Effect of mixed pinning landscapes produced by 6 MeV oxygen irradiation on the resulting critical current densities J_c in 1.3 µm thick GdBa₂Cu₃O_7-d coated conductors grown by co-evaporation

Highlights

• GdBa₂Cu₃O₇ coated conductors are irradiated with 6 MeV Oxygen.

• The in-field dependences of J_c are improved with H parallel and tilted from the c-axis

• Characteristic glassy exponents are analyzed.

Abstract

We report the influence of crystalline defects introduced by 6 MeV ¹⁶O³⁺ irradiation on the critical current densities J_c and flux creep rates in 1.3 µm thick GdBa₂Cu₃O_7-δ coated conductor produced by co-evaporation. Pristine films with pinning produced mainly by random nanoparticles with diameter close to 50 nm were irradiated with doses between 2 × 10¹³ cm⁻² and 4 × 10¹⁴ cm⁻². The irradiations were performed with the ion beam perpendicular to the surface of the samples. The J_c and the flux creep rates were analyzed for two magnetic field configurations: magnetic field applied parallel (H║c) and at 45° (H║45°) to the c-axis. The results show that at temperatures below 40 K the in-field J_c dependences can be significantly improved by irradiation. For doses of 1 × 10¹⁴ cm⁻² the J_c values at μ₀H = 5 T are doubled without affecting significantly the J_c at small fields. Analyzing the flux creep rates as function of the temperature in both magnetic field configurations, it can be observed that the irradiation suppresses the peak associated with double-kink relaxation and increases the flux creep rates at intermediate and high temperatures. Under 0.5 T, the flux relaxation for H‖c and H||45° in pristine films presents characteristic glassy exponents μ = 1.63 and μ = 1.45, respectively. For samples irradiated with 1 × 10¹⁴ cm⁻², these values drop to μ = 1.45 and μ = 1.24, respectively

Introduction

During the last years, a great amount of effort has been orientated to improve the superconducting properties of RBa₂Cu₃O_7−δ (RBCO; R: Sm, Dy, Y, Gd) coated superconductors (CCs) [1], [2], [3], [4]. One of the strongest aspects expected for applications such as motors and magnets is to be able to maintain high critical current density J_c at high magnetic fields [5], [6]. The current carrying capacity is determined by the interaction of vortex matter with crystalline defects [7]. The geometry and density of the pinning centers determine the in-field and angular dependence of J_c [7], [8], [9]. In addition, the vortex-defect interactions are not accumulative and the vortex pinning produced by the different crystalline defects depends on the temperature and on the magnetic field. While small defects improve pinning mainly below 40 K, large defects (size of a few coherence length ξ) do so throughout the whole temperature range. [10]. For the latter, the pinning can be isotropic (nanoparticles) or correlated (columnar defects). Innovative approaches have been used to design optimal vortex pinning by defect structures [8], [11]. Most of them are based on the inclusion of nanometric secondary phases [12], [13].

The formation of the desired pinning centers depends sensitively on the film deposition technique and substrate architecture [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. Despite the remarkable advance in the synthesis of desirable microstructures obtained at laboratory scale research, pinning landscapes including a high density of strong and weak pinning centers are usually difficult at industrial scale. This limitation in the synthesis of CCs has been recently overcome by including pinning centers by irradiation [15], [16], [17], [18]. Depending on the mass and energy of the ions and the properties of the superconducting material, irradiation enables the creation of defects with well-controlled density and topology such as points, clusters, or tracks [19]. The in-field dependence of J_c at temperatures below 40 K has been improved by irradiation with protons in CCs grown by metal organic deposition MOD and co-evaporation [15], [16]. Most recently, the feasibility to similarly improve the properties of CCs obtained by MOD in much shorter irradiation times using 3.5 MeV ¹⁶O³⁺ has been reported [17].

Here, we show the effects of random point defects and nanoclusters introduced by 6 MeV ¹⁶O³⁺ irradiation with doses between 2 × 10¹³ cm⁻² and 4 × 10¹⁴ cm⁻² on the magnetic field dependence of J_c 1.3 µm thick GBCO thin films grown by co-evaporation. In addition, the vortex dynamics displayed in pristine and irradiated samples is analyzed by performing magnetic relaxation measurements. The microstructure of the pristine tape displays random and irregular shape precipitates with typical size of 50 nm. Unlike ref. [17], the energy of the ions was selected to place the Bragg peak inside the substrate (instead of the superconducting layer). Several attempts of irradiation with 3.5 MeV ¹⁶O³⁺ showed that, although the in-field of J_c dependence is smoother than for pristine samples, the irradiation reduces considerably the low field J_c values. In comparison to our previous work [16], in which tapes irradiated with protons reached the optimal doses in tens of minutes, the irradiation with 6 MeV ¹⁶O³⁺ demands only a few seconds. While the irradiation significantly improves the in-field dependence of J_c for the H‖c and H||45° configurations, its absolute values are merely affected at low fields. From Maley analyses, we calculate the μ glassy exponent for the collective creep regime. For both configurations, the μ values decrease after irradiation, which increases the flux creep rates in comparison to pristine films.