Spin-flipping with Holmium: Case study of proximity effect in superconductor/ferromagnet/superconductor heterostructures

Superconductor/ferromagnet/superconductor heterostructures exhibit a so-called long-range proximity effect, provided some layers of conical magnet Holmium (Ho) are included in the respective interface regions. The Ho layers lead to a spin-flip process at the interface, generating equal-spin spin-triplet pairing correlations in the ferromagnet. These equal-spin spin-triplet pairing correlations penetrate much further into the heterostructure compared with the spin-singlet and unequal-spin spin-triplet correlations which occur in the absence of Ho. Here, we present calculations of this effect based on the spin-dependent microscopic Bogoliubov-de Gennes equations solved within a tight-binding model in the clean limit. The influence of the ferromagnet and conical magnet layer thickness on the induced equal-spin spin-triplet pairing correlations is obtained and compared to available experimental data. It is shown that, in agreement with the experiment, a critical minimum thickness of conical magnet layers has to be present in order to observe a sizeable amount of equal-spin spin-triplet pairing correlations.