Enhancement of Critical Current Density of Yttrium Barium Copper Oxide Thin Films by Introducing Nano dimensional Cerium Oxide Defects

The critical current density, J has been the most important parameter used in the design and engineering of effective devices which is one of the implementation of high temperature superconductors (HTSC). In this work, an effort has been made to further improve the critical current density of YBa2Cu3O7-x (YBCO) thin films by preventing the magnetic flux line lattice against the Lorentz force by pinning it in place with the aid of nano-dimensional defects. These defects were generated by distributing nano sized CeO2 islands after YBCO layer was created on LaAlO3 substrates perpendicular to the film using pulsed laser deposition (PLD) technique. Three samples with buffer layers of CeO2 were prepared. CeO2 with 50 pulses, 100 pulses and 150 pulses, after each 1000 pulses of YBCO were prepared five layers for each of the samples. The structural characterization of YBCO/CeO2 and YBCO pristine films were carried out using x-ray diffraction (XRD) and scanning electron microscopy (SEM). Superconducting proprieties were measured using a vibrating sample magnetometer (VSM). J for the pure YBCO and the YBCO/CeO2 films were calculated from magnetization (M) versus Field (H) loops using Bean’s model. J for the 50 pulses of YBCO/CeO2 films was found to be increased slightly by an order of magnitude of about 40% with respect to those of YBCO films without the nano dimensional defects.

YBa 2 Cu 3 O 7− x (YBCO) film has attracted a lot of attention in electrical power applications due to its high critical transition temperature T c (>90 K) and critical current density J c (>1 MA cm −2 ) (Lei, Zhao, Xu, Wu, & Chen, 2011;Haugan, Barnes, Brunke, Manrtense & Murphy, 2003).The growth of YBa 2 Cu 3 O 7-x (YBCO) thin films is of great interest for superconducting applications because of its power transmission magnetic shielding, low phase noise oscillators, magnetic resonance imaging receiver coils (Chen et al., 2016;Matsuunotoet al.,, 2004;Rejith, Vidya &Thomas, 2015;Zhao, Ito & Goto, 2014).Fine precipitates, dislocations, grain boundaries and vacancies are many kinds of crystalline defects, being considered as pinning centers (Zhao, Ito & Goto, 2014;Kujur, Sahoo, Panda, Asokan & Behera, 2013).The pinning centers depend on size, shape and concentration to achieve effective defects.The critical current density is highly influenced by flux lattice motion due to thermal fluctuations and Lorentz force due to applied magnetic fields.To maintain the necessary levels of high J c in high applied magnetic fields we need high spatial densities or naturally occurring growth defect to suppress the thermal fluctuations to stop the vortex mobility by pining them.Although nanodots of CeO 2 have been shown to induce additional flux pinning, this has always been due to strain in the YBCO lattice because of these inclusions.Such strain however, could have detrimental effect on the YBCO.Aside from the thickness dependence of J c , there is also a limitation placed on the thickness at which such films could be grown (Uzun & Avci, 2014;Zhao, Iton & Goto, 2014;Sueyoshi, Kotaki, Fujiyoshi, Mitsugi, Ikegami & Ishikawa, 2013).(Huang, Li, Wang, Qi, Sebastain, Haugan, &Wang, 2017), reported the enhanced flux pinning properties of YBCO thin films with various pinning landscapes a magnetic nanocomposite of La 0.7 Sr 0.3 MnO 3 (LSMO))x (CeO 2 ) 1-x was incorporated into YBCO as either a cap layer or a buffer layer but the defect pinning and magnetic pinning are introduced in the systems giving the J c at 77K to be around 4.66MA/cm 2 (Xu et al., 2012), reported the influence of CeO 2 -cap layer on the texture and critical current density of YBCO film, here the found that the texture and J c of YBCO film were largely dependent on the texture of CeO 2 -cap layers under optimized deposition conditions, with the increase of the degree of in-plane and out-plane texture of CeO 2 -cap layers, there was a decrease in the Jc of the YBCO thin films from 4.23MA/cm 2 to 0.47MA/cm 2 .(Lee, Kim, Park & Park, 1996), agreed that addition of CeO 2 into YBCO will increase the critical current density to an extent at 77K, they reported 2x 10 4 A/cm 2 an increased by lowering the measuring temperature.(Solovyov, Bagarinao, Li, Si, Win, Zhou, Wiesmann & Qing, 2010), used active (001) Ceria (CeO 2 ) buffer to modify the structure of the epitaxial high temperature superconductor YBCO which resulted with 0.8m thick film exhibiting strong enhancement of the critical current density of 4.2MA/cm 2 .(Feng et al., 2015), used CeO 2 cap layers for high temperature superconducting, were YBCO films were epitaxially deposited on the as-growth CeO 2 / YSZ (001) stack using water-free metal organic deposition (MOD) method to obtain 1.92MA/cm 2 as J c at 77K at 0.5Pa deposition pressure.
Pulsed laser deposition (PLD) which is one of the physical deposition methods has been a promising technique for thin film growth and for fabricating nano-dimensional/nano-dots within epitaxial, textured or polycrystalline thin film matrices in recent years (Bhaumik et al., 2017;Haque, Pant and Narayan, 2018;Haque and Narayan, 2018;Karnati, Haque, Taufique and Ghosh, 2018).In the present study, we have generated the nanostructures on LaAlO 3 substrates by depositing YBCO and YBCO/CeO 2 multilayer thin films.To investigate the effects of introducing nano dimensional CeO 2 defects on (current density) Jc and the micro structure of YBCO.

