Masashi Mukaida

High-J/sub c/ Gd-Ba-Cu-O epitaxial films prepared by pulsed laser deposition

For applications of high-temperature cuprate superconductors in magnetic fields at 77 K, a RBa/sub 2/Cu/sub 3/O/sub x/ (R123; R=Nd, Sm, Gd, etc.) film is one of the promising materials due to its high critical temperature T/sub c/ and high irreversibility field B/sub irr/. Especially, a substitution between R and Ba sites that results in a T/sub c/ degradation is not likely to occur in GdBa/sub 2/Cu/sub 3/O/sub x/ (Gd123) films, compared to the cases of Nd and Sm ions. This is an advantage for an application of Gd123 film. We studied the c-axis oriented Gd123 film for obtaining a high critical current density J/sub c/ in a magnetic field at 77 K. The substrate temperature to deposit the epitaxial films on MgO(100) was varied between 700-800/spl deg/C under an oxygen partial pressure of 200 mTorr. T/sub c/ values of the films were within 89.8-92.7 K. Interestingly, the J/sub c/ at 77 K for 5 T (B//c) reached 0.2 MA/cm/sup 2/. The values are three times higher than those of Y123 films, which indicate that Gd123 films have the superior potentiality in magnetic fields at 77 K.

Magnetic Field Dependence of Critical Current Density and Microstructure in

We have reported high-J_c Sm_1+x,Ba_2-x,Cu₃O_y (SmBCO) films on single crystal MgO prepared by pulsed laser deposition (PLD). In this study, we fabricate SmBCO films on metallic substrates using low temperature growth (LTG) technique, to investigate the availability of the LTG technique for coated conductors. The LTG-SmBCO films are deposited on PLD - CeO₂/IBAD - YSZ/Hastelloy tapes. The critical current density (J_c) is 4.7 MA/cm² (77 K, B//c, B = OT) and 0.23 MA/cm² (77 K, B//c, B = 5 T). Furthermore, at 65 K and B = 17 T, the Jc of LTG-SmBCO film (B//c) reaches 0.03 MA /cm . These values are higher than those of conventional PLD-YBCO and PLD-SmBCO films. From these results, we suggest that the LTG is an effective technique for improving the magnetic field dependence of J_c of coated conductors.

{\rm Sm}_{1+x}{\rm Ba}_{2-x}{\rm Cu}_{3}{\rm O}_{y}

Gold nanorods were introduced into GdBa2Cu3O7-delta (GdBCO) thin films to form artificial pinning centers. The incorporation of gold nanorods into the GdBCO films was achieved simply by attaching gold sheets to a GdBCO target in the pulsed laser deposition process. Transmission electron microscopy observation showed that gold nanorods grew parallel to the c-axis of the GdBCO Alms. Small angle X-ray scattering and scanning electron microscopy results indicated that matching field, BPhi was increased with the amount of supplied gold. Jc - B characteristics in the field aligned with the c-axis at 50 and 65 K exhibited an anomalous shoulder (peak effect) around at BPhi, and the peak position was not dependent on the temperature. This suggests that the peak effect was originated from the matching effect between vortices and the gold nanorods.

Films on Metallic Substrates

A novel technology to introduce artificial pinning centers (APCs) into YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) thin films prepared by pulsed laser deposition (PLD) was investigated for a drastic improvement of J/sub c/ in the films. Linear-like defects (one-dimensional APCs) were introduced perpendicular to the surface of the c-axis oriented films during the deposition process by using distributed Y/sub 2/O/sub 3/ nano-islands on substrates. The density of nano-islands was varied within 0.8-1.2/spl times/10/sup 10//cm/sup 2/ by PLD. A normalized resistivity as a function of temperature in magnetic fields shows a sharper resistivity drop for the film with APC, compared to pure YBCO film. J/sub c/ of the film with APC was also increased to 0.12 MA/cm/sup 2/ (77 K, B//c, 5 T), which was about two times higher than that of the pure YBCO film. The film had a very large J/sub c/ peak when the field was applied close to the c-axis. The peak J/sub c/ increased with the number density of Y/sub 2/O/sub 3/ nano-islands. This indicates that strong APCs parallel to the c-axis were incorporated into the YBCO film.

