Mesoscopic S/F/S Trilayers in Parallel Magnetic Fields

Abstract

We report on the fabrication and study of NbN/F/NbN trilayers with NbN films of 80-nm thickness, which is much larger than the coherence length but significantly smaller than the London penetration depth, and 50-nm-thick ferromagnetic (F = Ni or NiCu alloy) interlayers, along with 160-nm-thick single layers of NbN as a reference. The heterostructures were patterned into square shapes to enable nonlocal four-probe resistive measurements. This technique, previously applied to single-layer NbN films, allows for the simultaneous and independent probing of surface and bulk superconducting transitions in one measurement run. By comparing NbN and NbN/F/NbN trilayers, we demonstrate that the inclusion of a ferromagnetic interlayer leads to a significant shift in the onset of the superconducting transition, particularly in the near-surface region of the trilayer, and increases the overall transition width. This behavior is consistent with the concept of the electromagnetic proximity effect, predicted theoretically and, thanks to a new measurement strategy that distinguishes between surface and bulk contributions, provides evidence for its presence. The work provides insights into the delicate interplay between superconductivity and magnetism and opens pathways for engineering related interface-sensitive spintronic devices.