Revealing the magnetic proximity effect in EuS/Al bilayers through superconducting tunneling spectroscopy

Abstract

A ferromagnetic insulator in contact with a superconductor is known to induce an exchange splitting of the singularity in the Bardeen-Cooper-Schrieffer (BCS) density of states (DoS). The magnitude of the splitting is proportional to the exchange field that penetrates into the superconductor to a depth comparable with the superconducting coherence length and which ranges in magnitude from a few to a few tens of tesla. We study this magnetic proximity effect in EuS/Al bilayers and show that the domain structure of the EuS affects the positions and the line shapes of the exchange-split BCS peaks. Remarkably, a clear exchange splitting is observed even in the unmagnetized state of the EuS layer, suggesting that the domain size of the EuS is comparable with the superconducting coherence length. Upon magnetizing the EuS layer, the splitting increases while the peaks change shape. Conductance measurements as a function of bias voltage at the lowest temperatures allowed us to relate the line shape of the split BCS DoS to the characteristic domain structure in the ultrathin EuS layer. These results pave the way to engineering triplet superconducting correlations at domain walls in EuS/Al bilayers. Furthermore, the hard gap and large splitting observed in our tunneling spectroscopy measurements make EuS/Al an excellent candidate for substituting strong magnetic fields in experiments studying Majorana bound states.

Signatures of spin-triplet Cooper pairing in the density of states of spin-textured superconductor-ferromagnet bilayers

“…We see again that far from the wall center (pink curves) the DOS is practically the same as the spinresolved DOS for a homogeneous superconductor with = h e h x eff . The results for a narrow DW d w =0.5ξ S are qualitatively similar to the results, obtained in [27] for a infinitely narrow DW. In contrast to the spinindependent DOS, here the spin-resolved DOS does not follow the homogeneous shape of the local field even for the case of wide walls.…”

“…This is not always the case here: the relative height of the inner and outher peaks depend on the value of the effective spin-orbit coupling and the both cases can be realized (inner or outer peaks dominate). The fact that the inner peaks are higher due to the inhomogeneity of the ferromagnet was experimentally observed for Al/EuS hybrid structures [27] and attributed to the domain structure in EuS. Now we discuss the influence of the effective spin-orbit coupling or, in other words, the value of the helix spatial period on the DOS.…”

“…In the diffusive limit equation (27) can be further simplified. As it seen from equation (3), in this case =f f and =f f 0 0 and the Green's function does not depend on the momentum direction.…”

“…Recent experiments investigated tunneling in the superconducting side of spin-spiral structures [35]. Furthermore, the influence of the domain structure on the position-averaged superconducting DOS in S/FI bilayer was studied experimentally and theoretically [27]. The DOS in the ferromagnetic part of a metallic S/F bilayer with an infinitely sharp domain wall [36] and in the ferromagnetic part of a S/F heterostructure with a textured ferromagnet [37,38] has been investigated.…”

Spin-flip enhanced thermoelectricity in superconductor-ferromagnet bilayers

“…Alternatively, such a self-energy can originate from a magnetic interface with strong spin-dependent scattering [52][53][54] thereby influencing the DOS [55,56]. The spin-flip scattering time τ sf is the average time between changes of the spin state of an electron [48].…”