Large critical current density Josephson $\pi$ junctions with PdNi barriers

TL;DR

This paper demonstrates large critical current densities in Nb/PdNi/Nb Josephson junctions with perpendicular magnetic anisotropy, enabling efficient zero-field $ ext{pi}$-phase operation for superconducting devices.

Contribution

It reports the first observation of large $J_c( ext{pi})$ in PdNi-based junctions with intrinsic perpendicular magnetic anisotropy, suitable for passive $ ext{pi}$-shifters.

Findings

Achieved $J_c( ext{pi})$ of 410 kA/cm² at 4.2 K in 9.4 nm PdNi barriers.

Observed oscillations in critical current indicating $0$-$ ext{pi}$ transition.

Confirmed perpendicular magnetic anisotropy in PdNi films enabling zero-field operation.

Abstract

We report large $\pi$-state critical current densities, $J_c(\pi)$, in Nb/Pd$_{89}$Ni$_{11}$/Nb Josephson junctions at Pd$_{89}$Ni$_{11}$ thicknesses near the first $\pi$-state. We observe oscillations in the critical current with ferromagnetic barrier thickness consistent with a $0$-$\pi$ transition. For a junction with a 9.4~nm Pd$_{89}$Ni$_{11}$ barrier, we obtain $J_c(\pi) = 410~\mathrm{kA/cm^{2}}$ at 4.2~K, exceeding values reported in prior PdNi-based studies. Magnetization measurements on continuous films, together with coercivity tests on patterned arrays, confirm that Pd$_{89}$Ni$_{11}$ exhibits perpendicular magnetic anisotropy, enabling zero-field operation without magnetic initialization. The combination of large $J_c(\pi)$ and intrinsic anisotropy establishes Pd$_{89}$Ni$_{11}$ as a promising barrier material for passive $\pi$-shifters in superconducting digital logic and qubit architectures.