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

TL;DR

This paper investigates how Holmium layers in superconductor/ferromagnet heterostructures induce long-range equal-spin triplet pairing correlations, with calculations aligning with experimental observations of the critical layer thickness needed.

Contribution

It provides a microscopic calculation of the proximity effect in superconductor/ferromagnet/holmium heterostructures, highlighting the role of Holmium in generating equal-spin triplet correlations.

Findings

Holmium layers induce long-range equal-spin triplet pairing.

A critical minimum thickness of Holmium layers is necessary for observable triplet correlations.

Calculations agree with experimental data on layer thickness dependence.

Abstract

Superconductor/ferromagnet/superconductor heterostructures exhibit a so-called long-range proximity effect provided some layers of conical magnet Holmium 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 to 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 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.