4-fold enhancement in the critical current density of YBa2Cu3O7 films by practical ion irradiation

Abstract

We report an up-to-4-fold enhancement in the in-magnetic-field critical current density at 77 K of epitaxial YBa 2 Cu 3 O 7 films on CeO 2 -buffered SrTiO 3 substrates by 3-MeV Au 2þ irradiation. This indicates that irradiation using an industrially practical ion beam, which generally has kinetic energy less than 5 MeV, can provide a substantial increase in the in-field current performance of high-temperature superconductor films. Transmission electron microscopy results show that pointlike defects smaller than 6 nm in diameter were created in the films by the irradiation. V C 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4769836]One of the most significant hurdles in commercialization of electric-power devices using high-temperature superconductors (HTSC) is degraded critical-current-density (J c ) in magnetic fields. 1 In 2004, Macmanus-Driscoll et al. addressed this challenge by precipitating BaZrO 3 in YBa 2 Cu 3 O 7 (YBCO) films, 2 which work as artificial pinning centers (APCs) of vortices. 3 However, precise control of the secondary phase in shape 4 and alignment 5 has recently been recognized as vital to maximize J c and requires highly sophisticated synthesis techniques. Post-growth ion irradiation 6-12 is an easier route for such microstructure engineering, since it can control the properties of irradiation defects (size, shape, density, and alignment) by external parameters (ion mass, energy, fluence, and incident angle) without altering the growth conditions of a target material. 8,11 The following two reasons explain mainly why the irradiation approach to APC introduction has not received much attention during the recent development of second-generation HTSC wires, which consist of an epitaxial HTSC film on a metallic tape. The first reason is that previously observed J c improvements were substantial only in the bulk material but not in films, 1 which are a naturally pinning-center-rich form of the material. 13 The second is that measurable improvement in films was attained through the use of extremely high-energy ions (from hundreds MeV to GeV) generated by industrially impractical accelerators. 7,10-12 Indeed, previous ion-irradiation experiments for improving the current properties of HTSC heavily focused on the high-ion-energy range called electronic-stopping regime (>100 MeV), where incident ions lose their kinetic energy by the electronic excitation of target atoms. In that regime, continuous columnar defects can be fabricated along ion tracks 8,9,11 in perfect accordance with a long-accepted "consensus" that a cylindrical APC is most effective for increasing J c . 7,14 Conversely, despite its industrial affinity, ion irradiation in the nuclear-stopping regime (<5 MeV), where ions are decelerated by elastic collisions, has lacked an extensive investigation in the context of in-field-J c enhancement, 15-17 based on an assumption that expected point-like defects 9,11 are not as effective as continuous ones.In this letter, we propose reconsideration of these trends in the study...

Influence of middle-energy ion-irradiation on the flux pinning properties of YBCO films: Comparison between different synthesis methods

“…Essentially the same results were obtained at 0.1 and 1 T where we performed θ-dependent measurements. As we reported previously [1], MeV-ion irradiation on a FF-MOD film enhances J c more significantly at H // c than H // ab [figures 2(a) and 2(d)]. This basic anisotropy trend is also found in figures 2(b) and 2(e), suggesting that the same type of pinning centers were introduced by the irradiation in FF-MOD and TCE films, although the width of J c enhancement is largely different between the two.…”

“…J c data was collected by a nondestructive ac inductive method at the liquid nitrogen temperature [6]. All J c data of FF-MOD films was presented in our previous publication [1].…”

“…Recently, we observed a 4-fold enhancement of critical current density (J c ) in magnetic fields by 3-MeV gold ion irradiation in YBa 2 Cu 3 O 7 (YBCO) films prepared by fluorine-free metal-organic deposition (FF-MOD) [1]. This result was remarkable because it evidenced that ion irradiation in the so-called nuclear stopping regime (0.1-5 MeV in YBCO), which creates point-like defects in a target, can provide additional pinning force as strong as those by irradiation in the electronic stopping regime (> 100 MeV), which creates columnar defects [2].…”

Strongly enhanced vortex pinning from 4 to 77 K in magnetic fields up to 31 T in 15 mol.% Zr-added (Gd, Y)-Ba-Cu-O superconducting tapes

“…With respect to J c at self-field, the retained J c at 3 T is ∼19.5%, and ∼13.4% at 5 T, values much higher than the values of 10.9% at 3 T and 3.4% at 5 T measured in the 7.5% Zr-doped film. J c remains above 0.1 MA/cm 2 , a threshold value for applications, 4 up to 8 T. Meanwhile, J c of the 15% Zr-doped thin film exceeds that of Au-ion irradiated YBCO films at field above 2 T, 25 and the recently reported J c of proton-irradiated metal organic deposited (MOD) thin films above 6.5 T. sample is its extremely high H irr (77 K) ≈ 14.8 T, about 45% higher than that of the 7.5% Zr-doped film, and much higher than ∼9 T shown by nominally pure films.…”

A route for a strong increase of critical current in nanostrained iron-based superconductors

“…These defects produced by ion irradiation would account for the depression of T c in YBa 2 Cu 3 O y (YBCO), which is proportional to the average number of defects23. Recently, a low-energy ion irradiation has been revisited and suggested as a practically feasible approach to improve flux pinning in YBCO242526. In single crystals of iron-based superconductors, it has been found that ion irradiations, as a whole, improve J c and its J c enhancements persist up to much higher fluencies than in cuprate superconductors, although T c is suppressed with increasing irradiation doses272829.…”