Shun Miura

Characteristics of high-performance BaHfO3-doped SmBa2Cu3Oy superconducting films fabricated with a seed layer and low-temperature growth

BaHfO3(BHO)-doped SmBa2Cu3Oy (SmBCO) superconducting films were fabricated with a seed layer and low-temperature growth (LTG technique) via pulsed laser deposition. The LTG technique enables the fabrication of high-quality SmBCO films even at a low substrate temperature during the deposition, and also greatly increases the number density of BHO nano-rods. Transmission electron microscopy observations showed that the 5.6 vol% BHO-doped sample had a high number density [(4.8 ± 0.3) × 103 μm−2] of small (5.4 ± 0.7 nm) BHO nano-rods. This sample exhibits maximum flux pinning forces (Fp) of 405 GN m−3 in 9.0 T at 40 K and 105 GN m−3 in 9.0 T at 65 K. The high density and small size of the nano-rods in a REBa2Cu3Oy matrix (where RE is a rare earth element) is considered to be very effective for enhancing Fp at low temperatures under a high field.

Flux pinning properties and microstructures of a SmBa2Cu3Oy film with high number density of BaHfO3 nanorods deposited by using low-temperature growth technique

To enhance Jc in high magnetic fields, we fabricated a SmBa2Cu3Oy (SmBCO) film with a high number density (2830/µm2) of BaHfO3 (BHO) nanorods by pulsed laser deposition (PLD) adopting a low-temperature growth (LTG) technique (LTG-SmBCO + BHO). The LTG technique consists of seed layer deposition and subsequent film deposition at a lower temperature. The number density of BHO nanorods in the film was approximately 4 times higher than that in a BHO-doped SmBCO film fabricated by conventional PLD without a seed layer. The flux pinning performance of the LTG-SmBCO + BHO film (Fp = 23.8 GN/m3 at 77 K under 4.5 T) was comparable to the reported record high value of 28.3 GN/m3 at 77 K.

Improvement in Jc performance below liquid nitrogen temperature for SmBa2Cu3Oy superconducting films with BaHfO3 nano-rods controlled by low-temperature growth

For use in high-magnetic-field coil-based applications, the critical current density (Jc) of REBa2Cu3Oy (REBCO, where RE = rare earth) coated conductors must be isotropically improved, with respect to the direction of the magnetic field; these improvements must be realized at the operating conditions of these applications. In this study, improvement of the Jc for various applied directions of magnetic field was achieved by controlling the morphology of the BaHfO3 (BHO) nano-rods in a SmBCO film. We fabricated the 3.0 vol. % BHO-doped SmBCO film at a low growth temperature of 720 °C, by using a seed layer technique (Ts = 720 °C film). The low-temperature growth resulted in a morphological change in the BHO nano-rods. In fact, a high number density of (3.1 ± 0.1) × 103 μm−2 of small (diameter: 4 ± 1 nm), discontinuous nano-rods that grew in various directions, was obtained. In Jc measurements, the Jc of the Ts = 720 °C film in all directions of the applied magnetic field was higher than that of the non-dope...

Vortex pinning at low temperature under high magnetic field in SmBa2Cu3O y superconducting films with high number density and small size of BaHfO3 nano-rods

We have fabricated 5.6 vol.% BaHfO3 (BHO)-doped SmBa2Cu3O y (SmBCO) films at a low substrate temperature of 750 °C by using the low temperature growth (LTG) technique with a seed layer. Using the LTG technique, we can obtain a purely c-axis oriented SmBCO film even at low growth temperatures. The LTG technique also greatly increases the number density of BHO nanorods. The size and matching field of the BHO nanorods in the films were 5.4 nm and 9.9 T. Transport measurements were carried out, and a high irreversibility field (B irr) and strong flux pinning force density (F p) were obtained. The B irr was 15.1 T at 77 K for B//c. The maximum F p of 407 GN m−3 in 10 T at 40 K and 770 GN m−3 from 9 to 17 T at 20 K have been measured. We believe that the LTG technique can be considered as important for improving of flux pinning performance in BaMO3 (BMO: M = Zr, Sn and Hf)-doped SmBCO coated conductors.

Superconducting Properties in SmBa 2 Cu 3 O y Films With High Density of BaHfO 3 Nanorods Fabricated With a Seed Layer

A high density of BaMO₃ (BMO) nanorods within a REBa₂Cu₃O_y (REBCO) film can be generated by heavy doping of the BMO. This is very effective in improving superconducting properties in high magnetic fields. However, heavy doping causes a significant T_c reduction. Introduction of a high density of BMO nanorods into REBCO films without reduction in T_c is required. To this end, we deposited SmBa₂Cu₃O_y (SmBCO) films with various amount of BaHfO₃ (BHO) by a pulsed laser deposition (PLD) method using a seed-layer technique and a low-temperature growth (LTG) technique. Using this technique, we can fabricate a purely c-axis-oriented SmBCO film even at low substrate temperatures T_s. The SmBCO films with BHO were fabricated at low T_s of 750 °C without heavy doping of BHO. As a result, the BHO nanorods became thinner and denser than those within SmBCO films fabricated by conventional PLD methods at high T_s. The maximum density was 4780/μm², and the B_φ of the film reached 9.9 T. These samples showed high T_c (> 90 K). We have therefore successfully fabricated high-T_c SmBCO films with high densities of BHO nanorods using the LTG technique.

