Observation of Spin-Triplet Superconductivity in Co-Based Josephson Junctions

We have measured a long-range supercurrent in Josephson junctions containing Co (a strong ferromagnetic material) when we insert thin layers of either PdNi or CuNi weakly ferromagnetic alloys between the Co and the two superconducting Nb electrodes. The critical current in such junctions hardly decays for Co thicknesses in the range of 12–28 nm, whereas it decays very steeply in similar junctions without the alloy layers. The long-range supercurrent is controllable by the thickness of the alloy layer, reaching a maximum for a thickness of a few nm. These experimental observations provide strong evidence for induced spin-triplet pair correlations, which have been predicted to occur in superconducting-ferromagnetic hybrid systems in the presence of certain types of magnetic inhomogeneity.

Figure 1

(a) Schematic diagram of the Josephson junction samples, shown in cross-section. (b) Detailed sequence of the metal layers inside the Josephson junctions (labeled $F$ in a). The layers labeled $X$ are either PdNi or CuNi alloy. The functions of the various layers are described in the text. Only the thicknesses of the Co and $X$ layers are varied in this work. The Cu buffer layers play no active role in the devices, but are important to isolate the $X$ layers magnetically from the Co layers.

Figure 2

Critical current ($I_c$) vs applied magnetic field ($H$) for a $10\mu \mathrm{m}$ diameter Josephson junction with $d_{\mathrm{Co}}=13\mathrm{nm}$ and $d_{\mathrm{PdNi}}=4\mathrm{nm}$, measured at $T=4.2\mathrm{K}$. The excellent “Fraunhofer pattern” results from cancellation of the intrinsic magnetic flux in the junction, due to antiparallel exchange coupling of the two Co layers via the thin Ru layer. (The lines are guides to the eye.) The inset shows the current-voltage ($I\mathrm{\text{−}}V$) characteristic of the junction at $H=0$.

Figure 3

(a) Product of critical current times normal state resistance, $I_c{R}_N$, as a function of total Co thickness, $D_{\mathrm{Co}}=2d_{\mathrm{Co}}$. Red circles represent junctions with $X=\mathrm{PdNi}$ and $d_{\mathrm{PdNi}}=4\mathrm{nm}$, whereas black squares represent junctions with no $X$ layer (taken from Ref. 10). As $D_{\mathrm{Co}}$ increases above 12 nm, $I_c{R}_N$ hardly drops in samples with PdNi, but drops very rapidly in samples without. (The solid line is a fit of the data without PdNi to a decaying exponential, also from Ref. 10. In 10, data from multiple junctions with the same value of $D_{\mathrm{Co}}$ were represented by a single data point with an error bar; here, each device is represented by its own data point.) (b) $I_c{R}_N$ product as a function of $d_X$ for two series of junctions with fixed $D_{\mathrm{Co}}=20\mathrm{nm}$. Red circles: $X=\mathrm{PdNi}$; blue triangles: $X=\mathrm{CuNi}$. (The two squares at $d_X=0$ are taken from Ref. 10.) In both cases, $I_c{R}_N$ first increases, then eventually decreases with increasing $d_X$. Lines are guides to the eye.