The ability of large grain, REBa2Cu3O7- [(RE)BCO; RE = rare earth] bulk superconductors to trap magnetic field is determined by their critical current. With high trapped fields, however, bulk samples are subject to a relatively large Lorentz force, and their performance is limited primarily by their tensile strength. Consequently, sample reinforcement is the key to performance improvement in these technologically important materials. In this work, we report a trapped field of 17.6 T, the largest reported to date, in a stack of two, silver-doped GdBCO superconducting bulk samples, each of diameter 25 mm, fabricated by top-seeded melt growth (TSMG) and reinforced with shrink-fit stainless steel. This sample preparation technique has the advantage of being relatively straightforward and inexpensive to implement and offers the prospect of easy access to portable, high magnetic fields without any requirement for a sustaining current source.

The Fe-based superconductors (FBS) are an important new class of superconducting materials. As with any new superconductor with a high transition temperature and upper critical field, there is a need to establish what their applications potential might be. Applications require high critical current densities, so the usefulness of any new superconductor is determined both by the capability to develop strong vortex pinning and by the absence or ability to overcome any strong currentlimiting mechanisms of which grain boundaries in the cuprates are a cautionary example. In this review we first consider the positive role that grain boundary properties play in the metallic, low temperature superconductors and then review the theoretical background and current experimental data relating to the properties of grain boundaries in FBS polycrystals, bi-crystal thin films, and wires. Based on this evidence, we conclude that grain boundaries in FBS are weak linked in a qualitatively similar way to grain boundaries in the cuprate superconductors, but also that the effects are a little less marked. Initial experiments with the textured substrates used for cuprate coated conductors show similar benefit for the critical current density of FBS thin films too. We also note that the particular richness of the pairing symmetry and the multiband parent state in FBS may provide opportunities for grain boundary modification as a better understanding of their pairing state and grain boundary properties are developed.

Progress in superconducting bulk materials has been somewhat overshadowed by the considerable effort required to produce practical long-length conductors. There has, however, been steady progress in both the materials science of bulk superconducting materials and the technologies required to use them effectively in engineering applications. In particular, magnetised bulk superconductors are capable of acting as quasi-permanent magnets with the potential of providing magnetic fields of several tesla or greater from a small volume of material, they can act as magnetic shields and they can provide self-stabilised levitation. This roadmap, based on a workshop which involved the participation of a wide range of academic and industrial participants (see doi: 10.17863/CAM.586 for details of the workshop methodology), aims to explore some of the key potential domains of application of bulk superconductors. Detailed technological roadmaps are presented for four key applications that were identified as providing both good market opportunity and feasibility. These are: portable systems for bulk superconductivity; portable, high-field magnet systems for medical devices; ultra-light superconducting rotating machines for next-generation transport & power applications; and magnetic shielding applications for electric machines, equipment and other high-field devices.

We report a novel negative magnetocaloric effect in CoMnSi1−xGex arising from a metamagnetic magnetoelastic transition. The effect is of relevance to magnetic refrigeration over a wide range of temperature, including room temperature. In addition we report a very high shift in the metamagnetic transition temperature with applied magnetic field. This is driven by competition between antiferromagnetic and ferromagnetic order which can be readily tuned by applied pressure and compositional changes.