Determinant for self-organization of BaMO3 nanorods included in vapor-phase-grown REBa2Cu3Oy films

BaMO3 (BMO; M = Zr, Sn, and Hf) self-organizes into nanorod shape within REBCO films grown by vapor phase epitaxy. Controlling the number density of the BMO nanorods is effective for an improvement of critical current density of the REBCO films in magnetic fields. We focused on the growth mechanism of the BMO nanorods in this paper. BaHfO3 (BHO)-doped REBCO films were fabricated under various conditions such as volume fraction of BHO and substrate temperature. We checked the number density and diameter of the BHO nanorods. We found that the volume fraction of BHO increased the number density, whereas the diameter was not affected by the volume fraction so much. The low substrate temperature contributed to the high number density in the films. Additionally, we carried out the 2-D Monte Carlo simulations for the nucleation and the selforganization of the BMO. The REBCO and BMO nuclei occurred randomly. However, due to small amount of BMO relative to REBCO, it was difficult for the BMO nuclei to get fat. We changed the molar fraction of the BMO and the substrate temperature in the simulations and evaluated the number density and the diameter of the resulting BMO nanorods.

Improvement of critical current densities in SmBa2Cu3Oy, films with BaHfO3 nano-rods using low temperature growth technique

We studied growth substrate temperature dependent morphologies of BaHfO3 (BHO) nano-rods for SmBa2Cu3Oy, (SmBCO) films with BHO fabricated by pulsed laser deposition (PLD) method. The morphology of BHO nano-rods within the SmBCO matrix could be controlled by changing substrate temperatures adoping a Low Temperature Growth (LTG) technique. Using the LTG technique, we could fabricate SmBCO films showed c-axis orientation even at low substrate temperature during deposition without a degradation of superconducting properties, which is one of the most advantages of the LTG technique. In this study, we found an optimal morphology of the BHO nano-rods and the morphology was fine, straight and various growth directions. The BHO nano-rods were more effective to improve the Jc in a magnetic field than other morphologies of the BHO nano-rods.

Delocalization of vortex in SmBa2Cu3O7−δ superconducting films with BaHfO3 nano-rods

Transport measurements revealed flux pinning properties and vortex phases in SmBa2Cu3O7−δ (Sm123) superconducting films with BaHfO3 nano-rods on LaAlO3 substrates. The films have large matching fields BΦ up to 9.9 T, nano-rod diameters of ∼6 nm, and a slight Tc degradation with Tc ∼ 91.8 K by using the low temperature growth technique. According to the transport results, a small critical exponent ∼4 indicates the presence of a Bose-glass phase in the films. Double peaks of the flux pinning force density are unexpectedly observed at high temperatures over 80 K, which is accompanied by steep drops of the crossover magnetic fields between the single vortex pinning and the collective pinning states. The drops are explained by the delocalization of the vortex where the vortex is pinned by many nano-rods in the single vortex pinning state. From the viewpoint of the vortex delocalization, we conclude that BΦ should be less than 11 T for applications at liquid nitrogen temperature.

Flux Pinning Properties in BaHfO3-Doped SmBa2Cu3Oy Films on Metallic Substrates Fabricated with Low Temperature Growth

We have developed SmBa2Cu3Oy (SmBCO) superconducting films with BaHfO3 (BHO) inclusions on singlecrystalline substrates via a pulsed laser deposition method. We have controlled the morphology of the BHO nanorods within the SmBCO matrix by varying growth temperature during the depositions. In this paper, we fabricated 1.5 vol.% BHO-doped SmBCO films on metallic substrates at 750 °C [low-temperature growth (LTG) film] and 840 °C [high-temperature growth (HTG) film]. The LTG film showed lower flux pinning performance at a relatively high measurement temperature ranging from 65 K to 77 K than the HTG film. However, at a low measurement temperature of 20 K under high magnetic fields, the LTG film exhibited higher flux pinning performance in comparison with the HTG film. The maximum flux pinning force density was more than 520 GN/m3 at 20 K under 9 T. Even on the metallic substrates, LTG is also helpful in achieving an improved flux pinning performance of SmBCO films at low measurement temperatures under high magnetic fields.

Magnetic field of BG-VG transition depending on the nanorods shape in BaHfO3-doped SmBa2Cu3Oy films

The vortex state under the magnetic held around vortex glass (VG)-Bose glass (BG) transition held was investigated in the SmBa2Cu3Oy (SmBCO) films with BaHfO3 (BHO) nanorods. We prepared two films with different forms of the nanorods (thick-straight and thin-tilted), and we used the magnetic held angular dependence of flux flow resistivity (p - θ) to identify a vortex state. From a comparison between the p - θ curves in completely BG and VG phases, it was revealed that the existence of deep and sharp dip structure along the c-axis direction of the SmBCO matrix (B//c) was an evidence of the BG state in both the films. In the magnetic held region around VG-BG transition, shallower and broader dips than the dips in the BG state were observed at B//c in both the films. In addition, the dips became deeper with increase of the magnetic held in this region in each film. Therefore, we considered the vortex state of both the films was an intermediate and/or a transient state of BG and VG. Under all magnetic fields, the film with thick and straight BHO nanorods showed stronger c-axis correlated flux pinning force than the film with thin and tilted nanorods. We suggested the intermediate state of the hlm with thicker and straighter nanorods was similar to the BG state compared with another hlm from the both of experimental results and reported theory.