Magnetic patterns and flux pinning in Pd$_{0.99}$Fe$_{0.01}$-Nb hybrid structures

The magnetic properties of hybrid thin-film Pd$_{0.99}$Fe$_{0.01}$-Nb structures are studied by a magneto-optical technique. It is shown that, below $14$ K, the samples exhibit the ferromagnetic ordering corresponding to the formation of weakly coupled ferromagnetic nanoclusters. In the clusters, the effective spin polarization of Fe ions is about $4\mu$B, corresponding to that in the bulk Pd$_3$Fe alloy. The proximity of the ferromagnetic layer does not suppress the superconductivity in niobium. It does not affect the superconducting transition temperature but leads to an enhanced pinning and results in an increase in the critical current by about $30\%$. This behavior agrees well with the existence of the nanocluster structure in the ferromagnetic film.