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

TL;DR

This paper investigates how the critical supercurrent in ferromagnetic Josephson junctions depends on the junction area, revealing that magnetization aligns the supercurrent's sign across the entire junction.

Contribution

It provides experimental evidence that magnetization induces a uniform sign of the supercurrent across the junction area, clarifying the area dependence of spin-triplet supercurrent.

Findings

Critical current varies randomly in unmagnetized samples due to multi-domain ferromagnets.

Magnetization causes the critical current to become proportional to the junction area.

Evidence supports uniform supercurrent sign when ferromagnetic layers are magnetized.

Abstract

In 2010, several experimental groups obtained compelling evidence for spin-triplet supercurrent in Josephson junctions containing strong ferromagnetic materials. Our own best results were obtained from large-area junctions containing a thick central Co/Ru/Co "synthetic antiferromagnet" and two thin outer layers made of Ni or PdNi alloy. Because the ferromagnetic layers in our samples are multi-domain, one would expect the sign of the local current-phase relation inside the junctions to vary randomly as a function of lateral position. Here we report measurements of the area dependence of the critical current in several samples, where we find some evidence for those random sign variations. When the samples are magnetized, however, the critical current becomes clearly proportional to the area, indicating that the current-phase relation has the same sign across the entire area of the junctions.