Enhanced Supercurrents in Josephson Junctions Containing Nonparallel Ferromagnetic Domains

$\pi$ coupling may arise when a ferromagnet forms a link between two superconductors of an artificial Josephson junction. Using a trilayer $\mathrm{Fe}/\mathrm{Cr}/\mathrm{Fe}$ barrier in which the Cr thickness determines the alignment of the Fe layers, we show that the critical currents are substantially enhanced in the antiparallel configuration. The result agrees with existing superconductor-ferromagnet proximity theory according to which the phase-controlling effects of ferromagnets on Cooper pairs can be minimized by arranging their moments in a nonparallel way [Bergeret et al., Phys. Rev. Lett. 86, 3140 (2001); Blanter et al., Phys. Rev. B 69, 024525 (2004)].

Figure 1

(a) Electrons (●) and holes (○) crossing a $SFNFS$ Josephson junction with an AP magnetic configuration (adapted from 14): due to Andreev reflection at both $SF$ interfaces, a reflected hole gains the same energy as is lost by its associated electron so that the transmission resonance results in an enhanced supercurrent. (b) Illustrated $\mathrm{Nb}/\mathrm{Fe}/\mathrm{Cr}/\mathrm{Fe}/\mathrm{Nb}$ junction used in this work: both Fe layers have matched thicknesses ($d$) with the Cr-spacer thickness ($L$) controlling the alignment of the Fe layers.

Figure 2

Room temperature saturation field (a) and coercive field (b) versus Cr-spacer thickness for unpatterned $\mathrm{Nb}/\mathrm{Fe}/\mathrm{Cr}/\mathrm{Fe}/\mathrm{Nb}$ films (series S2). Peaks in the saturation fields suggest AP coupling between the Fe layers while the minima suggest $P$ coupling. Coercivity vanishes in the AP configuration (light yellow) but reaches a maximum value in the $P$-coupled state (dark blue). Arrows indicate possible Fe layer orientations. Inset: normalized magnetization versus external field for $\mathrm{Nb}/\mathrm{Fe}/\mathrm{Cr}/\mathrm{Fe}/\mathrm{Nb}$ films with different Cr-spacer thicknesses as indicated by (i), (ii), and (iii) labels which match-up with (i), (ii), and (iii) labels in the main panel.

Figure 3

Cr thickness-dependence of the maximum characteristic voltage (●) and critical current density (○) at 4.2 K for $\mathrm{Nb}/\mathrm{Fe}/\mathrm{Cr}/\mathrm{Fe}/\mathrm{Nb}$ junction series S1 (a) and S2 (b). Dark blue regions indicate high coercivity (i.e., $P$ coupling) and light yellow regions zero coercivity (i.e., AP coupling). Insets: Predicted characteristic voltage versus Cr-thickness dependence for the $P$ and AP Fe configurations (see main text).