Influence of randomly distributed magnetic nanoparticles on surface superconductivity in Nb films

TL;DR

This study investigates how randomly distributed magnetic nanoparticles embedded in Nb films influence surface superconductivity, revealing field-dependent effects linked to nanoparticle magnetization and orientation.

Contribution

It provides experimental evidence that magnetic nanoparticles can enhance surface superconductivity in Nb films, depending on magnetic field orientation and nanoparticle saturation.

Findings

Surface superconductivity field Hc3(T) is enhanced by magnetic nanoparticles.

Enhancement occurs only when magnetic field is normal to the film.

The effect correlates with the saturation field of the nanoparticles.

Abstract

We report on combined resistance and magnetic measurements in a hybrid structure (HS) of randomly distributed anisotropic CoPt magnetic nanoparticles (MN) embedded in a 160 nm Nb thick film. Our resistance measurements exhibited a sharp increase at the magnetically determined bulk upper-critical fields Hc2(T). Above these points the resistance curves are rounded, attaining the normal state value at much higher fields identified as the surface superconductivity fields Hc3(T). When plotted in reduced temperature units, the characteristic field lines Hc3(T) of the HS and of a pure Nb film, prepared at exactly the same conditions, coincide for H<10 kOe, while for fields H>10 kOe they strongly segregate. Interestingly, the characteristic value H=10 kOe is equal to the saturation field of the MN. The behavior mentioned above is observed only for the case where the field is normal to the HS, while is absent when the field is parallel to the film. Our experimental results suggest that the observed enhancement of surface superconductivity field Hc3(T) is possibly due to the not uniform local reduction of the external magnetic field by the dipolar fields of the MN.