Experimental Procedure
Several alternating layers of YBCO/CeO 2 thin films were grown on single crystal LaAlO 3 substrates using pulsed laser deposition technique.YBCO target of 99.9% with density of 5g/cm 3 and 99.9% of CeO 2 target, both were ordered from MTI corporation were used for this experiment.The substrate which is LaAlO with orientation (100) and size of (10 × 10) and (5 × 5 )mm were used for the deposition, cleaned with acetone, methanol, and ethanol for 10 mins each, under ultrasonic to remove dirt and oil, after that the substrate was dropped inside hydrogen fluoride to also remove oil particle if there is any left and then dry.The deposition chamber was cleaned with acetone and methanol to remove dirt particle inside it.We then mounted the cleaned substrate on the holder with silver paste.Ablation was performed using KrF laser source of a wavelength 248nm.The ablation was done at (300) mJ of laser energy with a repetition of 5Hz.The aperture size was 12 x 4 mm 2 and laser spot size was 1 x 4 mm 2 .The target-substrate distance was maintained at 4cm.A based pressure of 10 Torr was maintained before the deposition; the deposited films were cooled down naturally in 600 mTorr of oxygen gas.To determine the optimum oxygen pressure for YBCO depositions, experiments were carried out at oxygen pressures of 400 mTorr.For this study three samples were chosen based on their yield and structural quality.They are identified as 50 Pulses, 100 Pulses and 150 Pulses samples based on the number of laser pulses used for a layer of CeO 2 .The films were deposited on LaAlO 3 single crystal substrates with (001) direction.Starting with YBCO followed by CeO 2 , 10 layers of films were deposited for each sample tabulated below.

Multilayer Deposition of YBCO/CeO 2
The 50 Pulses sample contains five layers of YBCO and 5 layers of CeO 2 .The difference between the three samples is differences in the thickness of the CeO 2 layers.Single and multilayer samples were prepared using pulsed laser deposition.Also, a fourth sample of pure YBCO with 5000 pulses was made to be used for comparison of vital data such as the superconducting transition temperature and critical current density.The The (00l) peaks in the XRD-spectra indicate that YBCO thin films are highly c-axis oriented, which is commonly observed and corresponds to the natural growth of the material.The XRD pattern from the YBCO and YBCO/CeO 2 deposited on LaAlO 3 at 850℃ and 300mTorr of oxygen pressure, is shown in figure 2. From the graph above we can observe that (00l) peaks are (001) at 7.56°, (002) at 15.18°, (003) at 23.7°, (004) at 31.2°, (005) at 38.5°, (006) at 47° .The peaks of the substrate were also observed with corresponding degrees at 23.5° and 48.3°.All the samples of YBCO and YBCO/CeO 2 was highly c-axis oriented with good sharp and strong peaks.
The CeO 2 appears in the graph below as an independent phase.The peak of CeO 2 are observed only at 32.8° (200) orientation from the graph we can observe that the intensity increases as the number of pulses are added.The XRD pattern for the 50P YBCO/CeO 2 was not resolvable, probably due to the small amount of the CeO 2 size and low density.The presence of (00l) confirmed that the deposition parameters used for this work were good and suitable for the growth of YBCO on LAO substrates.No second phase such as Y 2 Cu 2 O and CuO were found in this case.
Only YBCO and CeO 2 phase was formed with a high intensity.From the XRD the oxygen deficiency was calculated for the samples using equation 1 ( The scanning electron microscopy (SEM) was performed on the samples, it revealed the surface morphologies of thin films.Figure 3 (a and b) shows the SEM image of pure YBCO and 50 pulses of YBCO/CeO 2 .Figure 3 (c and  d) shows the SEM of the YBCO/CeO 2 samples for (100 and 150) pulses respectively with nano dimensional defects, the pure YBCO reveals that they YBCO are well packed with long and extended grains randomly oriented in all direction with sizes vary from 1-5m.

