Abstract
The mixed-state superconducting properties of bulkMgB2+2 at.%TiO2, prepared by the in situ solid state reaction, have been investigated. Thehigh-resolution transmission electron microscopy study reveals that the samplehas combined microstructures, consisting of regions with small- and large-sizeMgB2 grains, and the MgO inclusions. The particular grains are separated by thinTiB2 interlayersalong the c axis. Analysis on the mixed-state parameters, such as the upper criticalfield, the coherence length and the Ginzburg–Landau parameterκ, proves thatthe MgB2+2 at.% TiO2 belongsto a high-κ type-II superconductor in the dirty limit. The non-uniformly scaled microstructure resultsin an unusual (close to the double peak) magnetic field dependence of the pinning forcedensity. The field-cooled temperature dependence of magnetic moment exhibits atransition of the sample to the paramagnetic state at certain applied magnetic fields,which is treated as a manifestation of the paramagnetic Meissner effect. Theexperimental results are discussed on the basis of modern theoretical approaches.
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