Area-dependence of spin-triplet supercurrent in ferromagnetic Josephson junctions

Abstract

Josephson junctions containing multiple ferromagnetic layers can carry spin-triplet supercurrent under certain conditions. Large-area junctions containing multiple domains are expected to have a random distribution of 0 or π coupling across the junction surface, whereas magnetized samples should have uniquely π coupling everywhere. We have measured the area dependence of the critical current in such junctions, and confirm that the critical current scales linearly with area in magnetized junctions. For as-grown (multi-domain) samples, the results are mixed. Samples grown on a thick Nb base exhibit critical currents that scale sub-linearly with area, while samples grown on a smoother Nb/Al multilayer base exhibit critical currents that scale linearly with area. The latter results are consistent with a theoretical picture due to Zyuzin and Spivak that predicts that the as-grown samples should have global π/2 coupling.

Controllable 0–π Josephson junctions containing a ferromagnetic spin valve

“…27 . This bottom wiring layer and all of the ferromagnetic layers inside the junction, including a 20-nm top layer of Nb and a final layer of Au to prevent oxidation, are deposited in a single sputtering run without breaking vacuum, to ensure high-quality interfaces.…”

Critical current oscillations of elliptical Josephson junctions with single-domain ferromagnetic layers

“…The low surface roughness of the [Nb/Al] multilayer [30][31][32] provides a smooth template for subsequent growth of the Cu spacer and ferromagnetic layer, where the Al is thin enough to be superconducting through the proximity effect with Nb. We independently measured the roughness of [Nb/Al] multilayer to be ≈ 2.3Å using atomic force microscopy (AFM), smoother than a single Nb(100) film (> 5Å).…”

Giant triplet proximity effect in superconducting pseudo spin valves with engineered anisotropy

“…For example, consider a S-F -F-F -S Josephson junction [7]: If the F and F layer magnetizations are collinearly aligned, spin-zero triplet pairs form which, like singlet pairs, are short ranged [8] with a coherence length in F metals of only a few nanometers (Ni [9][10][11][12], NiFe [13,14], Co [11,15,16], and Fe [17,18]). However, if F and F layer magnetizations are noncollinearly aligned spin-one triplet pairs form and the coherence length is greatly extended [19][20][21][22][23][24][25][26][27]. See also spectroscopic works on triplet S-F structures in Refs.…”