D =
. (2) Where,   Figure 5 shows T c as a function of number of pulses for the onset transition temperature.From the plot, we can deduce that the T c was high for Pure YBCO (0 pulses) and after the introduction of 50 pulses of CeO 2 the T c increased slightly.Similar changes were observed at 100 pulses and 150 pulses the T c decreases as the number of pulses were added.This is a clear indication that the CeO 2 defects were randomly introduced into superconducting matrix.Figure 6 shows that the obtained magnetic moment is in the emu units, which was converted to magnetization (emu/cm 3 ) by dividing the magnetic moment by the volume of the sample.The area of the thin film sample was (2 x 2) mm 2 .The thickness of the film was maintained at 100nm for pure YBCO.
The magnetic moment was obtained for the field swept from desired positive field to zero field and zero field to desired positive field.The value of the current density depends on several parameters including deposition temperature (T D ), the number of pulses (P), external magnetic field (H) and the temperature at which it is measured (T).i.e J c = (T D , P, H, T). (T D and P) are sample parameters that can be controlled during the sample growth.Keeping these two parameters fixed, J c depends on H and T. The most determining sample condition for the value of J c are the flux pinning centers introduced by the CeO 2 nano dots and randomly distributed in the superconducting matrix, the pinned flux experiences forces because thermal fluctuations and the Lorentz force that lead to flux lattice melting.Using Bean's formula, critical current density (J ), was calculated for pure YBCO and multilayer CeO nano dimensional defect.Bean's formula is given as (Matsuunoto, Horide, Osamura, Mukaida, Yoshida, Ichinose & Horii, 2004;Rejith, Vidya &Thomas, 2015;Zhao, Ito & Goto, 2014)   =  ∆  (3) where ∆M is the difference of magnetization at a field, in emu/cm 3 , a is the area of the inscribed circle, in cm and J is critical density in A/cm 2 .Figure 8 shows the plots of the current density J against the field in Tesla, at 77K it explains that the 50 pulses of the YBCO/CeO 2 in the multilayer gave highest J c which was about 4.1 MA/cm 2 this agrees with other report (Huang, Li, Wang, Qi, Sebastain, Haugan &Wang, 2017;Xu, Liu, Wang, Zhu, Zhu & Li, 2012;Lee, Kim, Park & Park, 1996;Feng, Zhang, Qu, Huang, Xiao, Zhu, Lu, Shi, &. Han, 2015) against the pure YBCO and we observed that the more CeO 2 was added in the multilayer the less the J c , and T c we got, and this did not affect its structural properties.Table 5 below shows some current density of other literature which agree with our result.(Zhu, Wei, Yan, Tei, Jing, Liang, Bin, He &Wei, 2016).Figure 8 shows critical current density J c as a function of measured temperature of all the samples.

Conclusion
In conclusion, we conclude that enhancing flux pinning in superconductors is central to improving their current carrying capability (J c ) and their ability to be used in magnetic fields but in this study only on 50 pulses that the J c increases slightly, Unlike other superconducting parameters, such as critical Temperature (T c ) and critical magnetic field.J c is not intrinsically limited to the material of system.In this work, the micro structure and critical current density o YBa Cu O films have been found to improve very slightly with the introduction of nano dimensional multilayer CeO .Evidence from magnetic properties and XRD, shows that no second phase such as Y 2 Cu 2 O and CuO were found.Only YBCO and CeO 2 phase were formed with a high intensity, while SEM shows that pure YBCO has long and extended grain randomly oriented in all direction with size vary from 1-5m with the grain very closely packed to each other.TEM analysis and X-ray diffraction texture analysis of CeO 2 layers will be incorporated in the future work.
Figure 1. on the first

Figure 4 .
Figure 4. Magnetization Versus Transition Temperature (K) of all the sample, pure YBCO at different deposition temperature and (50, 100.150) pulses of YBCO/CeO 2 at the same deposition temperature

Figure 5 .Figure 6 .
Figure 5. Transition Temperature Versus Number of pulses of all the samples, Pure YBCO and (50,100,150) Pulses of YBCO/CeO 2 Figure 6.Magnetization Vs Magnetic field (hysteresis loop at 77K of 50 pulses)

Figure 8 .
Figure 8.Current density Versus Measured Temperature (K) of all the samples at 10K, 50 K and 77K

Table 1 .
The multilayer deposition showing all the samples

Table 4 .
List of samples details and their superconducting properties

Table 5 .
Comparing the current density with other literatures