Role of disorder in the superconducting proximity effect in a-NdNi5/Nb bilayers

The search for optimal materials to be used in superconductor/ferromagnet hybrids is fundamental to the development of superconducting spintronic devices. One of the main issues which should be tackled is the optimum ferromagnetic material that should be used in these systems, i.e., a ballistic ferromagnet with a large value of the exchange energy or a weak ferromagnet with diffusive transport. Here, we investigate the proximity effect between Nb and amorphous NdNi5. We observe that the electrical transport is influenced by the strong disorder in the ferromagnetic layers. For small values of the ferromagnetic layer thickness, the depression of the superconducting critical temperature is stronger than as observed in metallic ferromagnetic layers with comparable values of the exchange energy. On the contrary, for a large thickness of the ferromagnet the interface transparency is very small and the effect of the ferromagnet on the superconductor becomes negligible. The results are in line with recent observations and confirm the prominent role played by the structural and morphological properties of the ferromagnet on the physics of superconductor/ferromagnet bilayers.