Revealing the role of magnetic domains in superconducting tunneling spectroscopy of EuS/Al bilayers.

A renewed interest in studying Ferromagnetic/Superconductor structures came with the development of superconducting spintronics [1]. Ferromagnetic insulators (FIs) attached to a superconductor are known to induce triplet superconducting pairing and an exchange energy splitting in the Bardeen-Cooper-Schrieffer (BCS) density of states [2].
Here we investigate this magnetic proximity effect in EuS/Al bilayers tunnel coupled to an Al probing layer. Remarkably, the tunneling spectroscopy of our devices reveals exchange splitting of the BCS peaks even in the unpolarized state of the EuS, with enhancement when magnetizing the sample. The high contrast in the low temperatures measurements (30 mK) allow to relate the line shape of the split BCS conductance peaks to the characteristic magnetic domain structure of the EuS layer [3]. The hard gap and large splitting observed in our measurements make EuS/Al an excellent candidate for substituting strong magnetic fields in experiments studying Majorana bound states.

[1] J. Linder and J. W. A. Robinson, Nat Phys, 11, 307, (2015).
[2] P. M. Tedrow, J. E. Tkaczyk, and A. Kumar, Phys. Rev. Lett., 56, 1746 (1986).
[3] E. Strambini et al., Phys. Rev. Materials, 1, 054402 (2017).