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The influence of CuFe2O4 nanoparticles on superconductivity of MgB2

The influence of CuFe2O4 nanoparticle [1] doping on superconducting properties of Fe-sheated MgB2 wires has been studied. The wires containing 0, 3 and 7.5 wt.% of monodisperse superparamagnetic nanoparticles (~7 nm) were sintered at 650°C or 750°C for 1 hour in the pure argon atmosphere. X-ray diffraction patterns of doped samples showed very small maxima corresponding to iron and small increase in the fraction of MgO phase indicating some interaction of nanoparticles with Mg. However, the measurements of magnetoresistance and the transport critical currents, Jc, performed in the temperature range 2-42 K and magnetic field up to 16 T showed strong deterioration of the superconducting properties upon doping with CuFe2O4. The transition temperatures, Tc, of doped samples decreased for about 1.4 K per wt.% of CuFe2O4. Also the irreversibility fields Birr(T) decreased progressively with increasing doping. Accordingly, also the suppression of Jc with magnetic field became stronger. The observed strong deterioration of superconducting properties of MgB2 wires is at variance with reported enhancement of critical currents at higher temperatures (determined from magnetization) in bulk MgB2 samples doped with Fe3O4 nanoparticles [2]. The probable reason for this discrepancy is briefly discussed. [1] D. Pajić et al, J. Magn. Magn. Mater. 281 (2004) 353 [2] B. Qu et al, Supercond. Sci. Technol. 22 (2009) 015027

MgB2 wires; magnetic nanoparticle doping; flux pinning; transport properties

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