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

• Rigorous study of the effects of magnetic particles on superconductivity in MgB2 wires.
• Detailed characterisation of magnetic state of nanoparticles embedded in MgB2 matrix.
• Matching effects in pinning of vortices on magnetic nanoparticles
• Consistent indications of possible contribution of magnetic pinning of vortices.

The results indicating magnetic flux pinning in MgB2 wires doped with three types of magnetic nanoparticles (MNP) are reported. The magnetic state of MNPs, both as-prepared and inside the MgB2 core, was determined with magnetization and ac susceptibility measurements. The competition between detrimental influence of doping (reduced connectivity, pair breaking) and enhanced flux pinning leads to deterioration of electromagnetic properties of doped wires at high MNP content, whereas light doping causes an enhancement of critical current density, Jc, and/or irreversibility field, Birr, for all our MNPs. For Ni and dextrin coated NiFe2O4 MNPs the enhancement of Jc was comparable to that achieved with the best nonmagnetic dopands. Detailed analysis indicates the contribution of magnetic flux pinning including the matching effects in flux pinning on MNPs.