Supercurrent in ferromagnetic Josephson junctions with heavy metal interlayers

The length scale over which supercurrent from conventional BCS, $s$-wave superconductors ($S$) can penetrate an adjacent ferromagnetic ($F$) layer depends on the ability to convert singlet Cooper pairs into triplet Cooper pairs. Spin-aligned triplet Cooper pairs are not dephased by the ferromagnetic exchange interaction and can thus penetrate an $F$ layer over much longer distances than singlet Cooper pairs. These triplet Cooper pairs carry a dissipationless spin current and are the fundamental building block for the fledgling field of superspintronics. Singlet-triplet conversion by inhomogeneous magnetism is well established. Here, we describe an attempt to use spin-orbit coupling as an alternative mechanism to mediate singlet-triplet conversion in S-F-S Josephson junctions. We report that the addition of thin Pt spin-orbit-coupling layers in our Josephson junctions significantly increases supercurrent transmission, however the decay length of the supercurrent is not found to increase. We attribute the increased supercurrent transmission to Pt acting as a buffer layer to improve the growth of the Co $F$ layer.

Figure 1

Magnetic hysteresis loops acquired at a temperature of 10 K, (a),(b) for (S-F-S)-type samples with the applied field oriented (a) in and (b) out of the sample plane, and (c),(d) for [$S$-Pt(0.5)-$F$-Pt(0.5)-$S$]-type samples (c) in and (d) out of the sample plane. $F$ is a Co(5)/Ru(0.6)/Co(5) multilayer in all cases. The diamagnetic contribution from the substrate has been subtracted and data are normalized by the saturated value of magnetization. Insets show the low-field switching.

Figure 2

Product of critical current times normal-state resistance vs Pt interlayer thickness ($d_{\text{Pt}}$) for Josephson junctions of the form $S-\mathrm{Pt}\left(d_{\text{Pt}}\right)$-Co(5)/Ru(0.6)/Co(5)-$\mathrm{Pt}\left(d_{\text{Pt}}\right)-S$. Each data point represents one Josephson junction and the uncertainty in determining $I_c{R}_N$ is smaller than the data points.

Figure 3

Product of critical current times normal-state resistance vs total Co thickness ($d_{\text{Co}}$) for Josephson junctions of the following forms: $S$-Pt(0.5)-$\mathrm{Co}(d_{\text{Co}}/2$)/Ru(0.6)/$\mathrm{Co}(d_{\text{Co}}/2$)-Pt(0.5)-$S$ (blue triangles) and $S-\mathrm{Co}(d_{\text{Co}}/2$)/Ru(0.6)/$\mathrm{Co}(d_{\text{Co}}/2$)-$S$ (red inverted triangles). Each data point represents one Josephson junction and where no error bars are shown, the uncertainty in determining $I_c{R}_N$ is smaller than the data point. The lines are fits to simple exponential decay in the range $8\le d_{\text{Co}}\le 16$ nm, with decay lengths of $1.73\pm 0.07$ and $1.7\pm 0.2$ nm, respectively.

Figure 4

Product of critical current times normal-state resistance (solid symbols) and Co $c$-plane spacing (open symbols) vs selected sample for the following: symmetric Josephson junctions containing no interlayers, two Pt(0.5) interlayers, or two Cu(0.5, 2.5) interlayers, and asymmetric Josephson junctions which contain only one Pt(0.5) interlayer. Each symbol corresponds to a set of samples grown in the same vacuum cycle, and our run-to-run variation in $I_c{R}_N$ is visible in the -$F$- only samples. All Josephson junctions contain a Co(5)/Ru(0.6)/Co(5) $F$ layer. Each solid data point represents one Josephson junction and the uncertainty in determining $I_c{R}_N$ is smaller than the data points. The open symbols show the Co $c$-plane spacing obtained by x-ray diffraction measurements on sheet films described in the text.