Superconducting triplet pairing in Ni--Ga-bilayer junctions.

Ni--Ga bilayers are a versatile platform for exploring the competition between strongly antagonistic ferromagnetic and superconducting phases. We characterize the impact of this competition on the transport properties of highly-ballistic Al/Al$ _2 $O$ _3 $(/EuS)/Ni--Ga tunnel junctions from both experimental and theoretical points of view. While the conductance spectra of junctions comprising Ni (3 nm)--Ga (60 nm) bilayers can be well understood within the framework of earlier results, which associate the emerging main conductance maxima with the junction films' superconducting gaps, thinner Ni (1.6 nm)--Ga (30 nm) bilayers entail completely different physics, giving rise to novel conductance-peak subseries that we term conductance shoulders. Detecting the paramagnetic Meissner response in Ga from polarized neutron reflectometry provides the essential experimental hint that these conductance shoulders are caused by superconducting triplet pairings that Ni's ferromagnetic exchange interaction induces near thin Ni--Ga bilayers' interfaces -- most likely owing to inhomogeneously magnetized interface domains. We further substantiate our findings by means of a phenomenological theoretical model, clarifying that induced superconducting triplet pairings around the interface of Ni--Ga bilayers can indeed manifest themselves in the observed conductance shoulders. Arranging our work in a broader context demonstrates that Ni--Ga-bilayer junctions have a strong potential for efficient triplet-pairing engineering in superconducting-spintronics applications.