Influence of NiFe<sub>2</sub>O<sub>4</sub> nanoparticle doping on properties of single-domain YBCO bulk superconductors

Abstract

NiFe<sub>2</sub>O<sub>4</sub> (NFO) nanoparticle doped YBCO bulk superconductors are fabricated by using a novel top-seed infiltration growth (TSIG) technique. The growth morphology, microstructure and superconducting properties are investigated. The results show that at low doping levels, the normal growth of YBCO single domain is not affected by the NFO doping, but at high doping levels, obvious random nucleation appears at the edge of the sample. The measurement of levitation force indicates that the maximum levitation force on the sample first increases and then decreases with the increase of the NFO doping amount, and the largest levitation force is obtained to be 33.93 N for the sample with a doping level of 0.2% (weight percent). Low-temperature magnetization measurement shows that the YBCO sample exhibits that <i>T</i><sub>c</sub> value decreases with NFO amount increasing, and the superconducting transition width (Δ<i>T</i><sub>c</sub>) also broadens gradually. The sample with the optimal doping (0.2% weight percent) presents an enhanced zero-field <i>J</i><sub>c</sub> value of 8.68 × 10<sup>4</sup> A/cm<sup>2</sup>, which is 31% higher than the sample without dopant. In addition, a more obvious secondary peak of 4.37 × 10<sup>4</sup> A/cm<sup>2</sup> at a field of 1 T is observed for the 0.2 wt.% NFO doped sample, which indicates the existence of enhanced <italic/>δ<i>T</i><sub>c</sub> pinning in the bulk. The SEM measurement shows that two types of particles are trapped in the Y-123 matrix for YBCO sample doped with 0.2% weight percent NFO: one is the large particle with a size mainly ranging from 0.5 μm to 2.0 μm, and the other is small nano-inclusion mainly ranging from dozens of nanometers to about several hundreds of nanometers. Such a microstructure is very similar to the microstructure of the undoped sample we reported earlier. So whether the NFO nanoparticles exist in the microstructure cannot be judged just from the morphology of the nano-inclusions. The electron probe microarea analysis (EPMA) result shows that different concentration distributions of Ni and Fe elements are observed in the sample doped with 0.2% weight percent NFO, which indicates the separation of NFO nanoparticles in the heat treatment process, and the dissolved Ni and Fe ions finally exist in the form of element substitutions in the YBCO bulk. Such element substitutions can introduce local lattice distortions and weak-superconducting regions into the superconducting matrix, which can act as effective flux pinning centers, and hence improving the properties of the samples.