Spin-triplet supercurrent in Co-based Josephson junctions

Abstract

In the past year several groups have reported experimental evidence for spin-triplet supercurrents in Josephson junctions containing strong ferromagnetic materials. In this paper we present several new experimental results that follow up on our previous work. We study Josephson junctions of the form S/X/N/SAF/N/X/S, where S is a superconductor (Nb), N is a normal metal, SAF is a synthetic antiferromagnet of the form Co/Ru/Co and X is an ferromagnetic layer necessary to induce spin-triplet correlations in the structure. Our work is distinguished by the fact that the generation of spin-triplet correlations is tuned by the type and thickness of the X layers. The most important new result reported here is the discovery that a conventional, strong ferromagnetic material, Ni, performs well as the X layer, if it is sufficiently thin. This discovery rules out our earlier hypothesis that out-of-plane magnetocrystalline anisotropy is an important attribute of the X layers. These results suggest that the spin-triplet correlations are most likely induced by noncollinear magnetization between the X layers and adjacent Co layers. (Some figures in this article are in colour only in the electronic version) 1 To be precise, only the spin-triplet components with m s = ±1 are long range in F, where m s is the projection of the spin onto the axis of magnetization. The m s = 0 triplet component, which is always generated in S/F systems even without noncollinear magnetization, is short range in F. In this paper we always mean the m s = ±1 components when we use the term, 'spin-triplet'.

Evidence for spin mixing in holmium thin film and crystal samples

“…However, this result is consistent with the view that whereas in Ho the spin triplet state is promoted by the intrinsic inhomogeneity of the spin state of the material, in the multilayer stack, the inhomogeneous magnetization has been created artificially and that a Ni 0.19 Pd 0.89 film would not necessarily be expected to support proximity induced spin triplet behaviour in isolation. Our inference concerning NiPd agrees with the recent publication from the Birge group 45 .…”

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

“…[16], so the rough base does not induce any globally anomalous magnetic behavior for Ni films at least 1 nm thick. In addition, there is evidence for generation of some spintriplet supercurrent even in samples containing F' and F" Ni layers only 0.5 nm thick, 23 indicating the presence of at least some ferromagnetic regions in those very thin layers. More importantly, one still has to explain how rough or discontinuous F' and F" layers would affect the area scaling of the junction critical current in the virgin state.…”

Optimization of Spin-Triplet Supercurrent in Ferromagnetic Josephson Junctions

“…1. The two layers labelled F' are either pure Ni or Pd 0.88 Ni 0.12 alloy in this work [11,18]. The inner Cu layers magnetically isolate the F' layers from the Co layers.…”