Addition of pyrochlore rare earth tantalate phases, RE 3 TaO 7 (RTO, where RE = rare earth, Er, Gd and Yb) to YBa 2 Cu 3 O 7−δ (YBCO) is shown to vastly improve pinning, without being detrimental to the superconducting transition temperature. The closely lattice matched to RTO phase provides a lower interfacial energy with YBCO than BaZrO 3 (BZO) and produces very fine (∼5 nm) particles with high linearity in their self-assembly along c. Critical current densities of 0.86, 0.38 MA cm −2 at 1 and 3 T (for fields) parallel to the c axis were recorded at 77 K in 0.5-1.0 μm thick films and a transition temperature of 92 K was observed even in the highest level doped sample (8 mol%).Improvement of flux pinning and thus the critical current, J c , that can be carried in YBa 2 Cu 3 O 7−δ (YBCO) is crucial for achieving widespread applications of this technologically important material. Practical pinning enhancement methods developed within the last five years, such as incorporating nanoinclusions in the film [1][2][3][4] or on the substrate surface [5,6], disorder effects from rare earth (RE) modifications [7] and microstructural modification, have all been successful in specific field and temperature regimes [8][9][10][11][12][13][14]. Barium perovskites (BaBO 3 ) nanoinclusions with B site ions from group IV and simple binary rare earth oxides have shown the best performance so far [15,16]. However, sometimes the superconducting transition temperature T c is reduced by the additions due to disordering or poisoning, thus limiting the usable 4 Author to whom any correspondence should be addressed.

Investigating, predicting and optimising practical magnetization techniques for charging bulk superconductors is a crucial prerequisite to their use as high performance 'psuedo' permanent magnets. The leading technique for such magnetization is the pulsed field magnetization (PFM) technique, in which a large magnetic field is applied via an external magnetic field pulse of duration of the order of milliseconds. Recently "giant flux leaps" have been observed during charging by PFM: this effect greatly aids magnetization as flux jumps occur in the superconductor leading to magnetic flux suddenly intruding into the centre of the superconductor. This results in a large increase in the measured trapped field at the centre of the top surface of the bulk sample and full magnetization. Due to the complex nature of the magnetic flux dynamics during the PFM process simple analytical methods, such as those based on the Bean critical state model (CSM), are not applicable. Consequently, in order to successfully model this process, a multi-physical numerical model is required, including both electromagnetic and thermal considerations over short time scales. In this paper, we show that a standard numerical modelling technique, based on a 2D axisymmetric finiteelement model implementing the H-formulation, can model this behaviour. In order to reproduce the observed behaviour in our model all that is required is the insertion of a bulk sample of high critical current density, J c. We further explore the consequences of this observation by examining the applicability of the model to a range of previously reported experimental results. Our key conclusion is that the "giant flux leaps" reported by Weinstein et al. and others need no new physical explanation in terms of the behaviour of bulk superconductors: it is clear the "giant flux leap" or flux jump-assisted magnetisation of bulk superconductors will be a key enabling technology for practical applications.

We discuss predictions of five proposed theories for the critical state of type-II superconductors accounting for both flux cutting and flux transport (depinning). The theories predict different behaviours for the ratio E y /E z of the transverse and parallel components of the in-plane electric field produced just above the critical current of a type-II superconducting slab as a function of the angle of an in-plane applied magnetic field. We present experimental results measured using an epitaxially grown YBCO thin film favoring one of the five theories: the extended elliptic critical-state model. We conclude that when the current density J is neither parallel nor perpendicular to the local magnetic flux density B, both flux cutting and flux transport occur simultaneously when J exceeds the critical current density J c , indicating an intimate relationship between flux cutting and depinning. We also conclude that the dynamical properties of the superconductor when J exceeds J c depend in detail upon two nonlinear effective resistivities for flux cutting (ρ c ) and flux flow (ρ f ) and their ratio r = ρ c /ρ f .

It has been well established that the critical current density J c across grain boundaries ͑GBs͒ in high-temperature superconductors decreases exponentially with misorientation angle beyond ϳ2°-3°. This rapid decrease is due to a suppression of the superconducting order parameter at the grain boundary, giving rise to weakly pinned Abrikosov-Josephson ͑AJ͒ vortices. Here we show that if the GB plane meanders, this exponential dependence no longer holds, permitting greatly enhanced J c values: up to six times at 0 T and four times at 1 T at ϳ 4°-6°. This enhancement is due to an increase in the current-carrying cross section of the GBs and the appearance of short AJ vortex segments in the GB plane, confined by the interaction with strongly pinned Abrikosov ͑A͒ vortices in the grains.