Spin-triplet supercurrents in Josephson junctions with Co/Py exchange-spring interfaces

Conventional (spin-singlet) s-wave superconductivity (S) and ferromagnetism (F) are incompatible. However, it is now established that s-wave S can coexist with F through the conversion of spin-singlet to spin-triplet Cooper pairs at a magnetically inhomogeneous interface [1]. Here we report Josephson coupling in Nb/Co/Py/Nb Josephson junctions in which the total bilayer (Co/Py) thickness exceeds the singlet pair coherence length [1nm in Py [2]). At a Co/Py interface, the interface exchange coupling is strong compared to the weak magnetic anisotropy of Py and so, depending on the direction and magnitude of an external magnetic field, an in-plane Bloch domain wall can form in Py creating the necessary magnetic non-collinearity for pair conversion [3]. We observe Josephson coupling for Py layer thicknesses up to 11 nm which greatly exceeds the singlet coherence length, strongly suggesting the propagation of spin-polarized triplet supercurrents. Furthermore, through magnetic field history we are able to programme the magnetic state, which offers the potential for active control of triplet supercurrents.
[1]. J. Linder et al. Nat. Phys. 11, 307 (2015).
[2]. J. W. A. Robinson et al. Phys. Rev. Lett. 97, 177003 (2006).
[3]. T. E. Baker et al. New J. Phys. 16, 093048 (2014).