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Enhancing Superconducting Properties of MgB2 by Addition of Magnetic Nanoparticles

Main results of ongoing comprehensive study of the influence of magnetic nanoparticles (MNP) on superconducting properties (transition temperature Tc, critical fields and critical current density Jc. etc.) of high quality Fe-sheated MgB2 wires are brieflv reviewed. In spite of conceptual importance (coexistence of superconductivity and magnetism, magnetic flux pinning in bulk superconductor, etc.) and tehnological relevance (enhancement of Jc and critical fields) of such studies, present experimental and reoretical results for MgB2 are quite inconclusive [1, 2]. In order to shed more light on this topic we prepared so far over 70 MgB2 wires doped with 12 different types of MNP, ( with contents up to 12 wt. 0/0), such as elemental magnets ( Fe, Co, Ni ), magnetic borides ( Fe2B, Co2B, CoNiB ). ferrites ( AFe2O4, A=Mn, Fe, Co, Ni, Cu) and rare earth borides ( DyB4, EuB6). Both. uncoated and coated (with carbon, silica or dextrin shell ) MNPs were used in order to assess the effects of co-doping and of interparticle interactions. The influence of the shape of MNP ( sphere or rod) and of annealing temperature ( < 750 C ) have been studied, too. Generally, doping with MNPs supresses Tc of MgB2 thus higher MNP contents cause deterioration of superconducting properties of wires. However, light doping (< 2.5 wt. %) with some types of MNPs (e.g. Ni, CoNiB [3] etc.) improved flux pinning and in-field J, of wires. Particulary promising seems co-doping with core-shell MNPs , where both critical fields and Jc were simultaneously enhanced [2] throughout the explored field (B <16T) and temperature range (T < Tc). Pobable origins of these improvements ( such as magnetic flux pinning, etc.) are briefly discussed. However, only the nanoprecipitates of EuB6 and DyB4 embeded in wires provided the evidence for magnetic contribution to vortex pinning in bulk MgB2 . The results are compared with the literature dana. 1.N.Novosel and E.Babić, Physica 493, 119 (2013) and reference therein ; 2.N.Novosel et al. Supercond.sci.Technol. 26, 105024 (2013). 3.N.Novosel et al. Supercond.sci.Technol. 26, 065004 (2013).

superconductivity; MgB2; nanoparticles

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