with a20 kV accelerating voltage.

Fig. 10b. SEM micrographs for pure MgB2 sample operating with a 20 kVaccelerating voltage.

Fig. 10c. SEM micrographs of the 6% n-ZnO doped MgB2 samples operating with a20 kV accelerating voltage.

I.A. Ansari et al. / Physica C 495 (2013) 208–212 211

vicinity of 10 kOe in comparison with n-alumina doped MgB2 sam-ple. Therefore, it is evident from Fig. 7 that magnetization for n-ZnO doped MgB2 sample appears to be more stable rather thann-alumina doped MgB2 sample for all temperature and doping le-vel against our previous reported paper [15].

Critical current density, Jc of our samples was determined byBean’s model as described elsewhere [15]. The present study re-veals the magnetic field dependence of Jc at different temperatureand concentration for the n-ZnO doped Mg1�xZnxB2 sample. Wenote that Jc is found to be decrease with the increase of appliedfield and doping level at all the temperature. At lower fields allthe sample exhibits the Jc value of the order of �106 A cm�2 at allthe temperatures. As shown in Fig. 8, we investigated that atT = 20 K, 2% doped sample shows the excellent Jc in comparisonwith other doped and undoped samples. At low temperature(T 6 10 K) the all samples shows the vortex-avalanches up to10 kOe applied field. In previous report, at higher temperature(T P 15 K) for n-alumina doped MgB2 sample the Jc value decreasesand no vortex instability observed (for comparison see Ref. [15]).Vortex instabilities observed in this n-ZnO sample are lesser incomparison with n-alumina doped sample in the vicinity of lowmagnetic field (for comparison see Ref. [15]). Interestingly, in com-parison with doped and undoped samples, the 2% doped MgB2

sample reveals the better Jc–H properties at all temperature andmagnetic field.

Fig. 9 represents the Hirr(T) plot for n-ZnO doped MgB2 system.The irreversibility line (Hirr) discussed in this paper was deter-

212 I.A. Ansari et al. / Physica C 495 (2013) 208–212

mined from M–H loop taken at discrete temperatures. These linesfor n-ZnO doped MgB2 sample become less inclined in comparisonwith n-Al2O3 doped MgB2 sample (see [15]). The Hirr value in-creases for 2% n-ZnO doped sample and shows the excellent valueof Hirr at all the temperatures.

Figs. 10a–10c observes the typical results from microstructuralanalysis for pure, 2% and 6% n-ZnO doped MgB2 samples. This SEMimages confirms the porous polycrystalline nature of the sample.These samples consist of larger grains from a �5 lm to as largeas �10 lm and agglomerates of fine grains. SEM image confirmthe presence of secondary phase of MgO detected by XRD. The typ-ical size of this impurity grains are �1 lm. These impurity phasesare uniformly distributed within the matrix and are included in thegrains as fine inclusions.

4. Conclusion

In Summary, result shows the substitution of n-ZnO in the stoi-chiometry of MgB2. The impurity MgO, as seen in the XRD plot, mightarise during the solid-state reaction of the starting materials. Slightvariation in Tc (onset) and Tc (q = 0) is observed from the tempera-ture dependence of resistivity plot for n-ZnO doped MgB2. Resultshows that magnetization for n-ZnO doped MgB2 sample appearsto be more stable in comparison with previous reported n-aluminadoped MgB2 sample for all temperature and doping level. We ana-lyzed that 2% ZnO doped sample shows the excellent Jc and Hirr atall temperature and magnetic field. Nano-particle inclusions ob-served by SEM are proposed to be responsible for the slight enhance-ment of flux pinning at all temperatures and higher magnetic fields.

Acknowledgement

This work was supported by NPST program by King SaudUniversity, Riyadh, Project Number 08-ADV397-2.

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