Vortex Pinning by Gold Nanorods in

A c-axis oriented epitaxial REBa/sub 2/Cu/sub 3/O/sub y/ (RE123) thin film performs excellent superconducting properties in magnetic field. Recently, we reported that a critical current density (J/sub c/) in the RE123 thin film was improved by a deliberate composition control. A Sm/sub 1+x/Ba/sub 2-x/Cu/sub 3/O/sub y/ (Sm123) thin film with a small amount of Sm/Ba substitution x showed the great J/sub c//spl sim/0.17 MA/cm/sup 2/ in 5 T at 77.3 K, while the J/sub c/ in (Yb/sub 1-z/Nd/sub z/)Ba/sub 2/Cu/sub 3/O/sub y/ (Yb/Nd123) thin films depended on an amount of mixed crystal ratio z. In this report, we studied J/sub c/ as a function of magnetic field and field orientation with respect to ab-planes. In low magnetic field, the J/sub c/ of Sm123 thin film was almost independent of the applied angle of the field. In the case of Yb/Nd123 thin film, extremely high J/sub c/ were observed when the magnetic field was aligned parallel or perpendicular to the surface of the film. Because compositional fluctuations in RE123 thin films were observed by transmission electron microscopy, we found that the pinning centers in RE123 thin films are strongly affected by the composition in the thin films.

{\rm GdBa}_{2}{\rm Cu}_{3}{\rm O}_{7-\delta}

In this work, we studied the optimum deposition conditions for SrTiO₃ (STO) buffers deposited on MgO single crystals substrates by PLD method. We found that 120 nm thick STO films, deposited at 730 ^oC , present the smoothest surface and a minimum FWMH of the (002) reflection. High quality, smooth and free-crack (YBCO) thick superconducting films have then been fabricated using a deposition temperature of 800 ^oC on STO-buffered MgO substrates. A 373 nm thick YBCO/STO/Mg₂O film can carry current densities (77 K, 0 T) of 1.4 MA/ cm² and J_c (77 K, 5 T) of 1.4 MA/ cm² The J_c in high field overcomes the current transported in same experimental conditions by 180 nm thick YBCO film grown on Y₂O₃ -decorated STO single crystal. For 1350 nm thick YBCO/STO/MgO film the J_c (77 K, 0 T) is about 0.8 MA/cm² and J_c (77 K, 5 T) is 0.09 MA/cm².

Thin Films

In this work, we studied the optimum deposition conditions for SrTiO₃ (STO) buffers deposited on MgO single crystals substrates by PLD method. We found that 120 nm thick STO films, deposited at 730 ^oC , present the smoothest surface and a minimum FWMH of the (002) reflection. High quality, smooth and free-crack (YBCO) thick superconducting films have then been fabricated using a deposition temperature of 800 ^oC on STO-buffered MgO substrates. A 373 nm thick YBCO/STO/Mg₂O film can carry current densities (77 K, 0 T) of 1.4 MA/ cm² and J_c (77 K, 5 T) of 1.4 MA/ cm² The J_c in high field overcomes the current transported in same experimental conditions by 180 nm thick YBCO film grown on Y₂O₃ -decorated STO single crystal. For 1350 nm thick YBCO/STO/MgO film the J_c (77 K, 0 T) is about 0.8 MA/cm² and J_c (77 K, 5 T) is 0.09 MA/cm².

J/sub c/ enhancement in YBa/sub 2/Cu/sub 3/O/sub x/ thin films by introduction of one-dimensional artificial pinning centers

Coated conductors for power applications require REBCO films with high critical currents (<i>I</i> _c). However, REBCO thick films were likely to be -axis oriented so that was saturated with increasing the film thickness. Previously we reported that Sm-rich (Sm_1+xB_2-xCu₃O_y; <i>x</i> = 0.08) films grown at low deposition temperatures on MgO substrates showed the complete <i>c</i>-axis orientation and the large terrace width as compared with SmBCO (Sm_1+xB_2-xCu₃O_y; <i>x</i> = 0.04 ) films. In this study, we will propose a novel multilayer technique in order to suppress the <i>a</i>-axis growth and keep large terrace width. In this technique thin Sm-rich layers are inserted among SmBCO layers. The multilayer structures [Sm - rich/SmBCO] _* N (N = 1,2,3...) was fabricated by PLD method on MgO substrates. Although <i>I</i> _c in the SmBCO single layer films at 77 K was saturated at about 350 A/cm-width over the thickness of about 1.3 mum, the SmBCO multilayer films with a thickness of 1.7 mum showed the complete <i>c</i>-axis orientation, and the <i>I</i> _c achieved about 800 A/cm-width at 77 K. We confirmed that the growth of the <i>a</i>-axis oriented regions was suppressed by the Sm-rich layers. It is considered that the Sm-rich layers act as a template layer for maintaining the <i>c</i>-axis orientation, even though the surface temperature decreased with the thickness.