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Magnetic nanoparticles in MgB2 : Vortex pinning, pair breaking and connectivity

Present status of ongoing study of the influence of single-domain magnetic nanoparticles (MNP) with size comparable to the coherence length of MgB2 and superparamagnetic (SP) at/below room temperature on electromagnetic properties (resistivity, magnetization, connectivity, Tc, Jc, critical fields etc.) of high quality MgB2 wires is reported. In spite of conceptual importance (coexistence of of magnetism and superconductivity, magnetic flux pinning) and technological potential (improvement of superconducting parameters) present results for the influence of MNPs on MgB2 are quite inconclusive [1]. In order to shed more light on this problem we measured over sixty MgB2 wires doped with different amounts of four types of MNPs: elemental magnets, magnetic borides, ferrites and rare earth oxides/borides. Both, coated (carbon, silica or dextrin shell) and uncoated MNPs were used in order to asses the effects of co-doping and interparticle interaction. The influence of the annealing temperature and of shape of MNPs (sphere or rod) were studied, too. The magnetic state of MNPs, both in as-obtained form and inside MgB2 was determined via magnetization and ac susceptibility measurements. Transport and magnetic measurements were used in order to determine reliably characteristic fields and Jc of doped and virgin wires [1]. The competition between detrimental influence of magnetic inclusions (reduced connectivity, pair breaking) and enhanced flux pinning in doped wires leads to deterioration of electromagnetic properties at high MNP contents, whereas at light doping (<3 wt%) an enhancement of these properties was observed for several MNPs. For Ni and dextrin coated NiFe2O4 MNPs the enhancement of in-field Jc is comparable to that obtained by the best nonmagnetic dopands. In the case of the enhancement of the properties a detailed analysis indicates the contribution of magnetic flux pinning, including the first observation of the matching effects in vortex pinning on MNPs. [1] N. Novosel and E. Babic, Physica C 493 (2013) 119

MgB2 wires ; Magnetic nanoparticle doping ; Flux pinning

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